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# PH 121 9.1-9.2

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## Richard Datwyler

on 21 June 2018

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#### Transcript of PH 121 9.1-9.2

Linear momentum
Hopefully we have seen the value of
Newtonian physics,
But also we note that they aren't the best
tool for every problem.
Now let us move into conservation laws.
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
# 5, what value of F_max gives an impulse of 6.0 N*s?

Meaning that whatever we begin with
will be constant through out the whole
motion.

#11 Far in space where gravity is negligible, a 425 kg rocket traveling at 75 m/s fires its engines. This picture shows the thrust force as a function of time. The mass lost by the rocket during these 30 s is negligible.
a. What impulse does the engine impart to the rocket?
b. At what time does the rocket reach its maximum speed? What is that speed?

A pitcher throws a .15 kg baseball with a speed of 35 m/s to a batter.
a. If the ball is hit straight back to the pitcher with a speed of 55 m/s, what was the impulse given to the ball?
b. If the impact time was 2.5 ms, what was the average force delivered to the ball?

1500 N
15,000 N s
30 s
110 m/s

13.5 N s
5400 N
A golfer is able to deliver 1500 N of force for a time of 4.0 ms to a 75 g golf ball. What is the speed just after the impact?
80 m/s
"Why is stop and think 9.1, F?"
"Can you go over drawing a before and after pictorial representation?"
"I didn't understand how and why we are using integrals in some formulas.. can we review them?"
"can you explain impulse and the momentum theorem?"
"Can you explain how inertia is related to momentum?"
Full transcript