MOMENTUM? CHAPTER 7

You might have heard people gaining momentum.

What is it and how do you get it???

MomentumStore

I’d like some please

Sure Thing

MOMENTUM?

You might have heard people gaining momentum.

What is it and how do you get it???

MomentumStore

Here you go

Momentum is basically how hard it is to stop something.

What would make a moving object hard to stop?

MORE MASS (INERTIA)

MORE SPEED

An object with a lot of mass moving quickly has a lot of momentum

When playing red rover, why do you run instead of walk when trying to break the chain

MOMENTUMThe faster you are moving the more momentum you have!!!!

Which person would you rather have on your team?

The more mass ( for a given speed) the more momentum!!

AB

If they are all running the same speed who has the most momentum?

Momentum = mass x velocity

p = m v

momentum of object Kg m

velocity of object(m/s)

mass of object(Kg)

s

Which are vectors

both velocity and momentum are vectors

What would happen to the momentum of a cannonball if: speed doubled& mass is constant

a.) NO CHANGE

b.) x2

c.) 1/2

d.) x4

e.) 1/4

What would happen to the momentum of a cannonball if: speed is constantmass is cut in half

a.) NO CHANGE

b.) x2

c.) 1/2

d.) x4

e.) 1/4

What would happen to the momentum of a cannonball if: speed is doubledmass is doubled

a.) NO CHANGE

b.) x2

c.) 1/2

d.) x4

e.) 1/4

Can a bus and a bug have the same momentum?

Momentum = mass x velocity

                  = m v

                        

     

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

Hint:  Which would have thelarger speed?

What is the momentum of an 10 kg object moving at 20 m/s?

How fast is a 40 kg object moving if it has the same momentum?

p = m v

What is the momentum of a 1200 kg elephant at rest?

What does this vector thingee have to do with it?What would direction mean????

= m v

THE DIRECTION of momentum is just theDirection of velocity!!!!

If a car (mass constant) had momentum vectors changing as follows what does it mean?

What about signs?

Positive

Negative

If a car (mass constant) had momentum vectors changing as follows what does it mean?

If a car (speed constant) had its momentum vector change as below what does it mean?

What is the velocity of 750 kg rhinoceros, if its momentumis -2800 kg m/s?

How do you change the momentum of an object?(like a car at constant mass)

p = m vChange the velocity --- Accelerate!

How do you accelerate an object?

Apply a Force!!

When you step on the gas and apply a forceto speed up your car, 2 things determine

how much your momentum (speed) changes

1.)

2.)

How far you push the pedal - Magnitude of force

How long you push the pedal - Duration of force (time)

A force applied for a time is called an: IMPULSE

Impulse = F t

Magnitude and direction of force

Time force is applied

An impulse changes the momentum of an object.More specifically Impulse = Change in Momentum

An object is accelerated from rest by an impulse.How would the change in momentum of the object compare if:

The Force was applied for half the time?

The Force is twice as strong over the same time

IMPULSE = CHANGE IN MOMENTUM

The equation relating Impulse and the change in momentum it causes can be derived very easily:

F = m a

First a force must be accelerating a mass:

What is the definition of acceleration?

a = vt plugging in for “a”

F = m vt Cross multiplying

yields form 1 of the equation

Applied force

time force is applied &time object accelerates

F = mvtImpulse = change in momentum

F = mvt

v = v2 - v1

F t = m(v2 - v1)

F t = mv2 - mv1

F t = p 2 - p 1

F t = p Form 2 of the equation

F t = pIMPULSE = change in momentum!!!!

All forces are applied over a time period(An Impulse)

Even things that appear to be just for an instant

a balloon decelerating due to an impulse

Impact of a racquetball with a wall.

When trying to change the momentum a ball two factors count:

1.) Magnitude of Force - how hard you swing

2.) Duration of Force - Follow through

F t = p

IMPULSE = change in momentum

A speeding car (say 60 mph) can come to a stop two waysBoth will have the same and the same impulse

Crashing into a wall

Gradually applying brakes

pFt

pF tBOTH HAVE THE SAME CHANGE IN MOMENTUM

You can stop an object QUICKLY requires a LARGE FORCE

Ft

F t

BOTH HAVE THE SAME CHANGE IN MOMENTUM

Or you can stop an object gradually with a small force

Why is a falling glass more likely to break when it hitsconcrete than carpet?

Objects that cushion, spread the force out over a longer time

What is the safest way to land from jumping off a roof?

What involves the greatest change in momentum

Slowing from 60 m/s to rest Quickly or Slowly

What involves the greatest IMPULSE?

What involves the greatest FORCE?

The puncher wants to deliver a large force over a short time (reducing the momentum of his fist)

The puncher could have gently pushed his opponent for a minute and changed his momentum just as much

The puncher wants to deliver a large force,How could the punchee reduce the force?If time of contact with the punch is doubled, what happens to the force of the punch?

Rolling with the punches

Other times you want a large force over a short period

Other examples:

Air bagsBungee JumpingCrumple Zones in Cars

Egg and bed sheet demo

What is the momentum of El Nino (6.0 kg) rolling alongat 2 m/s.

What is the change in momentum if El Nino comes to a stop?

What IMPULSE would be required to do this?

What force would be required to stop El Nino in 10 seconds?

A 25 N force accelerates a 1200 kg car from rest to 20 m/s?

What is the change in momentum

Impulse?

How long did it take?

Which has the greater change in momentum

A ball that hits and stops?

or one that bounces back??

Let’s look at the ball that hits and stops,Lets say it has a mass of 1 kg and is initially traveling at 1 m/s (to the right)

After it stops what was the change in momentum?

Now let’s look at the ball that bounces.Again 1 kg and initially moving at 1 m/s (right) and after it bounces it is moving 1 m/s (left)

Which will require a greater impulse ball

A ball which hits and stops

orA ball that hits and bounces off

An object which bounces backwards at the same speedrequires twice the IMPULSE.

to bring it to a stopaccelerate it backwards

Which will hurt more a flower pot which hits your head and breaks, or one which hits your head and bounces off?

ouchForceForce

continued

Increasing Impulse

                                                      

      

Curved bladesprolong contactwith water: Impulse = F t

Greater impulsemeans greaterchange in momentum of the wheel.

A 2 kg ball moving to the right at 4 m/s hits a wall and bounces to the left at 4 m/s. What is the change in momentum.

(if needed)

Impulse momentum problems

2, 4, 5, 7, 8, 9, 12, 19,

Read Sections 7-1 to 7-4 in book

Answer Questions on pages 100 - 102

14, 15, 21, 24, 27, 35

CONSERVATION OF MOMENTUM.

Could you, like, save some momentum for

the rest of us.

Which object changes momentum after firing?

What does it mean that momentum is conserved?

Which feels the greater force?

cannon ball same

Which feels the force for the longest time?

cannon ball same

Think Newton’s 3rd law!!!

After the cannon fires both the cannon and the ball experience the same IMPULSE (Force x Time)

So they both experience the same change in:_________

momentumvelocity speed

After firing?

Which has more momentum

p = m V

Cannon ball

p = m V

THEY BOTH HAVE THE SAME MOMENTUM

They both have the same amount of momentum but something is different….

p = m V

Cannon ball

p = m V

THE DIRECTION(consider signs)

+5 kgm/s- 5 kg m/s

What do you get if you add up the momentum of the ball and cannon?

Cannon ball

+5 kgm/s- 5 kg m/s

What was the initial momentum of the ball and cannon?

ZERO

The total momentum is the same

BEFORE Firing AFTER Firing

SUM = ZERO SUM = ZERO

- 5

+ 5

0

0

Conservation of momentum:

Without an EXTERNAL force the momentum a system doesn’t change.

initial momentum = final momentum

A system:Objects of interest

We looked at a cannon / ball “system”

No external forcesAn external force would be from something other than the ball or cannon

Here the cannon and ball had no initial momentum,but the outside force gave them both positive momentum

No external forces

Would friction be considered an outside force?

Which will change the momentum of a car

Pushing a car from the inside

Pushing a car from the outside

both

ONLY pushing the car from the outside changes the momentum because it is an external force

Which will change the momentum of a car

Pushing a car from the inside

Pushing a car from the outside

bothPushing from inside will not change the momentum of the car /person system

Initial momentum = Final Momentum

p1 = p2

Does this mean that the momentum of parts of a system don’t change?

NO… When you add them up (taking into account + and -) initial momentum = final momentum

Initial Momentum = Final Momentum

pjack1 + pjill1 = pjack2 + pjill2

Jack(80 kg) and Jill (40 kg) are at rest initially, After pushing off Jack is moving at +10 m/s, what is Jill’s velocity?

Car & Track Momentum Demos

Equal masses stationary

Unequal masses stationary

=mv

Say the cannon has a mass of 650 kg and the ball has a mass of 25 kg. After firing the ball flies out at 35 m/s to the right, what is the velocity of the cannon?

What would happen if 2 astronauts played catch?

Newton’s cradle applet

Astronaut Catch

All of the scenarios we have looked, the objects started from rest. So the momentum always had to add up to ZERO.

Like Explosions

Conservation of momentum in collisions

In a collision the initial momentum may not be zero.WHY?

2 Main Types of Collisions

ELASTIC- objects hit and bounce (no energy is lost)

INELASTIC- objects hit and stick together (energy IS lost)

BUT MOMENTUM IS ALWAYS CONSERVED

Car & Track Momentum Demos

Equal masses 1 moving

Elastic Collisions

Equal masses both moving opposite directions

Equal masses both moving same direction

UNEqual masses 1 moving

Car & Track Momentum Demos

Equal masses 1 moving

INElastic Collisions

UNEqual masses lighter moving

UNEqual masses heavier moving

Inelastic collision applet

A 4.0 kg car moving 15 m/s hits a 8.0 kg car at rest. What is their velocity after they hit and stick together.

A large 210 kg hobo jumps into to the back of a 670 kg truck moving at 14 m/s. Assuming the hobo has no initial momentum, what is final the speed of the truck and hobo?

WS In Class

A 3 kg car traveling to the right at 5 m/s hit and sticks to a 6 kg car moving to the left at 2 m/s. What is the final velocity of both after the collision?

Deep Space 1 was launched in 1998 and thrusted for 678 days straight. It reached a speed of 4,600 m/s.

Drawing of Deep Space 1Test fire of ion propulsion engine

Rocket Applet

Astronaut Catch

Conservation of Momentum Problems (1D)

Book Problems page 101-10322, 28, 32, 35, 37 (2parts)

Conservation of Momentum Lab

Conservation of Momentum LabRecap Applet

http://www.walter-fendt.de/ph11e/collision.htm

Car Crash 2D momentum demo

After the can of gas ruptures how will it move?a.) leftb.) rightc.) won’t

Initial

It ruptures

Conservation of Momentum in 2 Dimensions

Car

Car

A car and a bus collide and stick together.What direction does the wreckage slide?

Car

It moves off at an angle

But MOMENTUM is Still conservedCar

Conservation of Momentum in 2 Dimensions

Draw Vectors for the momentum

Car

Car

x

Y

Momentum is conserved. So the outcome is just the two momentums added together

CarCar

x

Y

total

Car

Car

x

YCar

Car

x

Y

total

Before After

X = 0

Y = 0

The X momentum is conserved

The Y momentum is conserved

Car Crash 2D momentum demo

Problems:

Honors PhysicsPage 205 38,39

Physicspage 220 44, 45, 47 b only

Which has the greater change in momentum a ball that:

Hits and sticks or bounces backwards at the same speed ?

Initial Momentum

Final Momentum

Change Momentum

= -mv = -mv

= 0 = mv

= 0 –(-mv)= mv

= mv –(-mv) = 2mv

ELASTIC COLLIONS (things bounce)& Conservation of momentum

ELASTIC COLLIONS (things bounce)& Conservation of momentum

ELASTIC COLLIONS (things bounce)& Conservation of momentum

ELASTIC COLLIONS (things bounce)& Conservation of momentum