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Sports Physics 6-8 / Week 08 / Game Day
6/6
Week 08 · Bounce, Roll and Stop

Game Day

Game Day: Bounce contest
// Where the energy goes
⏱ about 20 min

Game Day: Bounce Contest

Saturday morning, three soft balls sit in a row on the patio: a tennis ball, a foam ball and a playground ball.

"Same drop line, three balls, three floors," Rocket says. "Highest bounce share wins."

Raven tapes the drop line at shoulder height. "Predict first. Which ball keeps the most of its energy as height?"

"Tennis ball on tile," Rocket says. "Foam ball last. It squashes and stays squashed for a moment."

Nova hovers at the drop line like a tiny starter. "What do you listen for?" she asks.

"The thump," Rocket says. "Soft thump, low bounce."

The foam ball lands with almost no sound and barely rises. "What do you notice?" Raven asks. "Quiet, and low."

Nova's light glows. "Then where did its energy go?"

  1. The law of conservation of energy: the total stays the same while energy changes form or moves between objects.
  2. A dropped ball bounces back lower because some energy became thermal energy and sound. A new tennis ball on a court returns to about 72 percent.
  3. Friction turns kinetic energy into thermal energy. That is why a rolling ball stops and why rubbing hands warms them.
  4. Efficiency is the useful share. Cycling is about 20 percent, and the rest is mostly warmth.
  5. Bending your knees or rolling on landing spreads the energy change over more time, so the force is smaller.
◇ FAMILY BOUNCE CONTEST
Tape a drop line at the shortest person's shoulder height. Stand a tape measure upright beside it.
Each person picks a ball and a floor and predicts the bounce as a percent of the drop height.
Drop from the line without pushing. A partner reads the top of the first bounce. Do three drops each.
Score by the closest prediction. Then swap balls and floors and try again.
Talk it over. Which ball kept the most energy as height? Which floor took the most? What did the thumps tell you?
For a grown-up
Soft balls only: tennis, foam or playground. Drops from standing reach, never from a chair or step.
Choose a spot away from windows, roads and younger children. Let a ball finish bouncing before anyone steps in.
The energy facts this week come from OpenStax physics texts, which you can read at openstax.org.
Safety first
Drop, never throw. Stand on the floor for every drop.
Keep the drop spot clear while a ball is bouncing.
Wear shoes outdoors and drink water on a hot day.
THINK IT THROUGH
  • Read the question.
  • Tap your answer.
The foam ball lands almost silently and bounces low. Where did most of its energy go?
Two balls drop from the same line. Why do they start with the same energy?
Game Day checkTrue or false?
A ball that bounces lower turned more of its energy into thermal energy and sound.?
The total energy after the bounce is less than before.?
Friction and squashing both turn motion energy into warmth.?
WHY THIS EXERCISEThe contest is the whole week in one drop: energy in, a share back as height, the rest as warmth and sound.
Draw your three balls at the top of their bounces beside the drop line. Label the highest and the lowest with where the energy went.
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