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EXPERIMENT
DYNAMICS:
MOMENTUM
AND
ENERGY
CONSERVATION
Introduction:
“Dynamics” is the study of a body’s motion taking into consideration the effects of its mass,
momentum, energy and any external forces acting on it. Newton’s laws give us (directly or
indirectly) the concepts of inertia (mass), force (rate of change of momentum), the equivalence of
action and reaction, and the concept of conservation of momentum. These, combined with the
principle of conservation of energy, represent all the tools necessary for a complete dynamical
description of motion.
As an example of an interacting system let us consider two gliders with masses
m
1
and
m
2
on a level air track, moving with velocities
u
1
and
u
2
, respectively. The total momentum of the
system is the vector sum of the individual momenta
If the two gliders travel toward each other and collide elastically, they will bounce apart with new
velocities
and
. The total momentum of the system will now be
1
v
2
v
2
2
1
1
i
u
m
u
m
p
+
=
During an elastic collision, only internal forces act, so that according to the principle of
conservation of momentum:
. Hence,
2
1
1
v
m
v
m
p
f
+
=
2
i
p
p
=
f
If the two gliders are initially at rest then the left hand side is zero. Further, if the two gliders are
initially connected to each other and then forced apart by a sudden force between them (an internal
force), such as an explosion or the sudden release of a spring, the gliders will move apart with new
velocities
v
1
and
v
2
. Thus, since
p=0
, we must have
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 Spring '11
 WagihGhobriel
 Energy, Force, Mass, Momentum

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