Momentum

Chapter 7

Momentum

• Definition: The “oomph” a moving object has

• Notice – you must have 2 things…– An object (MASS)– Moving (VELOCITY)

• Equation - p=mv– Units =

s

mkg

(like a radio station…kg m s all hits..all the time)

Momentum

• How much momentum does a 2kg object moving at 3 m/s have:

• How much momentum does a 4kg object moving at -5 m/s have:

Momentum

• Which has more momentum:– A skateboarder going down the sidewalk OR– Northwest High School ???

– A bullet fired from a gun OR – A dropped book

Changing Momentum

• Look at the Equation: p=mv• Only 2 ways to change momentum– 1. change the mass (we won’t do that very often)– 2. change the speed/velocity

• How do you change the speed/velocity– Go Faster– Go Slower

Changing Momentum

• To Go Faster or to Go Slower you have to apply a FORCE for a period of TIME

• Brakes apply a FORCE for as long as you apply them (TIME)…this slows you down

• Gas pedal makes the engine apply a FORCE for as long as you push it (TIME) … this speeds you up

Changing Momentum

• Look at that equation… Force..time…– F*t = change in momentum… we’ll call that an

IMPULSE– IMPULSE = F*t (units are N*sec)

• If you have a Big Impulse, you either had a Big Force or a Large amount of Time…or both…

• If you have a Small Impulse, you either had a Small Force or a Small amount of Time..or both

Changing Momentum

• If you have 5N applied for 3 sec– The Impulse is:– The change in Momentum is:

• So Impulse is defined as the change in momentum.

Changing Momentum

• Can’t we figure out the change in momentum another way?

• What if we started with a momentum of 15 kgm/s and a bit later we had a momentum of 10 kgm/s….what’s the change in momentum then?

Impulse-Momentum Theorem

• That gives us the following equation:

• So a N*sec = kgm/s…either unit is appropriate

if mvmvtF

Impulse Consequences

• 2 ways to look at a situation, ONE way is to maximize the amount of Impulse– Think baseball/softball/golf

Impulse Consequences

• 2 ways to look at a situation, The OTHER way is to minimize the amount of impact Force, (the Impulse is constant)

– Think car crashes, egg drop, catching

Momentum is Conserved!!

• That means that if you look at the momentum of everything BEFORE something happens..and the momentum of everything AFTER something happens, they will be the same amount. We don’t lose any.– Think of bounces, crashes

Momentum is Conserved !!

• Examine a Cannon shooting a cannon ball…

Momentum is Conserved !!

• Examine a Rocket blasting off…

Notes Quiz

1. What is the equation for momentum2. What is the definition of momentum3. What are the units of momentum4. Which has more momentum, a semi truck

going down the highway or a baby sitting in a crib?

5. Which has more momentum, your car in the parking lot or a thrown baseball?

Notes Quiz

6. What is one way to change your momentum?7. What is the equation for the Impulse-

Momentum Theorem?8. If I apply 3N of force for 7 sec, what is the

Change in my Momentum?9. If my momentum was 4 kgm/s and then was

20 kgm/s, what probably caused my momentum to change?

10.By how much did my momentum change?