Transcript Momentum - Cloudfront.net

Chapter 7 – Momentum
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Inertia in motion!!!
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An object in motion will stay in motion until
a force acts to stop it.
Momentum = mass x velocity (kg * m/s)
p

=
mv
Does an object at rest have momentum???
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If a car and truck are moving at the same
speed, which one has more momentum?
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When would a car have a greater
momentum than a truck?
Force Changes Momentum
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For momentum of an object to change,
either the mass or velocity or both
changes.
force =
velocity =
momentum
When would the mass change?
Impulse Changes Momentum
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How long a force acts is also important.
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Force and Time are important in changing
momentum
impulse = Force x time interval
impulse = F x t
What would we measure impulse in?
N .s
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Impulse = change in momentum
Ft
=
(mv)
Impulse is a vector quantity
Case 1: When would you want to increase
momentum??
*pulling a sling shot back, driving a golf ball,
swinging a bat
Let’s look at the forces involved
with impulses
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Hitting a golf ball: a golf club exerts zero
force until it comes in contact; then the
force increases rapidly as the ball becomes
distorted; then the force diminishes as the
ball comes up to speed and returns to its
original shape.
We call the force on the ball the impact
force.
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http://video.google.com/videoplay?docid=4763744673566416153&pr=goog-sl
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http://video.google.com/videoplay?docid=8591879745880686794&pr=goog-sl
Another scenario
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Case 2 – Decreasing Momentum
When would you want to decrease
momentum and minimize the force of
something?
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v
m
=> F
t
vs.
v
m
=>
Ft
Other Examples
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Bending your knees when you jump off
something
Moving away from a punch rather than
towards it
Landing on carpet rather concrete
Safety net for circus acrobats
Bouncing
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Impulses are greater when an object
bounces. Why?

The impulse required to bring an object to
stop and then “throw it back again” is
greater than the impulse required to just
bring it to a stop
Think of a karate
expert breaking
boards – they deliver
a force in a short
amount of time and
let their hand bounce
back which yields
twice the impulse to
the target.
Internal vs. External Forces
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If you’re sitting inside your car pushing on
the dashboard, will the car move?
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Internal force
If you’re standing outside a car pushing on
it, will the car move?
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External force
*Law of Conservation of
Momentum
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In the absence of an external force, the
momentum of a system remains
unchanged.
m1v1 = m2v2
How does this go along with Newton’s
Laws of Motion?
Examples
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Cannon and cannonball – look at the mass
and velocity of each = momentum
A dropped rock in free fall
The rock falls to Earth, as Earth “moves
up” to the rock with equal and opposite
momentum. Momentum is conserved.
Collisions!!!

When objects collide in the absence of
external forces, the net momentum of both
objects before collision equals the net
momentum of objects after collision
Net momentum before collision = Net momentum after collision
Elastic Collisions
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3 types
 A moving object striking a ball at rest
 A head-on collision between two moving objects
 A collision of two objects moving in the same
direction
Look at the vector arrows.
Objects are not deformed and heat is not generated.
Two objects ‘bounce’ off each other
Almost all energy is transferred from the first object to the
second object
Elastic Collison Example
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A car's bumper works by using this principle to prevent damage.
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In a low speed collision, the kinetic energy is small enough that the
bumper can deform and then bounce back, transferring all the energy
directly back into motion.
Almost no energy is converted into heat or damage to the body of the
car, as it would in an inelastic collision.
However, car bumpers are often made to collapse if the speed is high
enough, and not use the benefits of an elastic collision.
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The rational is that if you are going to collide with something at a high
speed, it is better to allow the kinetic energy to crumple the bumper in
an inelastic collision than let the bumper shake you around as your car
bounces in an elastic collision.
Making their bumpers this way benefits the car companies: they get to
sell you a new bumper, and you can't sue them for whiplash.
Inelastic Collisions
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Momentum becomes shared in some way, but is
still conserved.
Colliding objects become distorted and generate
heat during collision.
Look at the freight train in Fig 7.10 on page 95
(net mv)before = (net mv)after
(m)(4 m/s) + (m)(0 m/s) = (2kg)(v after)
4kg*m/s = (2kg) Vafter
 Vafter = ???
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Air track collisions – elastic and inelastic
collisions
http://faraday.physics.utoronto.ca/PVB/Har
rison/Flash/ClassMechanics/AirTrack/AirT
rack.html
Recoil Collisions