1raftMotor Boat
V
V
1.1. Method : 1 (Relative approach)V = Relative speed of
motor boat w.r.t.
river which is constant
Observer on raft see that speed of motor boat is constant because duty of motor boat is constant.Hence if motor boat take 1 hrs in down stream journey then to reach again at raft motor boatwill take 1 hrs in upstream journey. Hence Total time in complete journey = 2 hrs.Motion of raft : u = speed of river. Then 2 u = 6 u = 3 km/hr. Ans.
Method : 2 (With frame of ground)
Motion of raft : u (1 + t1) = 6 (i)
Motion of motor boat : (v + u) 1 (v u) t1 = 6 v + u vt1 + ut1 = 6
v vt1 + u (1 + t1) = 6; from (i) v vt1 + 6 = 6 t1 = 1 hrs.
put t1 = 1 hr in (i) : u (1 + 1) = 6 u = 3km/hr Ans.
Q.1.2: Total distance travel by point is S. Then Time taken in first journey :
t1 = 0V2
S Time taken in second journey :
2t V
2t V
2S 2
22
1 t2 = 21 V VS
Mean velocity = 21 t t
S =
210 V VS
VS/2
S
Mean velocity = 021
2102V V V
)V (V 2V
Ans.
Q.1.3 Method : 1 (Graphical Approach) Given tan =
timeTotal
trapziumof Area tan
2t ) t (
21
Time
ntDisplaceme
4 1 t Ans.
Fixed point
6 km u
distance = 2 u
raft
1 hr.
t1
6 km
Part OnePhysical Fundamenatls of Mechanics
1.1 Kinematics
V
t
t
t2
t2
Compact ISC Physics (XII)2
Method : 2 (Analytical)
Total displacement (S) = 22
2t
21 t
2t
2t
21
Time taken = =
22
2t
21 t
2t
2t
21
S t =
4 1 Ans.
Q.1.4 (a)
S
2m
20 s t
average velocity = 202
= 0.1 m/s = 10 cm/s Ans.
(b) Velocity will be maximum when slope of S (t) curve will be maximum.
V = 10 140.4 1.4
m/s = 25 cm/s Ans.
(c) Instanteneous velocity may be equal to mean velocity whenslope of line joining final and initial point will be same toslope at point on curve. Ans. : to = 16 s
Q.1.5 Velocity of B with respect to A : 12A B V V V
Position of B with respect to A : 12ABA B r r r r r
A
B
VB A
Particle B will be collide with A if velocity of B with respect A is directed toward observer A.
Then A BA BA BA B r V r || V Then | r r |
r r | V V |
V V
12
12
12
12
Ans.
Q.1.6
N
EW
S
V = 15 km/hrW E
V = 30 km/hrS E60
Y
X
S
16 sapprox
t
S
1.4
14t
0.4
10
A
r1
v1v2
r2
B
Y
X
3E SV
= 30 i; j 60sin 15 i 60 cos 15 V E W
E W E SS W V V j 60sin 15 i 30) 60 cos (15 V
km/hr 40 60)sin (15 30) 60 cos (15 |V | 22S W
19 30 60 cos 1560sin 15 tan Ans
Method 2
30 km/hr
VW S15 km/hr
60
we know E SE W S W V V V
60 cos V V 2 V V |V| E SE W 2
E S2
E W S W
40 km/hr Ans.
from figure : tan = 60 cos 15 3060sin 15
= 19
Person : 1
Q.1.72 km/hr
d
2.5 km/hr
Time to cross the river : t = cos 2.5
d (i)
from figure : sin = 54
2.52
cos = 53
Put in (i) : t = 1.5d
(ii)
Person : B
2 km/hr
d
2.5 km/hr
x
Using trigonometry : x = d tan 1
Time to reach at destination point : t = t1 + t2 Here t1 = Time to cross the river = 2.5d
t2 = Time to walk on bank = u tan d
ux And tan
= 5
4 2.52
t = 54
ud
2.5d
(iii)
from (ii) and (iii) : 5u4d
2.5d
1.5d
u = 3 km/hr Ans.
Compact ISC Physics (XII)4
Q.1.8 Boat A : dd
time to reach again at same point.
tA = W Vd
W Vd
(i) where V = Speed of boat w.r.t. river
W = Speed of water w.r.t. earth.
Boat B : dW
V Time to reach again at same point. tB = 22 W V
2d (ii)
Also given V = (iii) now 1
2d
d
d
tt
2222
B
A
1
tt
2B
A
= 1.8s
1.9: Method : 1 d Vm r
x
Vr EA
Time to cross the river: t = cos | V |d
mr
Then Drift (x) = (Vm r sin Vr E) cos Vd
mr Since Vm r < Vr E . Hence
this is not possible that drift will be zero. Hence we should have to minimize
drift (x). Hence d
dx = 0
d sec2 mr
E r V
d )(V sec tan = 0 sin = 2
1 VV
E r
r m = 30
Angle made by boat with flow velocity of water = 30 + 90 = 120 Ans.
Method : 2 (Vector addition method)
Vr E
Vm r
E r r mE m V V V
Hence will taken any value between
(0 180) hence we can draw a semi circle of radius of length |V| r m
. Then.
30 90
5Vr EA
C4C3
C2C1
B
And resultant is given by C1, C2, C3 and C4, ....... Cn. But for minimum drift resultant must be
tangent at semicircle. Then cos = 21
VV
E r
mr = 60 Then = 180 60 = 120
1.10: Relative acceleration of particle (1) w.r.t. (2) = g g = 0; Relative velocity
of particle (1) w.r.t. (2) = V1 2 = ) (90 cos V 2V V V 0020
20
= V0 )sin (1 2 Where 90 is angle b/w two velocity..
Since there is no relative acceleration of particle (1) hence its relative velocity
does not change w.r.t time then. Distance b/w two particle at time t is : Distance = V0 t )sin (1 2
= 22 m Ans.
Q.1.11 : Method : 1 (Vector) + y
3 m/s 4 m/s x (+)1
2
Initial velocity in y direction
of both particle zero. Hence vertical velocity of particles (1) at time t : Vy = u + at Vy = g t; Velocity
of particle (1) at time t = : j t g i 3 V1
; Velocity of particle (2) at time t : j t g i 4 V2
Since 0 V . V V V 2121
Then 12 + g2 t2 = 0 t = 0.12 s. Hence distance between two
particle will be : Distance = Vrelative t = (4 (3)) 0.12 = 7 0.12 2.5 m Ans.
Method : 2 (Graphical) : Since 21 V V
+ = 90
= 90 tan = tan (90 ) tan = cot
tan tan = 1 (i)
And tan = 3gt
tan = 4gt
Vr E
C
Vmr
V0
= 60
g g
1 2V0
3 m/s 4 m/s
V1 gt gt
Compact ISC Physics (XII)6
put in (i): 1 4gt
3gt
t = 0.12
Distance = Vrel time = 7 0.12 2.5 m Ans.
1.12 : Method : 1 (Velocity of approach) Since particle A heading to particle B and B to C and Cto A. Then position of all particle at t = dt is as figure 2.
Again position of all particle at t = 2 dt is as figure 3.
Again position at t = 3 dt and so on ........ Since at any timeall particle travel same distance then at each moment of time,all particle will be at equilateral tringle. Then by symmetry youcan say that all particle will be met at centriod of tringle thenpath of each particle will as :Suppose at any instant of time, distance b/w particle A andcentriod P is r. Then, line joing particle A and P make 30 angle
with side of equilateral tringe then dtdr
will always constant and
equal to v cos 30 then dtdr
= V cos 30, ()ive sign because
r is dicreasing function. Finally r = 0 while initial / 3r a
t
0
0
3a
dt 30 cos v
dr t = 3V
2a A P =
2a
sec 30 = 3a
Ans.
a
60 60
60
A
B
C
a
V
V
V
a
Vdt
Vdt
Vdt
Vdt
Vdt
Vdt
Vdt
Vdt
Vdt
A
B
C
30
P30A
P
A 30a2/
/ 3a
7Method : 2 (Relative approach)
let distance b/w A and B at time t is r then. At any instant of time, rate of decreasement ofdistance b/w two particle A and B will be constant as shown in figure.
Then dtdr
= (V + V cos 60) = 2
3V dt
0
0
a
dt 3V dr 2 t = V
3a Ans.
1.13: Suppose at time t distance b/w A and B is r. Then rate of decreasement of r is : dtdr
=
v + u cos o t
0dt ) cosu (v dr
l = vt + u
t
0 cos dt.... (i)
Now since t
0 cos dt is not known then to find this integration,
we use rate of decreasement of component: Then dtdx
= V cos
+ u dt u) cos V( dx t
0
0
0 0 = ut V cos
t
0 dt
cos t
0 dt = V
ut now put in (1) : l = vt + u
Vut
t = 22 u V V
1.14: With frame of train : With frame of train, train appear in restthen distance b/ there two event is equal to l.
With frame of earth : When event (1) will happen. Velocity of train is V = u + at = wt. Sinceevent (2) will be happen after time then. Distance travelledby headlight (A) = Distance travelled by head light (B) =
u1 t1 +
a t21 = wt() +
ww
t Then distance
b/ two events is =
2 t w = 0.24 km. Ans.
AA
1
A
11
B11
C11
C1
C
B1
B
V co
s 60
V cos
60
V V
V
VV
V60
60 60V
V cos 60
B
A
W
AB
event 2
B A event 1
w (t + /2)
A
VB u x
y x
r
Compact ISC Physics (XII)8
It we want that both event will be happen at same point then velocity of reference frame
will be Vreference frame = 60km 24.0
= 4 m/s Ans.
1.15: (a) For observer inside lift at t = 2s. velocity of bolt = 0
Accn of bolt = 10 + 1.2 = 11.2 m/s2. Then assume t time is taken by
bolt to reach at floor. s =
at2 2.7 =
11.2 t2 t = 0.7s
(b) Velocity of bolt with respect to ground at t = 2 sec.
V =1.22 = 2.4 m/s Displacement then S= ut +
at2 S = 2.4 (0.7)
10 (.7)2 0.7 m
Distance : H = 2g2.4 .
= 0.288 Distance travelled = 2 0.288 + 0.7
1.3 m Ans.
1.16: Method : 1 (Relative velocity) from figure
tan = 1
2VV
cos = 22
21
1
V V
V
also tan = 12
1 VV y
y = 12
1 VV BC =
1
212 V
V
Shortest distance = CM = BC cos = 22
21
1
1
212
V V
V VV
shaft
a=1.2 m/s2
2.7m
2.4m/sH
0.7m
Shortest distance
= 22
21
2112
V V
|V V |
now t = 2
22
1
22122
21
122
21
22
21
V V V V
V V
tan CM sec V V
BM AB V V
AM
22
21
2211
V V V V
t
Ans.
1
2y
0V1
B
CM
V2
1
2V2
0V1
9Method : 2 (Velocity of approach)
At shortest distance velocity of approach = 0. Then V1 cos = V2 sin tan = 21
VV
In tringle B A O : tan = 2
1
1
2VV
tV t V
V22 t l2 V2 = l1 V1 V2
1 t t = 222
1
21
V VV V
Ans.
And Shortest distance is : ) t (V t)V ( 2 22
1
Shortest distance = 22
21
21
V V
V V
Method :3
V t11 1 V t
V1
V t 2 2
V2
) t (V t)V ( 2 22
1 ... (1). for shortest
distance dtdl
= 0 ) t (V t)V (
V ) t (V 2 V t)V ( 2 2
22
1
2 211
= 0
t = 22
21
21
V VV V
Ans. put value of t in (1) : 2
221
21
V V
|V V |
Ans.
1.17: Method : 1
Time to reach at point D : T = V/n sec
V tan m
. For
minimum time : ddT
= 0 Then V
sec2 + Vn
sec
tan = 0 sec = n tan sin = 1
. Then
distance BC = l tan BC =
Ans.
1 1 V t
V1
V t2
V2
V t1
A A1
B1
2
B
90
O
BCm tan
m
D
A
tan
Compact ISC Physics (XII)10
Method: 2 (Help of light wave)
We know that light travel via that path in which time will be
less. Then. (2) mediumin speed(1) mediumin speed
sin isin = V
V sin
90sin
sin length BC = l tan =
12
Ans.
1.18:
1.19: (a) Mean velocity in irodov is misprint and it is mean speed then mean speed = R
Ans.
(b) Mean velocity = R 2
Ans. (c) We know ....(i)
And t2
2 2
2 ii from
(i) and (ii) :
2
2
2 now V = RW then
V = R
2
. Average accleration :
cf V V =
0 R 2
Avg. Accn = 2R 2
Ans.
B C
medium (1)i
medium (2)
BR
A
Time taken =
WX
13
6
1m/s2
1m/s2
x
Distance
t
t
t
11
1.20: This is one dimension motion be cause direction of position vector r is same as constant
vector a . Then t) (1 t a r a t t a 2
(a) at 2 a dtrd V
a 2 dt
vd a cc
a V
(1 2 t) Ans.
(b) At initial position t = 0 r = 0 Now at final position r = 0 then. a t (1 t) = 0
t = 0 ro t = Ans.
Initial A B At point B velocity will be equal to zero so
that particle will be turn back. Then 0 = a (1 2t) t =
Then position B is :
r = a
4a 1 . The Distance travelled in up and down journcy is:
2a
4a
4a
Ans.
1.21: (a) V = V0 (1 t/) dt t 1 V dx
x
0
t
00
x = V0
2t
t 2
Position at time t is
x then : x = V0 t
2t t Ans.
(b) Henc we see that velocity will change dirn at t = because When t > v =
And t < v = ive. Then Case I : t < x VV0 t
2t l Ans.
Case II : t > Total distance travelled = AB + BC
= V0
2t
l t V 2
l V 2
l 00 =
2t l t V
2 V
2 V
000
Distance = V0 V0 t
2t l when t > Ans.
1.22 : (a) v = 1
x x differentiate w. r. t. time : 21
21
21
x x2 a
dtdx x
21
dtdv
a = 2 21
Since acceleration is constant; velocity will be : V = u + at V = 2 21
t
C
AB
t =
Compact ISC Physics (XII)12
(b) S = ut + 21
at2 S = 21
22
t2 t =
s2 Mean velocity = s2
s
ts
< V > = s
Ans.
1.23: Calculation of time : w = a v ( i ve sign is used to show deacceleration)
dtdv
= a v ot
0
0
0v
dt a v
dv t0 = a
V 21
0 Ans.Ans.
Calculation of distance : v a v a dxdv v
0
0
21
x
0
0
v
dx a dv v x0 = 3aV 2
3
0
1.24 : (a) j bt iat r 2 x = at y = bt2 y = b
2
ax
a2y = bx2 Ans.
(b) j2bt i a dtrd v
j2bt i a v 222 tb 4 a |v| j 2b dt
vd a
j 2b a
2b |a|
(c) Since direction of acceleration is toward y dirn then angle made by velocity vector with y axis
is known as angle b/w two vectors then. tan = 2bta
Ans.
(d)t
j bt iat ts V
2
mean
jbt i a Vmean
222mean tb a |V|
1.25: (a) x = at y = at (1 t) y = x
ax 1 Ans.
(b) vx = dtdx
= a ay = dtdvy = a 2 a t j t) 2a (a i a v
22 t) 2a (a a |v|
2t) 2 (1 1 a |v|
j 2a dtvd a
2a |a|
Ans.
2bt
x
y
a
13
Method : 1
(c) Velocity =
a (2a t)
a acceleration = 2a
cos = | v | | a |v . a
222 )(2a t) 2a (a a
t) 2a (a 2a 2
1
t = 1
Method : 2
t 2a aa 4 tan
a = a 2 a t t = Ans.
1.26 : x = a sin wt y = a (1 cos wt)
1
ay
= cos wt (i) ax
= sin wt (ii)
1 1 ay
ax
22
x2 + (y a)2 = a2 Equation of circle of radius a. j dtdy i
dtdx v
jsin wt aw i wt cos wa v wa |v| = const. Uniform circular motion.
(a) Distance travelled in time is = (aw) Ans.
(b) Since motion is uniform circular motion. Hence only radial acceleration ispresent then Angle b/w velocity vector and accn will be
Ans.
1.27 : y = ax bx2 differentiate w.r.t. time : dtdx2x b
dtdx a
dtdy
Vy = a VVx 2bx Vx (i)
At x = 0 Vy = a Vx (ii) differentiate equation (i) w.r.t. time : 2x
xxy 2bV dt
dV2bx dt
dV a dt
dV
ay = a ax 2bx ax 2b Vx2 At x = 0 Given ax = 0 and ay = w ay = | 2b Vx
2 | w = 2b Vx2 (iii)
Speed at origin will be : V = 2y2x V V = 2b
wa 2bw 2
V = 2bw
(1 + a2) Ans.
1.28 : (a) 2 ta 21 t u s
20 tg 21 t V s
g
V 0
Ans.
a
(0, a)
Compact ISC Physics (XII)14
(b) ts v
tg 21 v v 0
Ans.
we know T = gsin u 2
u sin = gg . v 0
g2
gg . v
2g v v 00
20
0 gg . v g v v
1.29 : (a) S = 0 in y direction 0 = (V0 sin ) T 21
g T2 T = gsin V 2 0
(b) At maximum height final velocity in y direction = 0 Vy2 = uy
2 + 2 aH
02 = (V0 sin )2 + 2 (g) H H = 2g
sin V 220 Range R = V0 cos
gsin 2V0
R = g 2sin V20 when H = R g
cos sin 2V 2g
sin V 2022
0
tan = 4 = tan 1 (4)
(c) x = (v0 cos ) t y = (v0 sin ) t 21
g t2 y = v0 sin 2
00 cos vx g
21
cos vx
y = x tan cos 2u
gx22
2
Ans.
(d) Radius of currature = onaccelerati normal
V2 R0 = cos g
V20
gV cos 0
A
V0 RA = g cos V 220 Ans.
1.30 : w = g sin andw
= g cos Here is first dicreasing and then
increasing and projection of total accn on velocity vector
will be ()ive then. wv = v. w1
= g sin
g
V
y
x
V cos 0
V0V sin 0
g
V0
g wn
w
V
t
wV
wg si
n w = w
V
w = g cos n
15
1.31 :
Time to collide with incline T = g
2g 2
cos g
cos2gh 2
cos gu 2 y
Length MO = (velocity in MO) T =
sin cos 2gh sin cos 2gh
Range OC = (MO) sin = cos
MO =
cos g
2gh 2 )sin cos 2gh 2 R = 8 h sin Ans.
1.32: We know R = g2sin u 2
5.1 103 = g sin )( 2
1 = 32.5. Also we know that range
will be same for 2 = 90 31.5 = 59.5. Then time of flight will be: T1 = gusin
T1 = 1031.5sin 240
= 24.3s = 0.41min Ans.
T2 = 1059.5sin 240
= 42.3s = 0.69 min Ans.
1.33: Method: 1
. For both particles x and y co-ordinate must be same then.
Particle (1): y = x tan 1 1
220
2
cos V 2 xg
.... (i)
Particle (ii): y = x tan 2 2
220
2
cos V 2 xg
.... (ii)
From (i) and (ii) : x tan 1 cos 2V
xg 1
220
2
= x tan 2
222
0 cos 2Vgt
V = 2gh0
Just before collision
2gh2gh
sin
Just after collision
2gh s
in
2gh c
os
O
C
x
M
y
5.110 m3
240m/s
250 m/s = v0
250 m/s = v 0
6045=
Compact ISC Physics (XII)16
x = 2
21
22121
20
cos cos cos cos ) (sin
gV
.... (iii) Time for particle (1) : t1 = 10 cos V
x
TIme for particle (ii) : t2 = 20 cos Vx
Then t =
210 cos1
cos
1
Vx
.... (iv)
Put value of x on (iv) : t =
21210 cos cos) (sin
gV 2
= 11s Ans.
Method: 2
Particle (1) : x = V0 cos (t + t) .... (i) y = V0 sin (t + t)
g (t + t)2 .... (ii)
Particle (2) : x = V0 cos (t).... (iii) y = V0 sin (t)
gt2 .... (iv) From (i), (ii),
(iii) and (iv): t =
21210 cos cos) (sin
g
2V= 11s Ans.
1.34 : (a) tan = ayV0
Time to reach at hight y : t = y/V0
Also dtdx
= ay dtdx
= a [V0 t] t
0
x
00 dt t aV dx
x =
0
02
0Vy
2V a
2 taV
x =
0V 2a
y2 Ans.
(b) ay = 0 ax = dtdy a
dtdVx = aVy = aV0 anet = aV0 Tangential acceleration = a
= aV0 cos a = 20
2
220
0
)(ay/V 1
y a (ay) V
(ay) V a
Ans.
Radial accn or normal acc
n : (an) = a V0 sin an =
00 V
y a V a Ans.
y
0 x x
v = ayx
v0
y
17
1.35: (a) tan = bxa
x = at .... (i) Vy = (a t) b
y
0
t
0dt t ab dy
2 tab y
2
Then y =
ax
2ab
y =
2ab
x2 Ans.
(b) ax = 0 ay = dtdx
b dt
dVy = ab anet = ab Then normal accn : an = anet cos
anet = ab Then an = ab 22 (bx) a
a
R =
b a) xb (a
aV
2
3/2222
n
2
R =
/
axb 1
ba
Ans.
1.36: Method: 1 (Work-Energy)
Tangential accn is given by :
. a |w| = a cos
Suppose at time t particle at posetion A and in smal l time dt particle displacementby ds.
Work done : dw = Ft ds = (ma cos) ds = ma (ds cos) dw = ma (ds cos) = ma
(ds) cos ma ds cos
Using work energy theorem : W = max
mV2 = max V = xa Ans.
y
x
a = v x
bx=Vy
v
a na net
0
a
ds
x
Compact ISC Physics (XII)18
Method : 2 (Kinematics)
Tangential accn : at = . a = a cos. Also since we know
that tangential accn is rate of change of speed then. dt|v| d
= a cos dsdv v
= a cos v dv = a (ds) cos = a dx
x
0
v
0dx a dv v
2v = ax v = xa Ans.
1.37: Angle travel by particle: = 2n. Speed of particle : v = Rw = at w =
t
0
n2
0
dt Rat ds
dtd
Rat
2n = 2Rat2
t2 = anR
t2 = anR
Now : Radial accn : ar = R ta R
V222
= a
Rn 4 Ra2
= 4na Tangential accn : at = dtdv
= a Total accn = 2t2r a a =
22 na)(4 a
Total accn = a 2n)(4 1 = 0.8 m/s2 Ans.
1.38: (a) At any time t : at = ar = RV2
Since at is rate of change of speed then.
V
V
t
0
22
0Rdt dV V
RV
dtdv () sign because
V is decreasing R
tV 1
V V R
t V
VV
0
0
0
1
(b) anet = 2r2t a a = at at = R
v2 = v ds
dv v dv =
Rv2
dt
Where ds = distance travel by particle in time dt. v dv = Rv2
ds
V
V
S
00Rds
vdv n
RS
VV
0 Then V = V0 e
S/R at = Re V
Rv 2S/R20
2
ds V
dx
a
R
v
R
atar
Givena = aat t = 0u = V
t r
0
19
anet = 2 RV
Re 2 V 22S/R20 anet = 2 R
V e R
2 V 22S/R
20 Ans.
1.39: Method : 1
v = a s Squaring both side : v2 = as
Compare with : v2 = u2 + 2 accS u = 0 acc = 2a
Hence motion is constant magnitude tangential accn. Then.
At any time t : at = a/2 ar = RV2
= Ras
Since we know that velocity vector and tangential accn is parallel then.
tan R2S
a/2 Ras
aa
t
r Ans.
Method : 2
v2 = as differiate w.r.t time : 2v = dtds a
dtdv
= av 2a
dtdv
= at
ar = Ras R
V2 tan = R2S
aa
tr Ans.
1.40 : (a) = a sin wt now = 0 t = 0
= a t = 23 ,2 , V = wtcos wa dt
d
a = sin wt wa dtdv 2 Then at = aw
2 sin wt
ar = R wtcos wa
RV 2222
anet = 2r2t a a
anet = aw2 wtcos
Ra
wt sin 422
2 (i)
at t = 0 = 0 at t = 23or 2 anet = R
wa 22 anet = aw
2 Ans.
at
ar
at
ar
x = a
sin wt
y
Compact ISC Physics (XII)20
(b) For minimum value of acceleration 0 dtdanet cos wt = a 2
R put in equation(i)
amin = aw2
2aR 1 =
a 2R 1 a = 2
2
a 2R 1 a Ans.Ans.
1.41: Speed of particle when distance is S. V2 = 2aS (i) (Because a = const.)
S = 21
at2 t2 = a2S
Then wn = b 222
a4bS
a2S
Radius of curvature : R = 22
n
2
4bS2aSa
wV
R = 2bSa3
Ans.
Net accn : w = 2n2T w a w =
2
2
22
a4bS a
Ans.
1.42: (a) y = ax2 diff. w.r.t. time : dtdx2ax
dtdy
Vy = 2ax VVx
Again diff. w.r.t. time : ay = 2ax ax + 2aVx2
At x = 0 Vy = 0 Then Vx = v ay = 2aV2
Since speed is const. its tangential acceleration will be zero. Then
ax = dtdV
= 0 anet = 2 a VV2 R = 2
2
n
2
2avV a
V R = 2a1
Ans.
(b) 22
2
2
by
ax
= 1 diff. w.r.t. time : 0 b
V2y
aV2x
2y
2x
Again diff. w.r.t. time : 0 b
2V
b
a2y
aV 2
a
a2x 2
2y
2y
2
2x
2x
At x = 0 and y = b : 0 b
V 2
ba 2
aV 2
2
2yy
2
2x
a = a
w = btn4
x
y
V0
21
As shown in figure : Vy = 0 and Vx = V0 0 ba 2
aV 2 y2
20 ay =
202 V a
b
Since speed is Const. tangential accn = ax = 0 at; t = 0 anet = 202 V a
b
Radius of curvature : R = n
20
aV R = 2
02
20
V a
bV
R = b
a2 Ans.
1.43 : Given dtd
= w = const. Then dtd 2
dt)d(2
= 2w = const.
Hence angular velocity of point A w.r.t. point P isconstnat then. WP = 2W and velocity will be perpendicularto position of A w.r.t. P then V = R WP = 2RW Ans.
Since WP = const. 0 dtdWP Then Tangential accn = 0
Radial accn = R WP2 = 4 RW2 = anet Direction is toward the centre. Ans.
1.44 : = at2 dtd
= w = 2at dtdw
= = 2a
Tangential acceleration : at = R = 2aR Radial acceleration: ar = RW
2 = R (2at)2 = 4 a2 t2 R
And. v = RW = R 2at 2at = Rv (i)
Now anet = 2r2t a a = 2aR
22 )(2at 1
from (i) : anet = 22 )(2at 1
tv
= 0.7 m/s Ans.
1.45 : Since acceleration is constant. l = 21
at2 and
V = at Then l = 2
Vt (i) and also. angular acceleration is const. then
= 21 t2 and w = at then =
21
wt and since particle taken n turn in its journey..
V
O
A
P
at
ar
l
Va
Compact ISC Physics (XII)22
= 2 n Now 2 = 21
wt (ii)
(II)(I) : W
V
2
W = v2
Ans.
1.46 : = at bt3 w = dtd
= a 3bt2 when body is in rest then w = 0 a 3bt2 = 0
t = 3ba
in = 0 and final = t (a bt2) =
3
2a 3ba
Wavg = t infinal
=
32a
3ba
32a 3b
a Ans.
Win = a and Wfinal = a 3b
3ba
= 0 avg = 3ab
3baa
t W W infinal Ans.
And. = dtdw
= 6bt = 6b 3ba
= 2 3ab Ans.
1.47 : Given = = at To find tangential accn : (at) : at = R = Rat
To find radial accn : (ar) : = dtdw
ar = Rw2
t
0
w
0 dt dw w = 2
at dtat 2 t
0 ar = 4
ta R 42
Then
at
ar
tan = t
r
aa
tan = ta R 4 ta R 42
t = s 7 tan a4 3 Ans.
1.48 : Given W = angular accn. = k W k = const.
Wk ddW W
()ive because W is decreasing.
0
0
W
21
dk dw W0
at
ar
y
x
R
23
k 3
W2 230 (i) = Angular displacement. Also = k W
dW = k W dt t
0
0
w2
1 dtk dw W
0 t
k2W 2
10 (ii)
Avg. angular velocity : Wavg = = t
= 3
W
k W2
k 3
W2 02
10
230
Ans.
1.49: (a) Given W = W0 a (i). At t = 0 = 0 W = W0
Also dtd
= W0 a t
0
0 0dt
a Wd
t )a (Wn a1
0 0
at Wa W
n 0
0 = )e (1 a
W at0 Ans.
(b) Put value of in equation (i) : W = W0 a
)e (1 a
W at0 W = WW0 e
at Ans.
1.50: Given = 0 cos = W ddW
= 0 cos
0 0 w
0 d cos dw w
2W2
= 0 sin
W = sin 2 0 Ans.
1.51: (a) Instanteneous axis of rotation is passing through that pointwhich velocity is zero always. If we observe carefully itspoint must be at line joining AB. Then. at time t :VP 0 = RW = Rt = yt
y
x
B
A
V
y
0
0
0
2 sin 2 w 0
sin 2 w 0
cos 0
Compact ISC Physics (XII)24
x = vt (i) Velocity of point P :
VP = 0 v = ty x = v
yv
t = yv put in (i)
y = xv2
Ans.
(b) Velocity of point O = 0
VO P = Rw VP = u + at = wt
Then yw = wt t = wwy
x = 21
at2 = 21
w 2
wwy
x =
wyw
21 22
Ans.
1.52 : Since there is no slipping at ground. VC = 0 V = RW where VC = velocity of contact point.
To find distance, we have to find speed of a particle of rim then. = wt Time to one complets
journey = 2/w VP = ) ( cos v v2 v v 22 = 2 v sin /2 = 2V sin (wt)/2 Again
dtds
= 2 v sin wt/2
w2
0
s
0 dt sin wt/2 v2 ds S = w
v8 = 8 R Ans.
y
xO
P (x, y) Vy t
y
x
O
P
(x, y)yw = RW
y
x
C
w
V
A
W
O
RC
B
P
v
v
1.53: (a) Acceleration of point C : aC = w Velocity of pointC at time t : VC = u + at = wt
Angular accn of ball about its centre:
= Rw
Rw
RaC Angular velocity of ball at
time t : w = w0 t = Rw
t Velocity of point AA
w.r.t. centre C : VA C = RW = w t i VC E = w t i VA E = 2 w t Ans.
25
Point B :
A
wt
wtC B
y
x
VBC = Rw = wt ( j) VCE = wt i VBE = wt (i j) VBE = wt Ans.
Point O : VOC = wt i VCE = wt i VOE = O Ans.
(b) Acceleration Calculations :
Point A:
A
arC
at
y
x
aCE = wi .... (i) aAC = at i ar j Where at = tangential acceleration. ar = radial acceleration.
at = R = w ar = R tw R
2(wt) Rv
222 aAC = wi R
tw 22 j .... (ii)
(i) + (ii) : aAE = 2 wi R tw 22
j aAE = 2w
2Rwt
2 Ans.
wt
wtC
0
A
wt
wt
y
x
Compact ISC Physics (XII)26
Point B : a
CE = wi .... (i) a
BC = (R) j +
Rv2
i =
w j R tw 22
i .... (ii)
(i) + (ii) : a
B =
R tw w
22
i + w j aB = w
2Rwt
2 Ans.
Point O: a
CE = w i .... (i) a
OC = (R) i +
Rv2 j =
w i R tw 22 j .... (ii)
(i) + (ii) : a
OE = R tw 22
j aOE = R tw 22
Ans.
1.54 : Velocity of point A = 2v Velocity of point B = v accn
of point A = Rv2 accn of point B = R
v2
Again Radius of curvature = onaccelerati normal
(speed)2 RA = /RV
(2V)2
2 = 4R
Again an = Rv2
cos 45 = R 2
v2 RB = R 2 2
R 2v
)2(V2
1.55 : Method : 1 (axis is rotating): w
10 = w1 i w
10 = w2 j w
12 = w1 i w2 j
w12 = 2221 w w
12 = dt
wd 21
= w1 dti d
w2 dtj d
direction of dt
i d is toward y axis and
this is rate of change of dirn of x axis.
y
x
RC B
y
x
C
0
A 2V
B v
v
V
x
B
an
v2
RV
45
V V 2
v2
A2V
R
27
Method : 2 (axis is not rotating)
w
1C = w1 cos i + w1 sin k w
C0 = w2 j w
10 = w1
cos i + w1 sin k + w2 j 222101 w w |w|
dtw d 01
= w1 sin
dtd
i + w1 cos
dtd
k
And dtd
= w2 = w1 w2 sin i + w1 w2 cos k
| | = w1 w2. Here x axis is directly attached with observer. Then
dti d
= w2 k d 1|i d| dtd
dt|i d| = w2
But dt
j d = 0 Because with frame of this observed dirn of axis does not rotating then.
12 =
w1 w2 k 12 = w1 w2
1.56 : w = at i + b t2 j dtdw
= a i + 2b t j (a)
t
ab 1at |w|
(b)
a t2ab 1at ||
w = w cos cos
w . w
cos 2224222
322
t4b a tb ta
t2b t a
= 17 Ans
y
x0
w2
w1
y
x0
w2
y
x0
w2
w1
w2z
C1
y
z passing
y axis 1 unit
1 unitd
jd ii
Compact ISC Physics (XII)28
1.57: Method : 1 (axis are moving): It we see carefullythen x axis is moving while y direction is not rotating.Angular velocity of disc with respect to centre M
is : i RV W OM
.... (i) Angular velocity of centre
M w.r.t. origen. j OMV W OM
OM = R cot
j tan RV
W OM
. Since angular velocity is vector
quantity it follow vector addition law ] j tan i[ RV W OD
cos R
V tan 1 RV |W| 2OD
Ans.
Again Angular acceleration () is : dtwd
dtj d
tan dt
i d
RV
And since y dirn is constant dt
j d = 0 But
dti d | id | = d dt
d dt
id = wMO k tan R
V dt
id
k tan RV
RV
k tan
RV 2
2
2
2
RV ||
tan
Method : 2 (axis are not rotating)
angular velocity of disc w.r.t. M : WDM = RV
cos i +
RV
sin j angular velocity of M w.r.f. O
: j tan RV j
cot RV W OM
angular velocity of disc
wrt O: W
DO = W
DM + W
M0 W
DO = RV
cos i
RV
sin j RV
tan j
cos RV
tan RV
sin RV
cos RV
|W| OM
Ans.
y
xR0
v
M
p
z
y axis
y axisd
jd ii
y
x0
w =0
D
z
VR
VM
29
0 j dtd cos
RV i
dtd sin
RV
dtwd ODOD
and dt
d is angular velocity of M w.r.t.
0 : d/dt = RV
tan 22
ODRV
sin tan i + 2
2
RV
cos tan j | OD
| = 22
RV
tan Ans.
1.58 : Method : 1 : (Direction of Co-ordinate axis is fixed)
At time t line AB rotate by angle then APW
=
w0 cos i + w0 sin k t j
| APW
| = 22022
022
0 t sin w cos w
| APW
| = w0
0
0w
t Now
j k dtd cos W
dtd ) i ( sin W
dtW d 000AP
= W0 0t sin i + W0 0 t cos k + 0 j 2022
020 t w ||
2200 t w 1 ||
Ans.
Method : 2 (x axis is moving) : Here we take line AB along
x dirn. jt i w W 00AP
20
2
0APw
t 1 w |W|
j dt
j d t dt
i d w dt
wd 000AP
Here 0
dtj d
But dt
i d =
t k Then k t k dt
ds dtdi
0 = w0 t k + j 212020 t w 1 ||
Ans.
y
xA
0
w0
w0
z
y
xA
0w0
z
P
Compact ISC Physics (XII)30
1.2 The Fundamentals Equation of Dynamcis
1.59: Where F is force due to air which will be constant then. mg F= mw .... (1). When m mass is taken then F (m m) g + (m
m) w .... (ii) From (i) and (ii) : m = w mw 2
Ans.
1.60 : Net pulling force = ( m) a m0g k m1 g k m2
g = (m1 + m2 + m0) a a =
021
210m m m
)m (mk mg Ans.
F. B. D of m2 :
m2
a
k m g2
T
T k m2 g = m2 a. Put value of a then T = 2100m m m
m k) (1
m2 g Ans.
1.61 : (a) Pulling force F = m1 g sin + m2 g sin k1 m1 g cos k2 m2 g cos . Then acceleration
a is : a = 21
2211m m
)m k m (k cos g m2) (m1 sin g
.... (i)
F. B. D of m1 : N = Normal force between two blocks. m2 g sin + N + k1 m1 g cos = m1
a .... (ii) put value of (a) in (ii) : N = 21
2121m m
cos m m )k(k Ans.
F
mg
m
F
(m m) g
m m w
km g2
m2m1
a
km g1m0
m g0
(b) For minimum value of acceleration will be zero and friction force will at maxm value then
0 = [g sin (m1 + m2) g cos (k1 m1 + k2 m2)] / m1 +m2 tan = 212211
m mm k m k
a
mg s
in
1
k mg c
os
22
k mg c
os
11
mg s
in
2
m 2
m 1
N
m g sin
1
k mg co
s
11
m 1
31
1.62 : Upward Journey : We know S = Vt 21
at2where V = final velocity in this situation Vf = 0
Then = 21
(g sin + g cos ) t2 (i)
t = Time taken in upward journey.
Downward journey : We know S = ut + 21
at2 u = initial
velocity. Then = 21
(g sin g cos ) (t)2 (ii)
and u = 0 now : (ii)(i)
= 21
cos ug sin g cos ug sin g
=
1 1
2
2
tan Ans.
1.63 : (a) Starts coming down. m2g > m1 g sin + fmax m2g > m1 g sin + k m1 g cos 12
mm
> sin + k cos (b) m1 g sin > m2 g + k m1 g cos g sin > 12
mm
+ k cos
1
2mm
< g sin k cos (c) At rest : Friction will be static : g sin k cos < 1
2mm
m1 g sin Block m2 has tendency tomove down ward.
Then. Pulling force = m2g m1 g sin k m1 g cos acceleration a = 21112
m m cos g mk sin g m g m
a = 1 ] cosk sin [ g
Ans.
1.65 : (a) Before no sliding b/w m1 and m2 :
F.B.D. of System : Acceleration of both block will be same. w1 = w2 = w = 2121 m mat
m m
F
But friction b/w m1 and m2 will be static then. m1fr
fr = m1 [w1] = 211
m mat m
< k m2 g t < 1212
m a)m (m g mk
w1 = w2 = 21 m mat (i)
If t > k m2 g 121
m a)m (m
. Sleeping b/w two block will be start then F.B.D. of m2 :
m2 F = atkm g2 w2 w2 = 2
2m
g km at (ii) Ans.
F.B.D. of m1 : m1k m g2 w1 = 1
2m
g mk (iii) Ans.
Assume to = 1212
m a)m (m g mk t > to : Slope of w2 >
Slope of w1 = w2 because of Slope of w2 =
2ma
from
(ii) [After sliding] Slope of w1 =
21 m ma
from (i) [Before sliding]
Slope w1 = 0 from (iii) [After sliding]
m 1
m2
m g2
m g
sin
1
f
= k m
g co
s
max
1
k
Given mm
1
2.
m2m1
fr = k m g2 F = at
m2m1
w F = at
w
t0t
w1
w = w1 2
w2
33
1.66 : F.B.D. of block : ma = mg sin k mg cos a = g sin kg cos
Time to reach at bottom : s = ut + 21
at2 sec = 21
(g sin kg cos ) t2
A
R
k = coefficient of friction mg sin
k mg cos
t2 = ] cosk cos [sin g
2 ) cosk (sin g
sec 22
t = ] cosk cos [sin g
22
(i)
To minimise t (sin cos k cos2 ) will be minimum also. Assume. x = sin cos
k cos2 Now d
dx = 0 sin sin + cos2 + 2 k cos sin = 0
cos 2 = k sin 2 tan 2 = k1 Ans.
Put value of k : = 49 Put value of in equation (i) :
tmin = 49] cos 0.14 49 cos 49[sin 102.10 2
2
1.0 s. Ans.
1.67 : When block pull up Direction of friction will be downward.F.B.D. of block : N = mg cos T sin fr = k (mg cos T sin ). At just sliding:
T cos = mg sin + k (mg cos T sin ) T =
sin k cos cos mgk sin mg
Tmin
m k
T
m
T
mg sin
mg
mg cos N
T cos T s
in
N
(cos + k sin ) min minimum value of cos + k sin = 2k 1 Tmin = 2k 1
) cosk (sin mg
Compact ISC Physics (XII)34
1.68 : (a) At time of breaking off the plane vertical component of
F
must be equal to weight mg. Then F sin = mg = at sin
t = sin amg . Motion equation of block : a1 = Accelration
of block. F cos = m a1 a1 = dtdv
m cosat
1t
0
V
0
dt t cos a
dV m 2t
cos amV 21
sin ag m
21
cos amV
22
22 V = =
sin a 2 cos g m
2
2 Ans.
(b) t
0
V
0
dt t cos a
dV m
2t
cos amV 2
1.69 : Motion equation : = aS 3mg
cos = m a1 a1 =
acceleration of block of mass m. a1 = m 3mg
cos = 3g
cos
a1 = 3g
cos as s
0
v
0
ds as cos 3g
dv v
s
0
v
0
2
aassin
3g
2v
v2 = a 3g 2
sin as v = assin a 3g 2
v = m 2
cos a t2
2 t cos m 2a
dtds
t
0
2s
0
dt t m 2
cos a ds s = 3
3
xt
m 2 cos a
s =
sin a 6 cos g m
32
32 Ans.Ans.
1.70 : Method : 1
Motion of mass 2 m : T k2mg = 2 mw T = 2mw + 2kmg
Motion of motor : T kmg = ma 2 mw + 2 k mg k mg = ma a = 2 w kg
Acceleration of 2 m w.r.t. m : a2m m = 2w kg + w = 3w kg Suppose in time t both
will be met then. = 21
(3w kg) t2 t = kg 3w 2
Ans.
m
F = at
= 0
m
F = m
g/3
2m wk2mg
m a
k mgT T
kk
35
Method : 2
On system : Tension will be internal force hence k2 mg k mg = 2 mw + ma
a = 2w kg a2m m = 3 w kg then = 21
(3w kg) t2 t = kg 3w 2
Ans.
1.71: For observer inside elevator : a12 = 21010212
m m wm wm g m g m
= 21
012m m
) w (g )m (m
But in form of vector : a1car = 21012
m m)w g( )m (m
Ans.
Acceleration of m1 w. r. t. shaft : a1shaft = a1car + acar = 21012
m m) w (g )m (m
+ w0 a1shaft = 21021112
m m wm m g m )m (m
Ans.
Tension in string : T m1 g = m1 a1shft T = 21021
m m) w (g m m 2
Force applied by pulley on ceiling = 2 T = 21
021m m
) w (g m m 4
as vector form:
2T = )w g( m mm m 4
021
21 Ans.
1.72: Motion of body (2) : mg T/2 = ma 2 mg T= 2 ma .... (i)
Motion of body (1): T mg sin = m a/2 .... (ii) from (1)
and (ii) : ]1 []sin [2 2g
Ans.
1.73 : Equationof motion : T = m0
2
a a 21 .... (i) m1g
= T/2 = m1 a1 .... (ii) m2 g T/2 = m2 a2 .... (iii)
from (i), (ii) and (iii) : a1 = )m (m m m m g ).m (m m m m 4
21021
21021
Ans.
m g1
m1T
T T
m2
m g2
2T
shaft
w0
m
mg sin
a/2T
T/2a
mmg
T/2T/2
m2m1
a1
mg
m g2
m0T
a + a1 2
a2
2
Compact ISC Physics (XII)36
1.74: Motion equation on system: Mg mg = M a1 + m a2 .... (i) Motion equation
of m : fr mg = ma2 .... (ii) Since length of rod is then. 21
(a1 + a2)
t2 .... (iii). From (i), (ii) and (iii) : fr = 2 tm) (M m m
Ans.
1.75: = 100 cm T + mg = m a1 .... (i) + 2T mg = m 2a1 .... (ii) from
(i) and (ii) : a1 = 4 g 2) (
arel = 2a 3 1 . Suppose t time is taken then
= 21
arel t2 t = g ) (2
4) (
Ans.
1.76 : Motion equations T mg = ma .... (i) mg 2T = m a/2 ....
(ii) from (i) and (ii) : a = 4 2) ( 2g
. When body
(1) travel h distance then in same time body (2) travel 2h distance in upward dirn using constraint relation. Nowvelocity of body (2) Justbefore string slack.
V2 = 2a (2h). It body (2) travel x distance again then V2 = 2a (2h) = 2gx
x = gah 2
. Total hight from ground : 4 = 4 h 6
h gah 2
Ans.
1.77: F. B. D. of N sin = m a2 .... (i) F. B. D. of rod : mg N cos = m a1 .... (ii) Constraints:
a2 sin = a1 cos .... (iii) From (i), (ii) and (iii) : a1 = cot 1g
2 a2 = cot tan g
T
2T
mg
nm
Ta1
m
nmg
a 2
1
mgMg
Ma1
ma2
mg
2mgT a
2T
h
a2
N
mg
a1
a1
a2
m a2
a1
N a2
37
1.78: F. B. D. of m : mg KN T = m a2 .... (i) N = m a1 .... (ii) F. B. D. of wedge : a1 = a2....
(iv) From (i), (ii), (iii) and (iv) : a1 =
mM k 2
g km M 2m
mg
Ans.
mg
T
m
KN
M
a1a2
1.79: F.B.D of bodies on frame of wedge: Since system is stationary on frame of wedge hence :
mg = kma + ma + kmg a = k 1k) (1 g
Ans.
a2
a1
mg
mN
TKN
1.80: F.B.D. of block (2) with frame of wedge: At maximum accn w : fr1 will be maxm then fr1 = k [mg
cos + mw sin ]. Since block is under rest with frame of wedge, then equilibrium equation
of block along incline. k (mg cos + mw sin ) + mg sin = mw cos w = k cot )cot k (1 g
am
ma
mg
2 m N
1 mma
m
ma
kmg kma
mg
mg
mw
fr1
2
mg
mw
w
m
1fr
1.81 : Method : 1
F.B.D. of System a2 = accn of bar w.r.t. incline. Since no force on system in horizontal direction
then O = ma1 + m [a1 a2 cos ] (1) F.B.D. of bar w.r.t. wedge : m2 a1 cos + m2 g
sin = m2 a2 (ii) from (1) and (2) : a1 =
sin m m cos sin g m
221
2 Ans.
Compact ISC Physics (XII)38
Method : 2
F.B.D. of bar : with frame of wedge, bar has zero accn in perpendicular to incline then. N + m2a1 sin = m2g cos (i) F.B.D. of wedge : N sin = m1 a1 (ii) from (i) and
(ii) : a1 =
sin m m
sin cos g m2
21
2
2m 2
1
m 1
a2
m 2m1 a1
m a2 1m 2
m g2
Method : 3
F.B.D. of bar with frame of ground observes : Motionequation in perpendicular dirn of incline then : m2g cos N = m2 a1 sin (i)
1.84: v = 360 km/hr = sm 100
36001000 360
At point A : N mg = R
mv2 N = mg +
Rmv2
= 70 g + 500[100] 70 2
= 70
g + 500100 100 70
= 70 g + 140 g = 210 g = 2.1 kN Ans.
At point B : N = R
mv2 (i) put value of m, V and R N = 1.5 kN Ans.
At point C : N + mg = mv2/R N = mv2/R mg (ii) put value of V, m, Rin (ii) N = 0.7 kN Ans.
a2m g2
N
a1
vC
v
BR
Av
N
mg
N
m a2 1
m g2
N
N
m1
a190
N
mg
39
1.85: Tangential acceleration (at) : mg sin = m at at = g sin
Radial acceleration (ar) : Energy conservation : mg cos
= 21
mv2 v = cos g 2 ar = 2v = 2 g cos
anet = cos 4 sin g a a22
rt 22 anet = g cos 3 12
Now : T mg cos = 2mv
= 2 mg cos T = 3 mg cos Ans.
(b) Component of velocity in y dirn: vy = v sin
vy = cos g 2 sin v2y = 2 g cos sin2
For vy maximum v2
y will be maximum. Then x = cos sin2 will
be maximum. ddx = 0 = 2 cos2 sin sin3 = 0 2 cos2
= sin2 tan = 2 sin = 32 cos = 3
1 v2y
= 2 g 32
3
1 3 3
g 4 vmaxy
T = 3 mg cos = 3 mg
31
= mg 3 Ans.
(c) If no acceliration in y direction then. ar cos = at sin 2 g cos2
= g sin2 tan = 2 cos = 31
Ans.
1.86 : Extreme position : Only tangential acceleration is prerent at extremeposition. at = g sin
Lowest Position : Energy conservation : mg [ cos ] = 21
m v2
v2 = 2 g (1 cos ) ar = 2v
= 2 g (1 cos )
A/C condition : ar = at g sin = 2 g (1 cos ) sin + 2
cos = 2 cos = 53 = 53 Ans.
m
T
mg
v
v
y dirn.
at
ar
mgv
Compact ISC Physics (XII)40
1.87: Particle will break off sphere when normal reaction will be zero.
mg cos = R
mv2 v2 = Rg cos . Energy conservation : mgR
(1 cos ) = 21
mv2 mgR (1 cos ) = 21
mRg cos
1 cos = 2
cos cos = 3
2
2 cos 3
= 1 v2 =
Rg
32
v = 3Rg 2
Ans.
1.88: Top view : Spring force = F = N = normal reaction on sleeve. Since no accelerationin tangential direction. N sin = cos (i). Equation in radial direction: N cos
+ sin = m r w2 = m ( + ) cosec w2 from (i):
sin cos x
(cos ) + sin
= m ( + ) cosec (w2) x = m + m w2 = 2mw x m
Ans.
A
A
v
mg
RR
N
w
r
v
l
w
1.89: k = k0 =
Rr 1 friction coefficient. Suppose cyclist at radius of r. then friction provide centripital
force to motion on circular path. Then where kmg = friction force. k0 rmv
mg Rr
12
v2
= k0 g
Rr
r 2
. To maximum value of v : x = Rr r
2 will be maxm. Then dr
dx = 0
0 R2r
1 r = 2R 4
R g k 4R
2R g k V 00
2max
Vmax = gR k 2
10 Ans.Ans.
O
R
r kmgmv /r2
41
1.90: Tangential and radial both acceleration is only provided by friction becausefriction is acting as external force. Then maximum value of friction = k mg.Velocity of car after d distance travel. v2 = 2 w
d. Then ar = Radial
acceleration = R
d w2
Rv2 at = Tangential acceleration = w
anet = 22
22
R4d 1 w w
Rd 2w
Fnet = m w 22
R4d
1 = k mg
Squaring :
2
22
R4d 1 w = k2 g2 d = 1 w
kg 2R
2
Ans.
1.91 : y = a sin x k = friction coefficient. Centripital force=
R vm 2
and centripital force will be provided by friction.
At limiting condition : mgk R vm 2
v2 k Rg.
For v maximum R will be minimum. And we know : speedis constant also we are secing that radius of curvature will be
minimum at maximum point of curve then. ay = 22
2
2 a v dt
y d
sin x . At maximum value of curve : sin x = 1
ay = 22
a v R = a v
v av
2
22
y
2 R = a
2 . Then v2 g ak
2 v
akg Ans.Ans.
1.92: F.B.D. of differential element of length dl. Equation of motion : 2T sin N cos = (dm) Rw2
(i) N sin = (dm) g put value of N in (i) : 2 T (dm) g cot = (dm) Rw2 2 T
R
v /R2
w
y
x
y
x
v
ay
mg
N
w
T T
(dm)
N
gT
T
= very small angle.sin =
Compact ISC Physics (XII)42
R 2m
(R 2 ) g cot = (R 2 ) R 2m
Rw2 T
2 wR m
2cot g m 2
T = 2
m [Rw2
+ g cot ] T = 2g m
gRw
cot 2
Ans.
1.93 :1
2mm
= 0 T1 m1 g = m1 a (i) m2 g T2 = m2 a (ii)
Relation between T1 and T2 : (T + dT) sin 2d + T sin 2
d = dN
Td = dN dfr = dN = Td (i)
Since is very small. sin d d cos d 1
In horizontal direction : (T + dT) cos 2d T cos 2
d = dfr
dT = dfr (i) dT = T d
0
T
T
d TdT
2
1
n (T2 T1) =
T2 = T1 e (iii)
from (i) and (ii) : a) (g ma) (g m
TT
1
2
1
2
from (iii) :
1
2 e TT
Then e =
a ga g
a ga g
mm
1
2 (iv). Since pulley is fixed : Before skiding a =
0 = 0 e = 0 =
1 n 0
(b) from equaiton (iv) : en0 =
a ga g
a ga g
0
a = g 0
0n
] [n
Ans.
T1T2m1m2
m g1m g2
a
dT + dT
dN
dfr
T
y
x
43
1.94: Hence velocity along y axis is not responsible for circularmotion only velocity along Z-axis i s responsible.
Then N = RV m 2Z VZ = V0 cos N = R
cos V m 220
1.95: x = a sin wt ax = 22
dtxd
= aw2 sin wt
y = b cos wt ay = 22
dtyd
= bw2 cos wt jay i a a xnet
= j wt cos wb isin wt wa 22 ]j wt cos b isin wt [a mw F 2
j wt cos b isin wt a jy i x r 2 wr m F
where r
= position vector of particle. F = mw2 22 wt)cos (b sin wt) (a
F = mw2 22 y x Ans.
1.96 : Method : 1 (Impluse equation)
(a) We know P
= Impluse in time t = t
0
dt F = t
0
dt mg P
= mg t Ans.
v0
xR
Z
O
y
V0
w
x
y
gV0
m
(b) T = gsin V 2 0
g . V0 = V0 g cos (90 + ) g
sin V 2 mg P 0
g . V0
= V0
g sin = 2 m V0 sin V0 sin = gg . V 0
gg . V
2m P 0
Ans.
Method : 2 (Kinematic)
(a) j sin V i cos V V 000
j gt) sin (V i cos V V 00f
where fV
= final veloci ty vecto r then 0f V m V m P
j t g m P
Ans.
V0
y
x
V0
90+
g
Compact ISC Physics (XII)44
(b) T = gsin V 2 0
j gsin V 2
g m P 0
P
= 2 m V0 sin j
From method : 1 : V0 sin = gg . V 0
j gg . V m 2
P 0
Ans.
1.97 : m
t) (T t a F
(a) w m F
w = linear acceleration. ] tt [ m
a w
dtVd 2
t
0
2 t
0
V
0
dt t dt t ma Vd
3
2 t
ma
V32
f3
P 6 a V m
Ans.
(b) ] t t [ ma
w 2
t
0
2t
0
V
0
dt t dt t ma Vd
3t
2t
ma
V32
0
3
0
2S
0
dt 3t dt
2t
ma dS
12
6 .
ma
S43
12
ma S
4 S =
m 12 a 4
Ans.
1.98: sin wt F F 0
a = dtdV sin wt
mF0
t
0
0V
0
dtsin wt mF
dV V = t
0
0 wt cos mwF
V = wt]cos [1 mwF0
t
0
0S
0
dt wt]cos [1 mwF
dS S =
sin wt w1 t
mwF0
S = 20
mwF
[tw sin wt] distance
distance will be increasing function w.r.t. time then
dtdS
= (+)ive or > 0 w w cos wt > 0
cos wt < 1 0 < wt < 2 < wt < 3 4 < wt < 5
wt
45
1.99: F = F0 cos wt mF a
dtdv 0 cos wt
t
0
00
0
dt wt cos mF
dV 0 = wmF0 sin wt
sin wt = 0 wt = t = w
Ans.
Now : t
0
00
0
wt cos mF
dV V = wmF0 sin wt ... . (i )
t
0
0s
0
dt sin wt wm
F ds
S = wt)cos (1 wmF
20
at t = w s = 2
0
wm2F
velocity will be maximum when sin wt = 1
Vmax = wmF0 Ans.
1.100 : (a) F = rV V mr
dtdV
t
0
V
V
dt m
r VdV
0 t
mr Vn VV0
V = V0 er/m t V will be zero when t Ans.
(b) a = mr
V = dsdv v
s
0
V
V
ds vmr dV v
0V V0 = m
r S V = V0 mr S
Total distance travel by particle is S1 then final velocity = 0 0 = V0 mr S1
S1 = rV m 0 Ans.
(c) 0V
= VV0 mr
S2 S2 =
1
rV m 0
(i) Also 0V
= VV0 2 tmr
e
t2 = n rm (i) =
n
1) ( V
tS 0
2
2 Ans.
1.101 : F v2 F = kv2 a = mkv 2
t
0
V
V2 dt m
k vdv
0
mkt
V
1 V
V 0 m
kt
V1
V1
0
t km
V V
V V
0
0
(i)
V0 V
h
Compact ISC Physics (XII)46
Also a = dxdv v
mkv 2
v dv = m
dx kv 2
h
0
V
V
dx k Vdv m
0 m n
0VV
= kh
k = hm
n 0V
V put in (i) : t =
VVn
h V V
V V00
0
Ans.
1.102: Suppose at time t distance travel is x then F = mg sin axmg cos mw = mg sin ax mg cos where w =acceleration of block. w = g sin ax g cos .... (i) v = g
sin ax g cos dxdv v
= g sin a x g cos
mx
0
0
0
dx ) cos axg sin (g dv v 0 = (g sin ) x 2
cos g xa 2 x =
cos g a
sin g 2 x = a
2
tan . Velocity will be maximum when dtdv = 0 = acceleration g sin = ax g cos
x = a1
tan now
tan ay
0
maxv
0
dx ) cos g x a sin (g dv v
maxV = g sin
tan a1
2 cos g 2 tan
a1
VVmax = a
g cos sin
ag 2
sin tan Ans.
1.103: K = friction coefficient Block start sliding when.
F = at1 = k m g t1 = ag mk . Assuming over time start from block start sliding then.
Suppose acceleration of block is w then. mw = at kmg = a (t1 + t) kmg = a t w
= ma
t 2
t
0
v
0
t 2ma V t)( dt (
ma dV
t)( d t 2ma ds
t
0
2s
0
S = 6ma
(t)3 = 6ma
(t t1)3 Ans.
mg
sin
K mg co
s
k = ax
t = 0 t = t1
t = t t1
t = t
mK F = at
47
1.104 : h
v = 0
V0Upward journey :
mg
F = Kv2V0
Fnet = mg + kv2 a = g +
mkv 2
m
kv mg dsdv v
2 mv dv = (mg kv2) ds
0
v
h
02
0
ds kv mgdv mv h = mg
mg KVn
2km 20 .... (i)
Downward Journey : Fnet = mg kv2
mg k v
2
v a = mkv mg 2
= v dv/ds
mgmg KV
n 2km
h ds kv mgdv mv
20
v
0
h
02
2
20
kv mgmgn
2km
mgmg KVn
2km
v =
mgKV 1
v20
0
Ans.
1.105 :(a) Position vector of particle : r
= r cos i + r sin j
At time t : = wt )j sin i (cos F r F F
]j sin i [cos mF a
t
0
t
0
V
0
dt (sin wt) mF idt wt)(cos
mF v d
]j wt)cos (1 i[sin wt mwF V
speed = 22 wt)cos (1 (sin wt) mwF |V|
Speed = 2
wtsin mw2F
Ans.
(b) Distance is calculated by speed. Then 2wtsin
mw2F V
dtds
dt 2wtsin
mw2F
ds t
0
s
0
S = Distance S = t
02 2
wt cos 2 mw
2F S =
2wt cos 1
mwF 4
2 .... (ii) velocity of particle
rw
m
F
0
Compact ISC Physics (XII)48
will be zero : 0 = 0 2wtsin 2
wtsin mw2F
put value of t = w2 in (ii)
S = 2mwF 4
(1 cos ) S = 2mwF 8
Average speed = /w2
8F/mw Time
Distance 2
Average speed = mw 4F
Ans.
1.106: K = tan = friction coefficient When = W = net
tangent ial accelerat ion/ Wx = Acceleration
in x direction. Now W =
m cos mgk cos sin mg
= g (sin cos K cos ) = g [sin cos sin ] W = g sin (cos 1)... (i)
Acceleration in x dirn: Wx = m cos cos mgk sin mg
WWx = g sin [1 cos ] .... (ii)
From (i) and (ii) : W = Wx dt
V d dtV d x V = Vx + cos at t = 0 V
= V0 and Vx = 0 V = Vx + V0 .... (iii) Then const = V0 Also Vx = V cos ....
(iv) From (iii) and (iv) : V = cos 1
V0 Ans.
1.107: Tangential force on system is F then F sin g (dm) = sin g )d R ( = Rg
t
0
d sin = R f in = 0 = Rgf cos
f = R
f = RF
=
Rg R cos 1 a = mg R
R cos 1 = x g R
R cos 1 a = R g
R cos 1
v
wmg sin
K mg cos
x
f
y
Rd
dm= R d
x
(dm)g
F
ddm
49
1.108: At time of break off. normal reaction will be zero. Where mw0
= Psuedo force because observer at sphere. R
mV20 = mg cos
m w0 sin V20 = Rg cos R w0 sin .... (i)
Energy equation: Wall forces = Kf Ki Wpsuedo + Wmg = Kf
Ki m w0 R sin + mg [R R cos ] = 21
m V20 0 V20 = 2w0
R sin + 2 g R [1 cos
].... (ii) From (i) and (ii) : V20 = 2R
RV 20
+ 2 g R V20 = 2 g 3R V0 = 3
g R cos
gw
1 3
gw g 5 3
w
0
00
Ans.
1.109 : Given F nr1
F = nrK
Where K = Constannt.
A particle is said to steady if we displace particle away from origin, particle want to regainits position and also it we displaced particle toward origin, particle want to regain its original
position. Then. At steady state (mean position) : F = rV m 20 Then r
V m
rK 20n .... (i)
It we increases r then. F rV m 2
> 0 K rn mV2 r1 > 0 Differentiate both side
with respect to r for small increase of r: n k rn1 dr + mV2 r2 dr > 0 and
r1
rVm
r1
r
Kn 20n
> 0 .... (ii) From (i) and (ii) n < 1 Ans.
w = acceleration0 R
R00
mw0
V0
mg
R
O F
v0
r
r
0
v
Compact ISC Physics (XII)50
1.110: At steady state: mg sin = m (R sin ) w2 cos cos
= 2 wRg
. Case (i) : If Rw2 > g then cos 2 wRg
is defined and
only one equilibrium position will be exist and will be steady.
Case (ii) : If Rw2 < g then only 2 wRg
= 0 will be equilibrium
position because tangential fore along arch of ring due to mgwill be greater than that of pseudo force and object will comeat lower position of ring.
RN
Y
mg
m (R sin ) w2
Y
90