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# PH 121 9.3-9.4

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## Richard Datwyler

on 8 June 2018

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#### Transcript of PH 121 9.3-9.4

Linear momentum
By definition linear momentum is the mass times
the velocity.
UNITS
Momentum can be thought
of as 'moving' inertia
This says that some force acting
in a given amount of time, will change
my momentum.
This force happening over a period of time is special.
We call it an impulse
Conservation
mv
mv
mv'
mv'
A
A
A
A
A
A
A
B
B
B
B
B
B
B
Consider the following collision
F
F
A from B
B from A
This says that initial
Momentum equals final
momentum
Stated nicely:
The total momentum of an isolated
system of objects remains constant.

impulse-momentum theorem
Now this was a nice example of linear
system. The text then jumps into a nice
discussion on multiple dimensions.

Which then proves that:
The total momentum of an isolated
system is constant, and interactions inside
the system do not change to total momentum.

no change when multiple dimensions are
considered.

When doing these type of problems
remember to:
choose an isolated system (if possible)
if not, choose a section of the problem that is isolated and break it into parts
think of the problem as before and after parts
REMEMBER vectors

Consider the following examples.
There are two main types of collisions
Elastic & Inelastic

We will address only inelastic in this
chapter and get to elastic after we have
discussed energy.

Inelastic collisions

When we looked at the tennis ball and racket
we noted that the ball and the springs were
compressed.

In all collisions there is a compression of atoms.
And as we have mentioned before atoms
are bound to each other through electromagnetic
interactions, that act like little springs
or elastics.

If a collision is elastic these springs bounce back.

If it is inelastic the springs do not.

A perfectly inelastic collision is when the objects
'stick' together after the collision.

We usually consider clay balls, or snow balls, or
car crashes, or trains coupling, etc

When they collide they stick to each other and
gain a common final velocity.

If equal masses of clay are considered
and initially they are moving at
the same speed in a head on collision
what will there final velocities be?
A. the negative of their initial speeds
B. depends on the time of the collision
C. Zero

If an average Force of 500 N acts on a stationary mass for a duration of 0.002 s
What is the momentum of the mass after the collision?
A. 0.1 N s
B. 1.0 N s
C. 10 N s

"Why does the block c move faster than block a if the ball transferred all of it's energy into block a and saved some for it's rebound with block c?"
"Would you mind going over some of the equations needed to solve questions from this and the previous section?"
"could you explain how in order to be a perfectly inelastic collision the masses stick together?"
"I don't understand the equations given on page 228. Can you go over them?"
"how can you have a perfectly inelastic collision?"
"how do you choose the system to apply conservation of momentum for a problem?"
A glider and passenger with a mass of 680 kg is gliding horizontally through the air at 30 m/s when a 60 kg skydiver drops out by releasing his grip on the glider. What is the velocity just after the skydivers lets go?
30 m/s
A 10,000 kg railroad car is rolling at 2.0 m/s when a 4000 kg load of gravel is suddenly dropped in. What is the car's speed just after the gravel is loaded?
1.43 m/s
demo
A 1500 kg car rolling at 2.0 m/s you would like to stop the car by firing a 10 kg blob of sticky clay at it. how fast should you fire the clay?
300 m/s
A 50 gram bullet shot with a speed of 200 m/s hits a 2.5 kg block at rest.
If the bullet is trapped in the block, how fast do they move after the collision?
If the bullet emerges with a speed of 75 m/s, how fast is the block moving after the bullet comes out?
3.92 m/s
2.5 m/s
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