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## PowerPoint Slideshow about 'Bouncing Balls' - efuru

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Presentation Transcript

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

Observations aboutBouncing Balls

- Some balls bounce better than others
- Dropped balls don’t rebound to their full height
- Balls bounce differently from different surfaces
- Ball bounce differently from moving objects

4 Questions about Bouncing Balls

- Why can’t a ball that’s dropped on a hard floor rebound to its starting height?
- Why does the floor’s surface affect the bounce?
- How does a ball bounce when it hits a bat?
- What happens to the bat when a ball hits it?

Question 1

- Why can’t a ball that’s dropped on a hard floor rebound to its starting height?
- What happens to ball’s energy as it bounces?

Bouncing from a Rigid Floor

- As it strikes a rigid floor, a ball’s
- kinetic energy decreases by the “collision” energy
- elastic potential energy increases as it dents

- As it rebounds from that surface, the ball’s
- elastic potential energy decreases as it undents
- kinetic energy increases by the “rebound” energy

- Rebound energy < collision energy
- A “lively” ball wastes little energy as thermal energy
- A “dead” ball wastes most of its energy

Measuring a Ball’s Liveliness

- Coefficient of Restitution
- is a measure of a ball’s liveliness
- is the ratio of outgoing to incoming speeds:
coefficient of restitution = outgoing speed / incoming speed

Question 2

- Why does the floor’s surface affect the bounce?

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!

Question 3

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

Bouncing from Moving Surfaces

- Incoming speed → approaching speed
- Outgoing speed → separating speed
- Coefficient of Restitution becomes:
coefficient of restitution = separating speed / approaching speed

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

Ball and Bat (Part 2)

- Approaching speed is 200 km/h
- Baseball’s coefficient of restitution: 0.55
- Separating speed is 110 km/h

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

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

Question 4

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

Bouncing’s 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

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

The Ball’s Effects on a Bat

- The ball pushes the bat back and twists it, too
- When the ball hits the bat’s center of percussion,
- the bat’s backward and rotational motions balance
- the bat’s handle doesn’t jerk

- When the ball hits the bat’s vibrational node,
- the bat doesn’t vibrate
- more energy goes into the ball

Summary about Bouncing Balls

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

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