Bouncing Balls

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Bouncing Balls
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Introductory Question

• If you place a tennis ball on a basketball and
  drop this stack on the ground, how high will the
  tennis ball bounce?
• To approximately its original height
• Much higher than its original height
• Much less than its original height

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Observations aboutBouncing Balls

• Some balls bounce better than others
• Dropped balls dont rebound to their full height
• Balls bounce differently from different surfaces
• Ball bounce differently from moving objects

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4 Questions about Bouncing Balls

• Why cant a ball thats dropped on a hard floor
  rebound to its starting height?
• Why does the floors surface affect the bounce?
• How does a ball bounce when it hits a bat?
• What happens to the bat when a ball hits it?

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Question 1

• Why cant a ball thats dropped on a hard floor
  rebound to its starting height?
• What happens to balls energy as it bounces?

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Bouncing from a Rigid Floor

• As it strikes a rigid floor, a balls
• kinetic energy decreases by the collision
  energy
• elastic potential energy increases as it dents
• As it rebounds from that surface, the balls
• elastic potential energy decreases as it undents
• kinetic energy increases by the rebound energy
• Rebound energy lt collision energy
• A lively ball wastes little energy as thermal
  energy
• A dead ball wastes most of its energy

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Measuring a Balls Liveliness

• Coefficient of Restitution
• is a measure of a balls liveliness
• is the ratio of outgoing to incoming speeds
• coefficient of restitution outgoing speed /
  incoming speed

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Question 2

• Why does the floors surface affect the bounce?

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Bouncing from an Elastic Floor

• Both ball and floor dent during a bounce
• Work is proportional to dent distance
• Denting floor stores and returns energy
• A lively floor wastes little energy
• A dead floor wastes most of its energy
• A floor has a coefficient of restitution, too
• A soft, lively floor can help the ball bounce!

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Question 3

• How does a ball bounce when it hits a bat?
• Do both the ball and bat bounce?

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Bouncing from Moving Surfaces

• Incoming speed ? approaching speed
• Outgoing speed ? separating speed
• Coefficient of Restitution becomes
• coefficient of restitution separating speed
  / approaching speed

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Ball and Bat (Part 1)

• Ball heads toward home plate at 100 km/h
• Bat heads toward pitcher at 100 km/h
• Approaching speed is 200 km/h

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Ball and Bat (Part 2)

• Approaching speed is 200 km/h
• Baseballs coefficient of restitution 0.55
• Separating speed is 110 km/h

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Ball and Bat (Part 3)

• Separating speed is 110 km/h
• Bat heads toward pitcher at 100 km/h
• Ball heads toward pitcher at 210 km/h

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Introductory Question (revisited)

• If you place a tennis ball on a basketball and
  drop this stack on the ground, how high will the
  tennis ball bounce?
• To approximately its original height
• Much higher than its original height
• Much less than its original height

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Question 4

• What happens to the bat when a ball hits it?

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Bouncings Effects on Objects

• A bouncing ball transfers momentum
• while stopping
• while rebounding
• A livelier ball transfers more momentum
• A bouncing ball can also transfer energy
• These two transfers together govern bouncing
• A ball transfers momentum and energy to a bat
• Identical elastic balls can transfer motion
  perfectly

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Impact Forces

• Harder surfaces bounce faster
• Momentum is transferred more quickly
• Time is shorter, so force is larger
• No one likes bouncing off hard surfaces

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The Balls Effects on a Bat

• The ball pushes the bat back and twists it, too
• When the ball hits the bats center of
  percussion,
• the bats backward and rotational motions balance
• the bats handle doesnt jerk
• When the ball hits the bats vibrational node,
• the bat doesnt vibrate
• more energy goes into the ball

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Summary about Bouncing Balls

• Each ball has a coefficient of restitution
• Energy lost in a bounce becomes thermal
• The surface can affect a balls bounce
• Surfaces bounce, too