MOMENTUM AND IMPULSE

What is Momentum???

What is Impulse???

WHAT HAS MORE MOMENTUM?a) A civic going down the highway at 30

mph or a semi going the same speed?b) A civic going 30 mph or a civic going

60 mph?c) A civic going 5 mph or a semi parked

along the road?

d) What factors do you think affect the momentum of something?

MOMENTUMMOMENTUM IS:

• Proportional to both mass and velocity

• p• A vector quantity• SI Units: kg • m/s

p = mvMomentum = mass x velocity

CONSERVATION OF MOMENTUM The total momentum of a system

remains constant unless acted on by an outside force!

http://www.youtube.com/watch?v=DF4hdDXSg-o&feature=related

http://www.youtube.com/watch?v=rKsaHMBjuTc

http://www.youtube.com/watch?v=oN0idQZw-Ao

http://www.youtube.com/watch?v=prpidlpBNsA

MOMENTUM AND NEWTON’S

2ND LAW• Prove that the two equations

shown below are equivalent.F = ma and F =

p/t

• Newton actually wrote his 2nd Law as F = p/t.–Force depends on how rapidly the momentum changes.

IMPULSE AND MOMENTUM

The quantity Ft is called impulse.SI units: N•m or kg•m/s

Impulse equals change in momentum.Another version of Newton’s 2nd LawChanges in momentum depend on both the force

and the amount of time over which the force is applied.

EGG IN BLANKET DEMO:a) How does the change in momentum in

both cases (smashed and saved) compare?

b) Why is the egg saved in the blanket? (which variable is affected?)

c) Why is the egg smashed on the floor (which variable is affected?)

STOPPING TIMEFt = p = mv

• When stopping, p is the same for rapid or gradual stops.

• Increasing the time (t) decreases the force (F).– What examples demonstrate this

relationship?• Air bags, padded dashboards,

trampolines, etc

• Decreasing the time (t) increases the force (F).– What examples demonstrate this

relationship?• Hammers and baseball bats are made of

hard material to reduce the time of impact.

CHANGE IN MOMENTUM Greater changes in

momentum(p) require more force (F) or more time (t) .

A loaded truck requires more time to stop.Same stopping

forceGreater p for

truck with more mass

CRASH!!! These two cars both get into an accident with the same initial conditions and this causes both cars to stop abruptly. What safety features does the newer model have and how do they work to keep the passengers/car safe?

Car from 1960’s Car from 2008

PRACTICE ONE

• A 65 kg passenger in a car travels at a speed of 8.0 m/s. If the passenger is stopped by an airbag in 0.75 s, how much force is required?– Answer: 6.9 x 102 N

• If the car does not have an air bag and the passenger is instead stopped in 0.026 s when he strikes the dashboard, by what factor does the force increase?– Answer: F = 2.0 x 104 N so it is

29 times greater

PRACTICE TWO

• A 1350 kg car has a velocity of 22.0 m/s to the north. When braking rapidly, it stops in 4.50 s.– What was the momentum of the car before

braking?– What is the magnitude of the force required to

stop the car?

• Answers: – 2.97 x 104 kg • m/s to the north– 6.60 x 103 N