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# S18 PH 121 11 introduction

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## Richard Datwyler

on 16 November 2018

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#### Transcript of S18 PH 121 11 introduction

Linear momentum
As we begin we need to place this information
somewhere we have been.
So we can make an association with what we have
learned.
Momentum and its relation to force
By definition linear momentum is the mass times
the velocity.
UNITS
This is saying that if I have net force
acting on an object for a period of time
then it will have a change in momentum.
Lets solve this equation for momentum.
What is momentum?
(not just inertia)
Momentum can be thought
of as 'moving' inertia
Review
Think inertia
Think 2 thing
1 Think mass
2 Motion and force (f=ma)
How much Momentum does a 100 kg
bear have moving at 8 m/s?

A. 200 kg(m/s)
B. 400 kg(m/s)
C. 600 kg(m/s)
D. 800 kg(m/s)
How much momentum does it lose
if it slows down to 2 m/s?
For my bear problem, something happened to slow the bear down.
Just like matter it can't just disappear.
This says that some force acting
in a given amount of time, will change
my momentum.
consider a
tennis ball
and racket
When the ball first hits the racket
not a lot of force happens
then it grows, and starts to move away
and the force shrinks
this force looks like this:
This force happening over a period of time is special.
We call it an impulse
impulse-momentum theorem
"I cannot for the life of me figure out the stop and think from 11.1"
"I didn't understand how and why we are using integrals in some formulas.. can we review them?"
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

Conservation
Consider the following collision
mv
mv
mv'
mv'
A
A
A
A
A
A
A
B
B
B
B
B
B
B
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.
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.
demo
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

An explosion is the opposite of a collision.
Maybe a bit better, it is the opposite of an
inelastic collision.

Instead of a few masses sticking together into
one mass
It is one mass breaking apart into many masses.
Again if the system is isolated,
(no external forces)
then the momentum will be conserved.

This is true even though there is an
extremely large force when it explodes.
because this force is an internal one.
In most explosions, the initial momentum is zero
so because momentum is conserved the final momentum
must be zero too.
If the problem is linear, and there are just 2 objects, then
the two momentum are equal and opposite.
Demo
http://media.pearsoncmg.com/aw/aw_0media_physics/vtd/video16.html
Momentum in multiple dimensions
Well, what can we say, momentum is a vector quantity, thus it can be broken down into parts.
If the total momentum is conserved,
each component will be too.
overview
Momentum and impulse
Rockets
2D Momentum
Explosions
Collisions
Conservation

Break the momentum down into the components.
Rockets
Full transcript