Bouncing Balls 1 Bouncing Balls

Bouncing Balls 2 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? If you place a tennis ball on a basketball and drop this stack on the ground, how high will the tennis ball bounce? A. To approximately its original height B. Much higher than its original height C. Much less than its original height

Bouncing Balls 3 Observations about Bouncing Balls Some balls bounce better than others Some balls bounce better than others Dropped balls don’t rebound to their full height Dropped balls don’t rebound to their full height Balls bounce differently from different surfaces Balls bounce differently from different surfaces Ball bounce differently from moving objects Ball bounce differently from moving objects

Bouncing Balls 4 4 Questions about Bouncing Balls Why can’t a ball that’s dropped on a hard floor rebound to its starting height? 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? Why does the floor’s surface affect the bounce? How does a ball bounce when it hits a bat? How does a ball bounce when it hits a bat? What happens to the bat when a ball hits it? What happens to the bat when a ball hits it?

Bouncing Balls 5 Question 1 Why can’t a ball that’s dropped on a hard floor rebound to its starting height? 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? What happens to ball’s energy as it bounces?

Bouncing Balls 6 Bouncing from a Rigid Floor As it strikes a rigid floor, a ball’s As it strikes a rigid floor, a ball’s kinetic energy decreases by the “collision” energy kinetic energy decreases by the “collision” energy elastic potential energy increases as it dents elastic potential energy increases as it dents As it rebounds from that surface, the ball’s As it rebounds from that surface, the ball’s elastic potential energy decreases as it undents elastic potential energy decreases as it undents kinetic energy increases by the “rebound” energy kinetic energy increases by the “rebound” energy Rebound energy < collision energy Rebound energy < collision energy A “lively” ball wastes little energy as thermal energy A “lively” ball wastes little energy as thermal energy A “dead” ball wastes most of its energy A “dead” ball wastes most of its energy

Bouncing Balls 7 Measuring a Ball’s Liveliness Coefficient of Restitution Coefficient of Restitution is a measure of a ball’s liveliness is a measure of a ball’s liveliness is the ratio of outgoing to incoming speeds: is the ratio of outgoing to incoming speeds: coefficient of restitution = outgoing speed / incoming speed

Bouncing Balls 8 Question 2 Why does the floor’s surface affect the bounce? Why does the floor’s surface affect the bounce?

Bouncing Balls 9 Bouncing from an Elastic Floor Both ball and floor dent during a bounce Both ball and floor dent during a bounce Work is proportional to dent distance Work is proportional to dent distance Denting floor stores and returns energy Denting floor stores and returns energy A “lively” floor wastes little energy A “lively” floor wastes little energy A “dead” floor wastes most of its energy A “dead” floor wastes most of its energy A floor has a coefficient of restitution, too A floor has a coefficient of restitution, too A soft, lively floor can help the ball bounce! A soft, lively floor can help the ball bounce!

Bouncing Balls 10 Question 3 How does a ball bounce when it hits a bat? How does a ball bounce when it hits a bat? Do both the ball and bat bounce? Do both the ball and bat bounce?

Bouncing Balls 11 Bouncing from Moving Surfaces Incoming speed → approaching speed Incoming speed → approaching speed Outgoing speed → separating speed Outgoing speed → separating speed Coefficient of Restitution becomes: Coefficient of Restitution becomes: coefficient of restitution = separating speed / approaching speed

Bouncing Balls 12 Ball and Bat (Part 1) Ball heads toward home plate at 100 km/h Ball heads toward home plate at 100 km/h Bat heads toward pitcher at 100 km/h Bat heads toward pitcher at 100 km/h Approaching speed is 200 km/h Approaching speed is 200 km/h

Bouncing Balls 13 Ball and Bat (Part 2) Approaching speed is 200 km/h Approaching speed is 200 km/h Baseball’s coefficient of restitution: 0.55 Baseball’s coefficient of restitution: 0.55 Separating speed is 110 km/h Separating speed is 110 km/h

Bouncing Balls 14 Ball and Bat (Part 3) Separating speed is 110 km/h Separating speed is 110 km/h Bat heads toward pitcher at 100 km/h Bat heads toward pitcher at 100 km/h Ball heads toward pitcher at 210 km/h Ball heads toward pitcher at 210 km/h

Bouncing Balls 15 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? If you place a tennis ball on a basketball and drop this stack on the ground, how high will the tennis ball bounce? A. To approximately its original height B. Much higher than its original height C. Much less than its original height

Bouncing Balls 16 Question 4 What happens to the bat when a ball hits it? What happens to the bat when a ball hits it?

Bouncing Balls 17 Bouncing’s Effects on Objects A bouncing ball transfers momentum A bouncing ball transfers momentum while stopping while stopping while rebounding while rebounding A livelier ball transfers more momentum A livelier ball transfers more momentum A bouncing ball can also transfer energy A bouncing ball can also transfer energy These two transfers together govern bouncing These two transfers together govern bouncing A ball transfers momentum and energy to a bat A ball transfers momentum and energy to a bat Identical elastic balls can transfer motion perfectly Identical elastic balls can transfer motion perfectly

Bouncing Balls 18 Impact Forces Harder surfaces bounce faster Harder surfaces bounce faster Momentum is transferred more quickly Momentum is transferred more quickly Time is shorter, so force is larger Time is shorter, so force is larger No one likes bouncing off hard surfaces No one likes bouncing off hard surfaces

Bouncing Balls 19 The Ball’s Effects on a Bat The ball pushes the bat back and twists it, too The ball pushes the bat back and twists it, too When the ball hits the bat’s center of percussion, When the ball hits the bat’s center of percussion, the bat’s backward and rotational motions balance the bat’s backward and rotational motions balance the bat’s handle doesn’t jerk the bat’s handle doesn’t jerk When the ball hits the bat’s vibrational node, When the ball hits the bat’s vibrational node, the bat doesn’t vibrate the bat doesn’t vibrate more energy goes into the ball more energy goes into the ball

Bouncing Balls 20 Summary about Bouncing Balls Each ball has a coefficient of restitution Each ball has a coefficient of restitution Energy lost in a bounce becomes thermal Energy lost in a bounce becomes thermal The surface can affect a ball’s bounce The surface can affect a ball’s bounce Surfaces bounce, too Surfaces bounce, too