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Sports Physics 6-8 / Week 09 / Game Day
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Week 09 · Action and Reaction

Game Day

Game Day: Rolling collisions
// Every push has a push back
⏱ about 20 min

Game Day: Rolling Collisions

Saturday morning, a long strip of masking tape runs down the hallway at the Field. Two soft balls wait at one end.

"One line only," Raven says. "Every roll stays on the tape. That keeps the test fair."

Rocket rolls a tennis ball into a resting foam ball. The foam ball shoots ahead and the tennis ball slows.

"They pushed each other," he says. "The light one moved more, but both felt the same push."

Nova hovers above the tape line. "What do you notice if you swap them?"

Rocket rolls the foam ball into the resting tennis ball. The tennis ball barely rolls and the foam ball bounces back.

"Same pair of forces," Raven says, writing fast. "Different masses, different motion."

  1. Whenever one object exerts a force on another, the second exerts an equal and opposite force on the first.
  2. The two forces in a pair act on different objects, so they do not cancel.
  3. Walkers push the ground backward, swimmers push water backward, rockets push gas backward. Each gets pushed forward.
  4. Thrust is the reaction force on a rocket or balloon from the gas it pushes out.
  5. In a collision, both objects feel equal pushes. The lighter one changes its motion more.
◇ FAMILY ROLLING COLLISION CONTEST
Lay a straight line of masking tape about 3 metres long on a hallway floor or patio.
Each person rolls a soft ball along the tape into a resting soft ball. Mark where each ball stops.
Try a heavier roller into a lighter resting ball, then the other way around. Keep every roll slow and on the line.
Talk it over: which ball moved farther after each collision? Did the pushes feel equal even when the motion was not?
For a grown-up
Soft balls only: foam, tennis or a rolled pair of socks. Nothing is thrown, and every roll stays on the floor.
Pick a hallway or patio away from stairs, windows and younger children. Clear the line before each roll.
The collision and force facts this week come from NASA Glenn Research Center and a college physics text at libretexts.org.
Safety first
Roll, never throw. Soft balls only.
Keep the rolling line clear of feet, pets and toys before every roll.
Stop if anyone is hurt or tired.
THINK IT THROUGH
  • Read the question.
  • Tap your answer.
A tennis ball rolls into a resting foam ball. Which ball feels the bigger push?
After the collision, which ball changes its motion more?
Game Day checkTrue or false?
Two colliding balls push on each other with equal force.?
The heavier ball always feels the bigger push.?
Keeping every roll on one line makes the test fair.?
WHY THIS EXERCISEEqual pushes with different results is the heart of this week and the start of week 12.
Draw the two balls at the moment they touch. Add two equal arrows pointing opposite ways, one on each ball.
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