Let’s Bounce

Let’s Bounce

May 11, 2020

When you drop a ball, it bounces back up, but not to the same full height. What happens if you drop a ball on top of a ball on top of a ball? Physics Girl stacked up a golf ball on top of a bouncy ball on top of a basketball. When she dropped the stack, the basketball and bouncy ball rocketed the golf ball way up into the sky – 28 feet! That’s more than 8 times the height the golf ball started from – or a tower of 35 basketballs, or 200 golf balls.
 
Wee ones: Find a ball of any size. Drop it without throwing it down, and see how high it bounces. Does it bounce as high as your hand?
 
Little kids: If 3 friends each drop a stack of 3 balls, how many bouncing balls is that? Bonus: If each story of a building is 10 feet tall and you stand on the roof of a 2-story house, does that golf ball bouncing to 28 feet stop above or below you?
 
Big kids: If this stack made the golf ball fly 8 times its starting height, how high would the golf ball fly if a stack was dropped from a 20-foot-tall diving board? (Onto the pavement, not into the pool!) Bonus: What if you could bounce 8 times your height? How high would that be in feet and inches?
 

 

 

 

 

 

 
Answers:
Wee ones: You can try any ball – a rubber ball, soccer ball, tennis ball – but no matter how bouncy it is, it will not bounce to the full starting height.
 
Little kids: 9 balls. Bonus: It will bounce higher than you, since you will be just 20 feet above the ground.
 
Big kids: 160 feet. Bonus: Different for everyone…You can either multiply the feet and inches each by 8 separately and then figure out the extra feet, or you can take the total in inches and multiply that by 8. A neat shortcut: you double the number, then double again, then double it one more time, because 2 x 2 x 2 = 8.

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About the Author

Laura Overdeck

Laura Overdeck

Laura Bilodeau Overdeck is founder and president of Bedtime Math Foundation. Her goal is to make math as playful for kids as it was for her when she was a child. Her mom had Laura baking before she could walk, and her dad had her using power tools at a very unsafe age, measuring lengths, widths and angles in the process. Armed with this early love of numbers, Laura went on to get a BA in astrophysics from Princeton University, and an MBA from the Wharton School of Business; she continues to star-gaze today. Laura’s other interests include her three lively children, chocolate, extreme vehicles, and Lego Mindstorms.

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