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Sports Physics 6-8 / Week 02 / Game Day
6/6
Week 02 · Pushes, Pulls and Balanced Forces

Game Day

Game Day: Tug of war, by the rules
// What changes a motion, and what leaves it alone
⏱ about 20 min

Game Day: Tug of War, by the Rules

Saturday at the Field, the family lines up on two ends of a soft rope. A tape line marks the middle.

"Rules," Raven announces. "Gentle pulls, no letting go, and we call out balanced or unbalanced every ten seconds."

Rocket counts down. Both sides lean back. The knot hangs over the tape and trembles. "Balanced!" everyone shouts.

Then a cousin digs in her heels and the knot slides. "Unbalanced!" Rocket calls. "Toward the cousins!"

Nova hovers above the tape line, flashing each time the knot crosses it.

"What do you notice?" Raven asks afterward. "The knot only moved when one side pulled harder."

Rocket nods. "Newton would have made a good referee."

  1. A force is a push or a pull. Every force has a strength and a direction.
  2. Adding all the forces on an object gives the net force.
  3. Newton's first law: an object keeps its motion unless an unbalanced force acts on it.
  4. Balanced forces leave motion unchanged. Unbalanced forces change speed or direction.
  5. Friction acts against motion and is why rolling balls slow down and stop.
◇ FAMILY TUG OF WAR
Lay a soft rope or a long towel on grass or carpet. Tape a center line on the ground.
Split into two teams. Pull gently until the middle holds still over the line. Call out "balanced".
Let one team pull a little harder. Watch the middle move. Call out "unbalanced" and which way it moved.
Swap one person from team to team and try again. Talk about what changed.
Finish with everyone letting the rope go slack slowly, together.
For a grown-up
Keep the pulls gentle and the ground soft. Nobody lets go suddenly, and the rope is never wrapped around a hand or neck.
Younger children join the balanced test only, with an adult beside them.
The force facts this week come from NASA Glenn Research Center and an OpenStax physics text, at nasa.gov and openstax.org.
Safety first
Pull slowly and steadily. No jerking, no sudden letting go.
Grass or carpet underfoot, away from stairs, roads and furniture corners.
Stop at once if anyone slips, is tired or asks to stop.
THINK IT THROUGH
  • Read the question.
  • Tap your answer.
The rope's middle holds still over the line. What is true about the two teams' pulls?
The middle slides toward the cousins. Which team is pulling harder?
Game Day checkTrue or false?
A tied tug of war has balanced forces on the rope's middle.?
The knot moves toward the weaker pull.?
An unbalanced force changes an object's motion.?
WHY THIS EXERCISEThe knot is a net force meter: still means balanced, moving means unbalanced.
Draw the rope with your two teams as arrows. Show one balanced moment and one unbalanced moment.
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