1
P123 F11
Lecture 12
18 Nov 2011
CENTER OF MASS MOTION & ROTATIONAL
KINEMATICS
REVIEW
•
LINEAR MOMENTUM:
•
IMPULSE OF A FORCE:
(Strong force acting for
short time)
•
Impulse-Momentum Theorem
•
CONSERVATION OF LINEAR MOMENTUM:
•
TYPES OF COLLISIONS:
•
LINEAR MOMENTUM CONSERVED IN COLLISIONS
(When collision force >> external force)
Elastic:
conserved
Inelastic:
only is conserved
Completely Inelastic:
only conserved & objects stick
v
m
p
dt
p
d
a
m
F
)
(
1
2
t
t
F
J
)
(
1
2
2
1
t
t
F
dt
F
J
t
t
x
x
x
1
2
p
p
J
then
0
If
I
F
ext
P
P
F
]
[
]
[
F
F
I
I
B
A
B
A
p
p
p
p
KE
P
&
P
P
t
1
t
2
t
x
F
x
F

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2
Many nuclear reactors use
Li
(
M
= 7
m
o
) to moderate (
reduce
the
KE
) of neutrons (
M
n
=
m
o
)
produced in nuclear reactions
by elastic collisions with
Li
nuclei..
An alternative is
Na
(
M
Na
= 23
m
o
).
Li
is a better choice because:
A.
P
tot
is lower after
n
Li
collision
B.
n
has less
KE
after
n
Li
collision compared
to
n
Na
collision
C.
P
tot
is higher after
n
Na
collision
D.
KE
of
n
does not change after
n
Na
collision
E.
Na
is better choice (trick question)
i-Clicker
Li
n
Na
n
- or -
P
final
=
P
initial