Physics 102: Chapter 6 - Momentum
1.
2.
3.
4.
Momentum and Energy views of force
Impulse-Momentum Theorem
What does conservation mean
Conservation of Momentum
Exam 2 - On Wednesday
Exam notes
1. Must be handwritten on 8.5” x 11” piece of
paper
2. Cannot be torn out of a notebook
3. No worked out examples, derivations, or
solutions
4. Corrections and Notes from Exam 1 available in
your boxes > Noon tomorrow
Review Session tomorrow night
from 7-9 PM in RH 114
Whenever an interaction occurs in a system, forces
occur in equal and opposite pairs. Which of the
following do not always occur in equal and opposite
pairs?
1. Impulses.
2. Accelerations.
3. Momentum changes.
4. All of these occur in equal and opposite pairs.
5. None of these do.
Whenever an interaction occurs in a system, forces
occur in equal and opposite pairs. Which of the
following do not always occur in equal and opposite
pairs?
2. Accelerations.
Because the time interval for each interaction part is the same, impulses and
momentum changes also occur in equal and opposite pairs. But not
necessarily accelerations, because the masses of the interaction may differ.
Consider equal and opposite forces acting on masses of different magnitude.
Which would be more damaging?
1. Driving into a massive concrete wall.
2. Driving at the same speed into a headon collision with
an identical car
traveling
toward
you at the
same speed.
3. They are
equivalent.
Which would be more damaging?
3. They are
equivalent.
Your car decelerates to a dead
stop either way. The dead stop
is easy to see when hitting the
wall, and a little thought will
show the same is true when
hitting the car. If the oncoming
car were traveling more
slowly, with less momentum,
you’d keep going after the
collision with more “give,”
and less damage (to you).
But if the oncoming car had more momentum than you, it would keep going and
you’d snap into a sudden reverse with greater damage. Identical cars at equal
speeds means equal momenta—zero before, zero after collision.
Reading Quiz
1. Impulse is
A. a force that is applied at a random time.
B. a force that is applied very suddenly.
C. the area under the force curve in a
force-versus-time graph.
D. the interval of time that a force lasts.
Slide 9-2
Answer
1. Impulse is
C. the area under the force curve in a
force-versus-time graph.
Slide 9-3
Reading Quiz
2. The total momentum of a system is conserved
A. always.
B. if no external forces act on the system.
C. if no internal forces act on the system.
D. never; momentum is only approximately
conserved.
Slide 9-4
Answer
2. The total momentum of a system is conserved
B. if no external forces act on the system.
Slide 9-5
Momentum - Key Equations
2. The total momentum of a system is conserved
B. if no external forces act on the system.
Slide 9-5
Impulse Effects of Time and Force
Large time => small force
Small time => Large force
Impulse Effects of Time and Force
1.
2.
3.
4.
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?
A. Throw the clay ball
B. Throw the superball
C. Throw either ball, it doesn’t matter
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:


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
Jack and the Skateboard -- Example 1
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
Bullet and Block -- Example 2
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
Professor on Ice -- Explosion Example
A professor of physics is going ice skating for the first time. He has
gotten himself into the middle of an ice rink and cannot figure out
how to make the skates work. Every motion he makes simply
slips on the ice and leaves him in the same place he started. He
decides that he can get off the ice by throwing his gloves in the
opposite direction.
(a) Suppose he has a mass M and his gloves have a mass m. If he
throws them as hard as he can away from him, and they leave his
hand with a velocity v. Explain whether or not he will move. If he
does move, calculate his velocity, V.
(b) Discuss his motion from the point of view of the forces acting on
him.
(c) If the ice rink is 10 m in diameter and the skater starts in the
center, estimate how long it will take him to reach the edge,
assuming there is no friction at all
Slide 9-5
Reading Quiz
3. In an inelastic collision,
A. impulse is conserved.
B. momentum is conserved.
C. force is conserved.
D. mechanical energy is conserved.
E. elasticity is conserved.
Slide 9-6
Answer
3. In an inelastic collision,
B. momentum is conserved.
Slide 9-7