impulse & momentum

Get the books off the cart and silently read pp 86-92.

Momentum• All objects have mass; so if an object is

moving, then it has momentum - it has its mass in motion.

• The amount of momentum which an object has is dependent upon two variables:

– how much matter is moving?

– how fast the matter is moving?

• Momentum: "mass in motion”• Equation: p = m x v• Unit: kg*m/s

Momentum

• Momentum is a vector quantity.

Momentum

Consider a Mack truck and a roller skate moving down the street at the same speed. The considerably greater mass of the Mack truck gives it a considerably greater momentum. If the Mack truck were at rest, which would have the greater momentum?

If an object is at rest, the momentum of that object is “0” because there is no mass in motion.

Momentum Questions

1. Determine the momentum of a ...

a.) 60 kg halfback moving eastward at 9 m/s.

b.) 1000 kg car moving northward at 20 m/s.

c.) 40 kg man moving southward at 2 m/s.

p = 540 kg*m/s, east

p = 20,000 kg*m/s, north

p = 80 kg*m/s, south

Momentum Questions

2. A car possesses 20,000 units of momentum. What would be the car's new momentum if ...

a.) its velocity were doubled b.) its mass were doubledc.) both its velocity and mass were

doubled

p = 40,000 units

p = 40,000 units

p = 80,000 units

If the boulder and the boyhave the same momentum,will the boulder crush the boy?

Hint: think about the momentum formula!

p = mv

DEMONSTRATIONS

• Egg and the Blanket

• Bowling Ball

IMPULSE – A force applied for a period of time which results in a change of momentum.

Impulse = change in momentum

F∆t = ∆p = ∆(mv)

F∆t = mvf - mvi

F∆t = m (vf – vi)

To change the momentum of a body, a force must be applied to the mass. The longer this force is applied to the mass, the greater effect it will have on changing the momentum.

The Wall The Haystack

Impulse

• A change in momentum in a short time requires a large force.

• A change in momentum in a long time requires a small force.

Greatest velocity change?Greatest acceleration?

Greatest momentum change?Greatest Impulse?

Therefore, a larger Force and impulse occurs!

Before the space shuttle lands, why

does it take giant S curves?

To increase landing time and decrease the force of the landing

1. An 80 kg skier loses control and demolishes a snow bank. If it takes the skier 3 seconds to come to rest from an impact speed of 9 m/s, find:

(a) the impulse on the man

(b) the average force exerted on him by the snow bank

-720 kg*m/s

-240 N

Lab Activity

Materials

dynamic cart

force sensor

motion sensor

Plot and calculate Impulse

Plot Fvs t graph

plot v vs t graph

Area is equal to impulse

change in momentum is equal to impulse

F vs t graph

• Area under the curve

7500

t (s)

20

F (N)

• Constant F

• Find Area:

=L x W

= F x t

= 7500 x 20

= 150,000 Ns 10

Impulse from an F vs t graph

• Area under the curve

7500

t (s)

20

F (N)

• Impulse = ∆F * t

• Find Area:

= 2(1/2 *b*h)

= 2(1/2*10*7500)

= 75,000 Ns10

Lab Activity-DemonstrationCollision Lab

• Conservation of Momentum i = f

• Purpose: Find relationship of cars before, and after collision

• Bouncy a) same mass, one not moving

b) same mass, both moving

c) different. mass one not moving

• Sticky a) same mass, one not moving

b) same mass, both moving

c) different. mass one not moving

• Explosion a) same mass

b) different mass Plot: i vs. f

Find: m, t1, vi, tf, vf, mvi, mvf for both cars

Conclusion: summarize what you have learnt from the lab

The momentum of a system

remains the same unless acted upon by an

external force.

Hey! It’s the Law!

To change momentum, exert an impulse on it.An outside push or pull is required to change momentum.

Newton’s Balls

Shooting PoolFirecracker

Can you think of any examples where you think momentum is conserved?

both sides gain momentumnet momenta = zero

momentum is conserved

Conservation of Momentum

What happens to the speed of a fighter aircraft chasing another

when it opens fire? What happens to the speed of the pursued

aircraft when it returns the fire (from the rear guns)?

Bored Astronauts Suppose there are 3 astronauts outside

a spaceship. Two of them decide to play catch. All the astronauts weigh the same and are equally strong. The game begins with the first astronaut throwing the second astronaut to the third. How long will the game last?

ΣΣ((mm11vv11)) i i= = ΣΣ((mm22vv22))ff

Watch out for negatives!!!

m1v1i + m2v2i = m1v1f + m2v2f

Tennis and Cannon Activity

• Conservation of Momentum-explosion interaction

pi = pf

• m1v1i + m2v2i = m1v1f + m2v2f

-0.75 m/s

Follow-up Question: Would you want to fire a rifle that has a bullet ten times as massive as the rifle? Explain.

Find the velocity of the rifle.

Three Types of Interactions

1. Bouncy Interaction – two objects collide and bounce off each other. No permanent deforming damage.


2. Sticky Interaction – two objects collide and stick together. Final velocities are the same.

m1v1i + m2v2i = (m1 + m2)vf

2. Explosion Interaction – two objects explode and move apart from each other.


The Next Michelle Kwan…? A softball player wishes to determine

her mass. She glides without friction along on some ice skates at 1.5 m/s, and throws a ball of 0.8 kg mass at 27 m/s. She then determines that she has slowed to 1.2 m/s. What is her mass?

Ans: m = 68 kg

Momentum Railroad Problem of Doom

A 500 kg railroad car moving at a speed of 30 m/s collides and sticks together with a 1000 kg railroad car initially at rest. What will be the resulting speed of the cars after the collision?

Sample Problem

A truck and a car collide head on. The speed of the truck was 20 m/s, and that of the car was 30 m/s. The truck has a mass of 5 times that of the car. If they stick together after the collision, how fast are they moving, as a unit, just after the collision?

The Power of Cheese Salami!

A 30 g bullet traveling at 300 m/s rips through a 0.65 kg salami and exits at 236 m/s.

How fast does the salami move after the bullet leaves?

More MomentumIf a Mack truck and a Volkswagen

have a head-on collision, which vehicle will experience the greatest

force of impact? The greatest impulse? The greatest change in

momentum? The greatest acceleration?

t (s)

F (N)

10

20

30

40

50

60

2 4 6 8 10 12 14 16 18 20 22

Find the impulse for the following graph.


A golf ball is moving with 1 kgm/s and bounces of a bowling ball initially at rest; after the collision, what is the momentum of the bowling ball?


Farmer Joe shoots a bullet of mass 4 g from a gun of mass 7 kg with a speed of 1420 m/s at his collection of coke cans. What is the speed with which the gun recoils?

Answer: V = -0.811 m/s

Net ∆mv (before collision) = Net ∆mv (after collision)

The total momentum before a collision is equal to the total momentum after a collision – Conservation of Momentum

Inelastic Collision:If the mass of each railroad car is the same, determine the velocity after the collision.

5 m/s

m/s m/s

You guessed it… MOMENTUM

Would a head-on collision between two cars be more damaging to the

occupants if the cars stick together or if the cars rebound upon

impact?

elastic

inelastic

Sailing, Sailing..

Gilligan’s super escape ice sail craft is stalled on a frozen lake on a windless day. His only piece of auxiliary equipment is a large fan that blows air into the sail. If all the wind produced by the fan strikes and bounces backward from the sail, the craft will move:

• Backward• Forward• Not at all

Review You jump off a table. When you land on the

floor you bend your knees in order to…

a.) Decrease the impulse on you by the floor

b.) Decrease the force applied to you by the floor

c.) Increase the time it takes to stop

d.) Both a and b

e.) Both b and c

f. ) All of the above

Review

A Mack truck and a Volkswagen have a head-on collision. Which vehicle will experience the greatest force of impact?

a.) Mack truckb.) Volkswagonc.) impossible to figure outd.) same

t (s)

F (N)

10

20

30

40

50

60

2 4 6 8 10 12 14 16 18 20 22

Find the impulse for the following graph.

impulse & momentum

Documents