Transcript Physics 102: Chapter 6

Physics 102: Chapter 6 - Momentum
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4.
Momentum and Energy views of force
Impulse-Momentum Theorem
What does conservation mean
Conservation of Momentum
Impulse
The force of the foot on
the ball is an impulsive
force.
Slide 9-8
Momentum
Momentum is the product of an object’s mass and its velocity:
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
p = mv
Slide 9-10
The Impulse-Momentum Theorem
Impulse causes a change in momentum:
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J =pf - pi = ∆p
Slide 9-11
Slide 9-15
Example
A 500 kg rocket sled is coasting at 20 m/s. It then turns on
its rocket engines for 5.0 s, with a thrust of 1000 N. What
is its final speed?
Slide 9-17
Starting and Stopping
A car with a mass of 1 metric ton speeds up to
highway speed from rest on a strait section of
Central Blvd. A little while later, the car comes to
a stop as it approaches a red light.
Part 1 - Determine the net impulse and average
net force on the car as it goes from rest to
highway speed.
Impulses and Car Crashes
1. Consider a car going at highway speeds
colliding in a front-end collision with a brick
wall. Compare the impulse needed to bring
the passenger in the front seat to a stop if
they are stopped by each of the following:
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Their seatbelt
The dashboard
An airbag
2. Rank the force being applied for each case.
Impulse Effects of Time and Force
Large time => small force
Small time => Large force
Impulse Effects of Time and Force
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2.
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5.
Pulling your hand back while you catch a ball
Bending your knees and rolling when you fall in
Self defense class / Sky-Diving
Falling on a wooden floor is safer than falling on a
cement floor
Railroad car couplings are loose => slow to
accelerate or stop
Movies
(Nail on head / hammer on hand/Inertia Balls)
(Table top pull movie)
Table top friction pull
Shut the Door
You are sitting on your bed in your dorm room, and
suddenly you hear the voice of your ex coming down the
hall. You really want to avoid any contact (you broke things
off a week ago), and so you want to shut the door. But you
don't have time to get up and shut it and act like it wasn't
on purpose. You need something fast. Sitting beside you,
you happen to have a super ball and a ball of clay that you
fidget with when you're studying on your bed. What do you
do?
Explain your answer and show why you chose one and not
the other..
(Demonstration movie =>http://groups.physics.umn.edu/demo/collisionframe.html)
Pelton Wheel
Enriching yourself with Physics
During the California Gold Rush, Lester Pelton
designed a water wheel that caused the water to
make a U-turn, i.e. causing the water to bounce
off the paddle. He made a lot of money on this
invention, more than the miners.
Bouncing off the Wall
In the overhead view shown below, a 290 g ball with a speed v of
4.6 m/s strikes a wall at an angle of 30° and then rebounds
with the same speed and angle. It is in contact with the wall
for 11 ms.
Overhead View
What does conserved mean?
Oil and Water example
Examples of Collisions and Explosions
The Law of Conservation of Momentum
In terms of the initial and final total momenta:
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
Pf = Pi
In terms of components:
Slide 9-18
Perfectly Inelastic Collision Example
Defining your system - system schema
Isolated system is when the sum of all external forces
is zero.
Slide 9-19
Example
A curling stone, with a mass of 20.0 kg, slides across the ice
at 1.50 m/s. It collides head on with a stationary 0.160-kg
hockey puck. After the collision, the puck’s speed is 2.50 m/s.
What is the stone’s final velocity?
Slide 9-20
Inelastic Collisions
For now, we’ll consider perfectly inelastic collisions:
A perfectly elastic collision results whenever the two objects
move off at a common final velocity.
Slide 9-21
Example
Jack stands at rest on a skateboard. The mass of Jack and
the skateboard together is 75 kg. Ryan throws a 3.0 kg ball
horizontally to the right at 4.0 m/s to Jack, who catches it.
What is the final speed of Jack and the skateboard?
Slide 9-22
Example
A 10 g bullet is fired into a 1.0 kg wood block, where it
lodges. Subsequently, the block slides 4.0 m across a floor
(µk = 0.20 for wood on wood). What was the bullet’s speed?
Slide 9-23