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Chapter 8: Momentum
A simple way to understand what happens in a
collision
is to use a quantity called
momentum
.
We’ll find that the total momentum of a system
is conserved if no external forces act on the
system
Collisions and Momentum
An 18wheeler moving at 30mph collides with a Honda Civic moving at
30mph  who comes off worse? Why?
We intuitively know the Civic is in trouble and the reason is that it has much
less mass.
Which is more dangerous  a bullet moving at 1mph or one moving at
50mph?
It is clear that both
mass
and
velocity
are important in these circumstances,
so we define a quantity that includes both,
momentum,
(18 wheeler has more momentum,
faster bullet has more momentum)
Momentum and Newton’s 2
nd
Law
recall the mathematical formulation of Newton’s 2
nd
law
and recall that the acceleration vector was defined as the instantaneous
change in the velocity vector
if the mass is constant with time then
so forces actually cause
changes in momentum
and this even works when the mass changes with time
(an example would be rocket propulsion)
Total momentum
if we have a system of particles, the total momentum of the system is simply
the vector sum of the momenta of the particles
Example 8.1  Preliminary analysis of a collision
A compact car with a mass of 1000kg is traveling north at speed 15m/s when it collides with a
truck of mass 2000kg traveling east at 10m/s. Treating each vehicle as a particle, find the total
momentum just before the collision.
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Conservation of momentum
Last time we learned of an example of a conservation law. We found that
provided no external forces acted on a system to do work, the total
energy in the system was conserved.
With momentum we have another conservation law:
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 Spring '08
 Sukenik
 Physics, Center Of Mass, Force, Kinetic Energy, Momentum, total momentum

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