Momentum

1
Chapter 5

• Momentum

2
Momentum

• The inertia of motion is called momentum
• A truck has more momentum than a car
• p m x v or mass times velocity
• What has more momentum, a high velocity bullet or
  a slow moving train?

3
Concept Check

• Can you think of a case where a car and a truck
  with twice the cars mass would have the same
  momentum?

4
Impulse Changes Momentum

• If momentum changes the mass, velocity or both
  change
• Most of the time the mass does not change so the
  velocity does
• If the velocity changes, it is accelerating and
  accelerations are produced by forces
• The greater the force, the greater the velocity,
  and the greater the momentum

5
Increasing Momentum

• To increase momentum you can increase the force,
  the time or both
• If I get my car stuck and I begin rocking it the
  momentum is only changed for a short amount of
  time
• Forward motion will have the greatest momentum if
  the forces is allowed to act over a longer period
  of time
• I might choose to put a piece of wood under the
  tire or perhaps put sand around the wheel on ice
  to that I can increase the force for longer

6
Tell Me Why

• Would we want to use a very long cannon?
• Should a wrestler relax their muscles before
  hitting a mat during a throw?
• When you jump out of the back of a truck you land
  with your knees bent?
• Use a bungee cord instead of regular rope when
  jumping off a bridge?
• Perform ballet on wooden floors rather than
  concrete?
• Cushion an egg backwards in the egg toss?
• Boxers rolls with a punch?

7
Contact Time

• If you are playing basketball and you are moving
  your hand forward to receive a pass and jam a
  finger, why does this result in more damage than
  if your finger was hit in a still position?
• Never move into a punchWhy?
• How can a black belt in Karate break a stack of
  boards or concrete?

8
Concept Check

• If a boxer rolls with a punch and makes the punch
  last 3 times as long, by how much is the force
  reduced?
• If the boxer instead moves into the punch and
  shortens the contact time by half, by how much is
  the force increased

9
Bouncing

• If you catch a falling object it will require a
  certain amount of force to zero it out
• It will take even more for to zero it and throw
  it back in the opposite direction (bouncing)
• Any object that bounces has a greater momentum
  due to this greater force

10
Smarter Than The Average Bear

• Lester A. Pelton was a gold miner and wasnt
  impressed with the force produced by a paddle
  wheel
• The water simply hit the flat paddles and
  produced moderate force for the wheel
• He found a way for water to bounce by shaping
  the paddles in a U shape so that the water poured
  into the U cups, hit the bottoms and bounced back
  up
• Long story shortHe made more with his patented
  Pelton wheel than most gold miners

11
Concept Check

• How does a karate experts force on a pile of
  concrete bricks compare with the force exerted on
  their hand?
• How will the impulse differ if their hand bounces
  back when striking the concrete (time stays
  constant)?

12
Net Momentum

• Again perspective is important
• If you look at a cannon ball and cannon as a
  system the momentum of the ball leaving is equal
  to the recoil of the cannon (leaving out gases
  expelled etc)
• Overall the change in momentum is zero or
  conserved
• Notice it isnt created or destroyed

13
Law of Conservation of Momentum

• In the absences of an external force, the
  momentum of a system remains unchanged is called
  the Law of Conservation of Momentum
• If a tennis bal hits a racket, cars collide, or
  stars explode the net momentum of the system is
  the same before as it is after
• The momentum is conserved

14
Concept Check

• A high-speed bus and a bug have a head on
  collision. The sudden change of momentum for the
  bug splatters it all over the windshield. Is the
  change in momentum of the bus greater, less or
  the same as the change in momentum of the bug?

15
Elastic Collisions vs. Inelastic Collisions

• Elastic Collisions
• The Law of Conservation of Momentum is conserved
• Objects collide without being deformed or
  generating heat
• They bounce perfectly
• Ex A billiard ball hits another billiard ball.
  The first one stops and the other continues with
  the momentum

• Inelastic Collisions
• The Law of Conservation of Momentum is conserved
• In a perfect collision they would stick together
• The objects can collide in the same direction
  such as a fast moving vehicle hits a slow moving
  car from behind
• The objects can hit head on

16
Concept Check

• Two gliders have the same mass and move towards
  each other at the same speed. They experience an
  elastic collision. What would their movement look
  like after the collision?
• What if they had an inelastic collision?

17
In A Perfect World

• Perfect elastic collisions are rare
• We can try to simulate them with an air track
• It works a lot like a air hockey table but carts
  slide back and forth on a track
• At an atomic level, collisions cause heat
• Bouncing a ball illustrates how the heat
  transferred to the floor decreases the height a
  ball can bounce to