• No lecture, no quiz, no minilabs, no homework next
week.
• But: Mandatory Post-Test!
• Pre-test, pre-survey, post-test, post-survey count 2
points toward your course grade.
• Mo, Thu, Fri sections take post-test during recitation on
Mo, Tu, We next week.
• Tu and We sections take post-test during last week of
classes (Tu Dec. 9 and We Dec. 10).
• Make-up sessions for post-test:
1. During lecture periods on We next week.
2. On Mo Dec. 8.
Diagnostic Post Test
CENTER OF MASS MOTION & ROTATIONAL KINEMATICS
REVIEW OF LAST LECTURE:
• MOMENTUM:
• IMPULSE OF A FORCE:
(Strong force acting for short time)
• Impulse-Momentum Theorem
• CONSERVATION OF MOMENTUM:
• TYPES OF COLLISIONS:
• MOMENTUM CONSERVED IN COLLISIONS
(When collision force >> external force)
Elastic: conserved
Inelastic: only is conserved
Completely Inelastic: only conserved & objects stick
vmp
dt
pdamF
)( 12 ttFJ
2
1
t
tdtFJ
12 ppJ
then 0 If IFext PPF
][][FFII BABA pppp
KEP &
P
P
t1 t2 t
xF
xF
Many nuclear reactors use Li (M = 7mo) to reduce the KE of
neutrons (M n = mo) produced
in nuclear reactions by elastic
collisions with Li nuclei.. An
alternative is Na (M Na = 23
mo).
Li is a better choice because:
A. Ptot is lower after n Li collision
B. n has less KE after n Li collision compared
to n Na collision
C. Ptot is higher after n Na collision
D. KE of n does not change in an elastic collision
E. Na is better choice (trick question)
i-Clicker
Li n
Na n
- or -
Pfinal = Pinitial
CENTER OF MASS
Point in an extended mechanical system that
moves as though all the mass were concentrated
at that point
Consider a collection
of (different) masses
distributed on x-axis.
Define
center of mass: ...
...
321
332211
mmm
xmxmxmxcm
i.e.:
Similar expression for and
If mass is a continuous distribution:
cmycmz
M
xm
m
xmx
ii
i
ii
cmi
i
i
M
rmr
ii
cmi
dmrrMcm
1
EXAMPLE:
mm
mmxcm
3
)2(3)4(
The center of mass is the mass-weighted
average position of the particles.
2
1
4
2
4
))6(4(
x
LOCATING CM
• If body is homogeneous and has a geometric center
(e.g., uniform sphere or cube) then CM is geometric
center
• CM of symmetric body is along axis of symmetry
(cylinder, wheel, dumbbell)
• For collection of extended bodies, find CM of each
body, then CM of collection is CM of those point
masses
EXAMPLE: Two meter sticks, each with a mass of
0.25 kg, form a “T”. Where is the CM?
m 25.0)kg 5.0(
)m 5.0)(kg 25.0(0)kg 25.0(cm
y
0cm x
A wire is bent into a U-shape. Where is the CM?
i-Clicker
An L-shaped profile is cut from sheet metal.
Where is the CM?
i-Clicker
The right half of a meter stick has twice the density
of the left half. Where is the CM?
i-Clicker
A B C E D
MOTION OF CM
• Under influence of external force CM moves like
an imaginary particle of mass M.
• Total momentum of collection of masses can only
be changed by an external force. P
M
rmr iii
cm
i
iicm rmrM
i i
iiicm PpvmvMdt
d
extcm F
dt
PdaM
dt
d
again
dt
PdaMF cmext
Mass m moves along x-axis with speed v
towards mass 3m which is at rest.
They collide and stick.
What is the motion of the CM before and
after the collision?
Before
After
No External Forces:
After the collision CM moves with the combined mass:
Before collision:
IF PP
33 i
fif
vvmvmv
3v
cmv
3
2
3
2 2121 xx
m
mxmxxcm
EXAMPLE:
33
)0(2
3
2 21
cmcm
iidt
dx
dt
dxvv
dt
dxv
Why does the velocity of the CM not change?
A rocket ship moves in a gravity-free region of space
with constant velocity. It fires a short burst of gas
from the rear engine. Afterwards, the CM of the
rocket and gas system has:
A) Sped up
B) Slowed down
C) Has same constant velocity
D) Changed but can’t tell how
E) Insufficient info to tell anything
i-Clicker
cmext aMF
00 cmext aMF
ROCKET PROPULSION
Conservation of momentum of rocket and propellant !!
No external forces:
Rocket is propelled by recoil !
fi PP
exf
exf
exf
mvvvM
mvMvMv
vvmMvvmM
)(
)()(
M + m
m
RIGID BODY ROTATION
• All points rotate about same
axis (through 0, to page)
• If we follow motion of point P as
body rotates, only q changes
Use polar coordinates
Measure q in radians (fractions of 2p )
arc length: (dimensionless) r
srs qq
rad 22
360 pp
r
r
3.572
360rad 1
p
deg][ 180
[rad] :or qp
q
ANGULAR VELOCITY )( z
Rate of change in angle (in xy plane)
Average Angular Velocity:
Instantaneous:
about z - axis
tttavz
qqq
12
12
dt
d
ttz
lim0
zcw)(ccw)(
ANGULAR ACCELERATION )( z
• Average:
• Instantaneous:
Rigid Body Rotation: Every particle has the same
Define vectors:
• Point along axis of rotation
• Have magnitude of and • Orientation define by Right Hand Rule
Sign of : (+) ccw
(-) cw
Sign of :
Units of : s
rad 2
s
rev1 p
dt
d from
zz and
ttt
zz
zav
12
12
2
2
dt
d
dt
dz
q
and
RIGID BODY ROTATION WITH CONSTANT
ANGULAR ACCELERATION
Form of equations same as 1-D motion:
Leads to analogous Equations of Motion:
2
2
)(
dt
xd
dt
dva
dt
dxv
tx
xx
x
1D: Fixed Axis Rotation
tvvxx
xxavv
tatvxx
tavv
a
oxxo
oxox
xoo
xox
x
x
x
x
)(
)(2
const
21
22
2
21
2
2
)(
dt
d
dt
ddt
d
t
zz
z
q
q
q
t
tt
t
ozzo
ozozz
zozo
zozz
z
)(
)(2
const
21
22
2
21
RELATING LINEAR AND ANGULAR MOTION
Each point, P, moves in circle
Magnitude of
Total acceleration
dt
dr
dt
dsv
q
ta
rdt
dr
dt
dvat
ra
22
rr
var
rt aaa
rv
Magnitude of centripetal acceleration
EXAMPLE: A bike tire is rotated by 120o to get the stem
vertical. By how many radians was the tire rotated?
EXAMPLE: Your bike tires have a diameter of 0.75 m.
You ride from CAC BC at a speed of 18 km/hr.
What is the angular velocity of your tires?
• in rev/s?
• in RPM?
3
2)120(
180rad
pp
m/s 5s/hr 106.3
m/hr 10183
3
v
r
v
rad/s 3.13m 325.0
m/s 5
rev/s 12.2rad/rev 2
rad/s 3.13
p
rpm 127s/min) 06rev/s)( 12.2(
EXAMPLE (Cont.): Suppose it took you 4 sec to reach
the speed of 18 km/hr. What was the (constant)
angular acceleration of your tires during this time?
• How many REVs did the tires make while accelerating?
to
sec) 4(0rad/s) 3.13(
2rad/s 33.3
2
21 ttoo qq
22
21 s) 4)(rad/s 33.3(q
rad 6.26q
rev 24.4rad/rev 2
rad 6.26
pq
The graphs below show the angular velocity of two
objects during the same time interval. What is true
about the objects final angular displacement?
A. Object 1 has a greater angular displacement.
B. Object 2 has a greater angular displacement.
C. Object 1 and Object 2 have the same final angular
displacement.
D. It cannot be found with the information given.
i-Clicker
Sketch an angular velocity versus time graph given
the angular acceleration graph shown for the same
time interval, assuming the initial angular velocity is
zero.
i-Clicker
Α
C
Β
D
Sketch an angular acceleration versus time graph given the
angular velocity versus time graph shown for the same
time interval.
Α
C
Β
D
Ε
i-Clicker
23
i-Clicker
rv
• No lecture, no quiz, no minilabs, no homework next
week.
• But: Mandatory Post-Test!
• Pre-test, pre-survey, post-test, post-survey count 2
points toward your course grade.
• Mo, Thu, Fri sections take post-test during recitation on
Mo, Tu, We next week.
• Tu and We sections take post-test during last week of
classes (Tu Dec. 9 and We Dec. 10).
• Make-up sessions for post-test:
1. During lecture periods on We next week.
2. On Mo Dec. 8.
Diagnostic Post Test
Happy Thanksgiving!