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Sports Physics 6-8 / Week 02 / Wednesday
3/6
Week 02 · Pushes, Pulls and Balanced Forces

Wednesday

Field Lab: The backpack tug
// What changes a motion, and what leaves it alone
⏱ about 20 min

Wednesday: Field Lab: The Backpack Tug

Rocket has tied two loops of string to the handle of a backpack. "One for each of us," he says. "Pull it two ways."

Raven takes her loop. "Predict first. If we pull equally hard, what does the backpack do?"

"Nothing," Rocket says. "Balanced forces. It stays."

They pull. The strings go tight. The backpack sits on the tape line and does not budge.

Nova hovers above. "Now one of you pull a little harder."

Raven leans back. The backpack slides toward her across the grass and stops when she eases off.

"What do you notice?" she asks. Rocket grins. "It moved toward the bigger pull, and it stopped when we matched again."

Your mission

Today you test the first law with a backpack. You will pull it with balanced forces, then with unbalanced forces.

You will also push a soft ball with one tap and watch what stops it.

Predict each result before you try it. Then record what you see.

  • A backpack with a few books inside, zipped shut
  • Two loops of string or two scarves tied to the handle
  • A soft ball: a tennis ball or a foam ball
  • Masking tape for a center line
  • A partner, and your Field Log
Safety first
Pull gently and slowly. Nobody lets go suddenly, and nobody wraps string around a hand or wrist.
Work on grass, carpet or a smooth floor, well away from stairs, roads and furniture corners.
Only a soft ball is used. It is rolled or tapped along the ground, never thrown.
Stop if anyone is tired. A grown-up picks the spot.
  1. Put a strip of tape on the ground. Set the backpack on it with one string loop on each side.
  2. Each partner holds one loop. Pull gently and equally until both strings are tight. Watch the backpack for ten seconds.
  3. Record whether the backpack moved. Write "balanced" or "unbalanced" beside the result.
  4. Now one partner pulls a little harder. Record which way the backpack moves and how far.
  5. Swap roles so the other partner pulls harder. Record the result.
  6. Set the ball on the tape line. Tap it once, gently, so it rolls across the floor.
  7. Watch until it stops. Record how far it rolled and what you think slowed it.
  8. Repeat the tap on a different surface (carpet, grass or tile) and record the distance again.
PREDICT BEFORE YOU PULL
  • Read the question.
  • Tap your answer.
Both partners pull the backpack equally hard in opposite directions. The backpack will:
One partner pulls harder. The backpack will move:
After one tap, the ball rolls, slows and stops. What slowed it?
TestMy predictionWhat happenedBalanced or unbalanced?
Equal pulls on the backpack
Partner A pulls harder
Partner B pulls harder
One tap, ball on smooth floor
One tap, ball on carpet or grass

Reading the result

With equal pulls, the backpack stayed on the line. The forces were balanced, so its motion did not change. It stayed at rest.

With one stronger pull, the backpack moved toward that pull. The forces were unbalanced, so the motion changed.

The ball needed only one tap to start. Nobody kept pushing, yet it kept rolling for a while. That is inertia.

It slowed anyway, because friction pushed against its motion. On carpet or grass, friction is bigger, so it stopped sooner.

Both results match Newton's first law. Motion changes only when the forces are unbalanced.

What the lab showsTrue or false?
Equal and opposite pulls leave the backpack at rest.?
The backpack moves toward the stronger pull.?
The ball kept rolling after the tap because someone kept pushing it.?
The ball rolled farther on the rougher surface.?
WHY THIS EXERCISEEach test is the first law in action: balanced forces, no change; unbalanced forces, a change.
How many string loops were tied to the backpack? Type a number.
WHY THIS EXERCISETwo opposite pulls on one object is the simplest way to see balanced and unbalanced forces.
Draw the backpack from above with two arrows. Make one longer to show the unbalanced test.

Careful testing. Tomorrow you will read a table of real forces from a physics text and NASA.

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