momentum? chapter 7
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I’d like some please. Momentum Store. MOMENTUM? CHAPTER 7. You might have heard people gaining momentum. What is it and how do you get it???. Sure Thing. MOMENTUM?. You might have heard people gaining momentum. What is it and how do you get it???. Here you go. Momentum Store. - PowerPoint PPT PresentationTRANSCRIPT
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