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Lesson: Chapter 4 — Newton's Laws of Motion

Newton's second law 3 of 4

Testing it with a trolley

Both halves of the second law can be confirmed with a simple experiment.

Figure to be added

A trolley on a horizontal table. One picture shows a single rubber band attached to the trolley and stretched by a hand; the other shows two rubber bands stretched by the same amount.

Part one — changing the force

  1. Place a trolley on a horizontal table and attach a rubber band to it.
  2. Hold the free end of the rubber band and stretch it until it reaches the far end of the trolley.
  3. Move your hand along with the trolley so that the stretched part of the band stays unchanged.
  4. Observe that the trolley moves with an acceleration.

Now attach two identical rubber bands, stretch both by the same amount as before, and repeat. The force applied is doubled. Then use three identical bands — three times the force.

Observation

Each time the number of bands goes up, the trolley's acceleration goes up too.

That is, a ∝ F.

Part two — changing the mass

  1. Place some mass on top of the trolley.
  2. Using one rubber band only, stretched as before, repeat the experiment.
  3. Add a further mass and repeat again.

Observation

Each time mass is added the acceleration falls — even though the force applied is unchanged.

That is, a ∝ 1 ÷ m.

Why stretch the band by the same amount?

How far a rubber band is stretched shows how much force it applies. Same stretch, same force. It is what makes sure that only one thing at a time changes in the experiment.