Bouncing Balls the tennis ball bounce? To approximately its - PDF document

Bouncing Balls 1 Bouncing Balls 2 Introductory Question If you place a tennis ball on a basketball and � drop this stack on the ground, how high will Bouncing Balls the tennis ball bounce? To approximately its original height A. Much higher than its original height B. Much less than its original height C. Bouncing Balls 3 Bouncing Balls 4 Observations about 4 Questions about Bouncing Balls Bouncing Balls � Some balls bounce better than others � Why can’t a ball that’s dropped on a hard floor rebound to its starting height? � Dropped balls don’t rebound to their full height � Why does the floor’s surface affect the bounce? � Balls bounce differently from different surfaces � How does a ball bounce when it hits a bat? � Ball bounce differently from moving objects � What happens to the bat when a ball hits it? Bouncing Balls 5 Bouncing Balls 6 Question 1 Bouncing from a Rigid Floor � As it strikes a rigid floor, a ball’s � Why can’t a ball that’s dropped on a hard floor rebound to its starting height? � kinetic energy decreases by the “collision” energy � elastic potential energy increases as it dents � What happens to ball’s energy as it bounces? � 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 •1

Bouncing Balls 7 Bouncing Balls 8 Measuring a Ball’s Liveliness Question 2 � Coefficient of Restitution � Why does the floor’s surface affect the bounce? � is a measure of a ball’s liveliness � is the ratio of outgoing to incoming speeds: coefficient of restitution = outgoing speed / incoming speed Bouncing Balls 9 Bouncing Balls 10 Bouncing from an Elastic Floor Question 3 � Both ball and floor dent during a bounce � How does a ball bounce when it hits a bat? � Work is proportional to dent distance � Do both the ball and bat bounce? � 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! Bouncing Balls 11 Bouncing Balls 12 Bouncing from Moving Surfaces Ball and Bat (Part 1) � Incoming speed → approaching speed � Ball heads toward home plate at 100 km/h � Outgoing speed → separating speed � Bat heads toward pitcher at 100 km/h � Coefficient of Restitution becomes: � Approaching speed is 200 km/h coefficient of restitution = separating speed / approaching speed •2

Bouncing Balls 13 Bouncing Balls 14 Ball and Bat (Part 2) Ball and Bat (Part 3) � Approaching speed is 200 km/h � Separating speed is 110 km/h � Baseball’s coefficient of restitution: 0.55 � Bat heads toward pitcher at 100 km/h � Separating speed is 110 km/h � Ball heads toward pitcher at 210 km/h Bouncing Balls 15 Bouncing Balls 16 Introductory Question (revisited) Question 4 If you place a tennis ball on a basketball and � What happens to the bat when a ball hits it? � drop this stack on the ground, how high will the tennis ball bounce? To approximately its original height A. Much higher than its original height B. Much less than its original height C. Bouncing Balls 17 Bouncing Balls 18 Bouncing’s Effects on Objects Impact Forces � A bouncing ball transfers momentum � Harder surfaces bounce faster � while stopping � Momentum is transferred more quickly � while rebounding � Time is shorter, so force is larger � A livelier ball transfers more momentum � No one likes bouncing off hard surfaces � 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 •3

Bouncing Balls 19 Bouncing Balls 20 The Ball’s Effects on a Bat Summary about Bouncing Balls � The ball pushes the bat back and twists it, too � Each ball has a coefficient of restitution � When the ball hits the bat’s center of percussion, � Energy lost in a bounce becomes thermal � the bat’s backward and rotational motions balance � The surface can affect a ball’s bounce � the bat’s handle doesn’t jerk � Surfaces bounce, too � When the ball hits the bat’s vibrational node, � the bat doesn’t vibrate � more energy goes into the ball •4