A Bouncing Ball Like You've Never Seen

Vicki Cobb
The “Julia Child” of kids’ hands-on science

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     You can’t play tennis unless you know where the ball will be after it bounces.  You can’t pass a basketball unless you understand how to angle a bounce so that it goes where you want it to go.  As long as the court surface is smooth and flat, a ball’s bounce is very predictable.  Its path depends on gravity and on the strength and direction of the force that sets the ball in motion. Thanks to high speed photography we can get a closer look at a bouncing ball.
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     This is a multiple exposure photograph of a bouncing ball.   It was taken in complete darkness with the camera shutter open while a high-speed flashing light, called a stroboscope or strobe, flashed 30 times a second.  Each flash produced an image.

     Here’s what you can learn from this photo:  The ball is moving fastest where the images are farthest apart and slowest where they are closest together.  When the ball is falling, it speeds up.  After it bounces and moves opposite the pull of gravity, it slows down at exactly the same rate as it sped up when it was falling until it stops for an instant and starts falling again.  Each time it collides with the ground, some energy is lost.  That’s why each bounce loses altitude.  If the bounce were perfect, no energy would be lost, every bounce would be as high as the last and the ball would bounce forever.

​     A strobe also captures the split second when a tennis ball is struck by a racket.  The collision flattens the ball, and stretches the strings and distorts the frame of the racket,  all in  .005 seconds. If these objects kept their distorted shapes, most of the force of the collision would be absorbed.  But they are elastic—they restore themselves to their original shapes after they collide.  This restoring force is transferred to the ball to change its direction and help add to the speed of the athlete’s swing. The fastest serve leaves a racket at 130 miles an hour.  In a rally, a ball-racket collision changes direction of the ball so it is not as fast as a serve, maybe 70 miles per hour. Since the distance between images  made by a  strobe tells how fast an object is moving, strobes are  part of the instruments used to measure the speed of balls from a tennis racket and a baseball pitcher.

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Photo credit: C. E. Miller, MIT

In this MIT YouTube,  a ball is dropped in front of a meter stick and lit by a strobe light. A long exposure photograph captures the position of the ball at each evenly spaced flash of light. The acceleration of the ball can then be measured from the photo.

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Would you believe that you could throw an egg across the room without breaking it? Burn a candle underwater? Vicki Cobb's We Dare You! is a gigantic collection of irresistible, easy-to-perform science experiments, tricks, bets, and games kids can do at home with everyday household objects. Thanks to the principles of gravity, mechanics, fluids, logic, geometry, energy, and perception, kids will find countless hours of fun with the selections included in this book.  If you would like to make a We Dare You Video, click here.
Vicki Cobb is a member of iNK's Authors on Call and is available for classroom programs through Field Trip Zoom,  a terrific technology that requires only a computer, wifi, and a webcam.  Click here to find out more.

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MLA 8 Citation
Cobb, Vicki. "A Bouncing Ball Like You've Never Seen." Nonfiction Minute, iNK
     Think Tank, 5 Feb. 2018, www.nonfictionminute.org/the-nonfiction-minute/
     a-bouncing-ball-like-you've-never-seen.