41
CONSERVATION OF MOMENTUM
Object:
To study the law of conservation of momentum by analyzing collisions in
two dimensions.
Apparatus:
Ramp, two steel balls, one glass ball, Cclamp, plumb bob, shelf paper
(50cm x 50cm), masking tape, meter stick, protractor, carbon paper (2 sheets).
Theory:
The momentum of an object is defined as
p
= m
v
where m is the mass
and
v
the velocity. If ball A with momentum
p
(figure #1) strikes ball B at rest, and if
the momenta of balls A and B are p
A
and p
B
after the collision, then conservation of
momentum requires that
p = p
A
+ p
B
(1)
Note:
This is a
vector
relation so directions are involved. Since momentum is
conserved this implies that the vector quantities (i.e.
Σ
p
x
’s and
Σ
p
y
’s ) must be
conserved. A typical collision is sketched in the figure below.
Before
After
Figure #1
The vector diagram for the collision looks like:
Figure #2
In this experiment equation #1 will be verified for several different kinds of collisions. In
order to do so,
p
,
p
A
, and
p
B
must be measured. Consider first a collision in which balls
A
and
B
have the same mass. Since both
A
and
B
have the same mass, and
B
is initially
at rest it can be shown that after the collision
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 Spring '09
 mccall
 Physics, Momentum, Collision, SB

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