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Session Objective
1. Rolling of a body – Description
2. Rolling of a body - Mathematical expression
3. Instantaneous center of rotation
Rolling of a body
Rolling without slipping : Motion is combined rotation and translation.
Point of contact has no relative motion with surface
There is no relative motion between body and surface
VP
Q surface
Rolling of a body
Friction (f) is responsible for rolling without slipping
No work is done by f when body rolls without slipping
f = 0slipping
21KE MV
2
spinningno translation
21KE I
2
P
f 0rolling
2 2cm
1 1KE MV I
2 2
Relationship between linear & angular velocity
The point in contact with surface is momentarily at rest. Linear velocity is velocity of center of mass (o).
Or P Vcm
Linear distance moved by centre of mass = S
It is also the distance PQ along the edge of circular body
VCMr
r
S Q
P
Relationship between linear & angular velocity
S r (r : radius of circular cross section)
CM
ds dv r r
dt dt
CMV r
Dynamics of rolling
Rolling without slipping = Linear motion of center of mass+ circular motion about center of mass.
Point P is at rest with respect to the surface at this instant.It is the instantaneous center of rotation .
VCM
VCM
VCM
P
2VCM
VCM
V = 0
+ O
rr
r r
Kinetic Energy
When considered with respect to P
2 2p p CM
1KE I But I I Mr
2
2 2CM CM
1 1KE I MV
2 2
CM( v r ) 777777777777777777777777777777777777777777
2 CMCM 2
I1v M
2 r
O VCM
P
Q
pr
r
r
Kinetic Energy with respect to instantaneous axis of rotation
Velocity vQ of the point Q, with respect to the instantaneous centre rotation
p CM pv v r r 777777777777777777777777777777777777777777777777777777777777777777777777777777777777
ppand a r 7777777777777 7
So torque (external) = (I measured from P)
I
21KE I and L I
2
7777777777777 7
are valid about instantaneous point of rotation.
O VCM
P
pr
r
r
pV77777777777777
Class Exercise - 1
A disc, a ring and a sphere of same radius and same mass are placed on a rough horizontal table so that they can roll over the table. An equal impulsive force is imparted to each through their centres of mass and they start rolling and slipping. When the slipping stops the linear velocity is greatest for
(a) ring (b) disc
(c) sphere (d) All are equal
Solution
2 2 22
1 1 1 IKE I mv v m
2 2 2 r
Ring: I = mr2 2 2r
1KE v (2m) mv
2
21Disc : I mr
2
2 2d
1 1 3KE v m m mv
2 2 4
22Sphere : I mr
5
2 2s
1 2 7KE v m m mv
2 5 10
For same mass and KE, sphere has maximum linearvelocity.
Hence, answer is (c).
Class Exercise - 2
A solid sphere rolls on a horizontal plane without slipping, percentage of KE which is rotational is approximately
(a) 28 (b) 3
(c) 72 (d) 100
Solution
For a sphere,
2 2
2 2 2
2
1 1KE I mv
2 2
1 2mr mv
2 5
1mv (0.4 1)
20.4
Rotational KE percentage 100%1.428%
Hence, answer is (a).
Class Exercise - 3
A hoop of radius r rolls over a horizontal plane with constant velocity v without slipping. The velocity of any point t seconds after it passes the top position is
v
vt vt(a) v cos (b) 2v cos
r rvt vt
(c) 2v sin (d) 2v cosr 2r
Solution contd..
2
r r cos r(1 cosr '
cos(CPB) cos2
r 1 2cos 12
2r cos2cos
2
v ' 2 r cos2t
2v cos t2vt
2v cos2r
Hence, answer is (d).
Class Exercise - 4
A cord is wound round the circumference of a wheel of radius r. The axis of the wheel is horizontal and moment of inertia about its axis is I . A weight mg is attached to the end of the cord and falls from rest. After falling through a distance h, the angular velocity of the wheel will be
2
2
2gh 2mgh(a) r (b) r
mr mr
2 mgh(c) (d) 2gh
2mr
Solution contd..
2 2wheel
22
2
22 2
2 2
2
1 1mgh I mv
2 2
1 v 1I mv
2 2r
1 I 1 I mrm v v
2 2r r
2mghv r
I mr
Hence, answer is (b).
Class Exercise - 5
A mass M is supported by a massless string wound round a uniform cylinder of mass M and radius R . On releasing the system from rest, the acceleration of mass M is
R
M
(a) g (b)
(c) (d) depends on R
g2
2g
3
Solution contd..
2 2
22 2
2
2
2
1 1Mgh Mv I
2 2
1 1 1 vMv Mr
2 2 2 r3
gh v4
3vh
4g
2v 0 2ah 2v 2
a g2h 3
Hence, answer is (c).
Class Exercise - 6
A ring and a cylinder of the same mass m and the same radius R are released with the same velocity at the same time on a flat horizontal surface such that they start rolling immediately on release towards a wall equidistant from both. The rolling friction is finite. Which of the two will reach the wall first?
Solution
For rolling,
2 21 1KE mv I [v = r ]
2 2
2
21 I
mv 12 mr
As m and v are same, KE is same for both.For the ring: I = mr2
2r
KEmv KE v
m
Solution contd..
21For the cylinder : I mr
2
2c
3 4 KEKE mv v
4 3 m
c rv v
So the cylinder will reach first.
Class Exercise - 7
A body of mass m and radius r is rolled up on irregular surface with finite but negligible friction so that it attains a
maximum vertical height of .
What is the moment of inertia of the body and what may be its shape?
25 v
6 g
v
h
Solution
2
2 2 22
1 1 1 1 vKE mv I mv I
2 2 2 2 r
v for rolling
r
2
21 I
v m2 r
As friction is negligible,Loss in KE = Gain in PE
25 mgv
mgh6 g
22
2
5 mgv 1 Iv m 1
6 g 2 mr
2
5 II
3 mr 22
I mr3
The body is probably a hollowsphere.
Class exercise - 8A cylinder AB of radius R, length L and mass M has two cords A and B wound around it. The ends of the cords are attached to a fixed, horizontal support and the cords are vertical. When the cylinder is released, the cords unwind and the cylinder moves down while rotating. What is the angular acceleration of the cylinder and the tension in the cords as the cylinder falls?
R
A B
L
Solution
The equations of motion:
Mg – 2T = Ma (Translational)
2TR = I (Rotational)
2MR
I2
and (Motion of cylinder is rotational) a
R
21 a 1Then 2TR MR MRa
2 R 2
1
2T Ma2
1
Mg Ma Ma2
2 2g
a g3 3R
1
T Mg6
Class Exercise - 9
A disc of radius R is spin about its axis so that its angular velocity is w. Then it is slowly placed on a rough horizontal surface of coefficient of friction m. How long will the disc takes to stop? (Plane of the disc is parallel to surface all through the rotation.)
Solution
Take a concentric ring of radius r andwidth dr (as one of a series ofconcentric rings making up the disc).
rdr
Its mass: 2 2M M
dM 2 rdr 2rdrR R
Solution contd..
Torque on the ring due to friction:
d = dMrg
So total torque on the disc opposing the motion:
R R
22
0 0
2 Mg 2d r dr MgR
3R
d 2I MgR
dt 3
21
I MR2
t 0
0
3 Rdt d
4 g
3 Rt
4 g