Newton's 3rd Law: Spring Scale Pull

1. Pick a trial with the tabs at the top. In Trial 1, Scale A is hooked to a post that does not move. In Trial 2, a partner pulls on each end.
2. Pull by moving a slider. You can also drag a hand in the picture. In Trial 2, each partner has their own slider.
3. Read both scales in the close-up boxes. The colored marker shows the force in newtons (N).
4. Click Record reading. Both scale readings go into the data table for the trial you are on.
5. Try at least 3 different pulls in each trial. In Trial 2, try equal pulls and unequal pulls. Compare Scale A and Scale B each time.
6. Turn on Force arrows to see the force pair. Turn off Show numbers to practice reading the scales yourself.
7. Click Let go to drop the pull to 0 N. Click Clear table to start over.

Newton's third law says forces always come in pairs. When one object pulls on another, the second object pulls back. The two pulls are the same size. They point in opposite directions.

Scale B pulls on Scale A. At the same moment, Scale A pulls back on Scale B. Each scale measures the pull on its own hook. So the two readings always match.

Trial 1: The post is not trying to pull. But it still pulls back on Scale A just as hard as Partner B pulls. That is why Scale A reads the same as Scale B.

Trial 2: Both partners pull. If the pulls are equal, nobody moves. If one partner pulls harder, both partners slide toward the stronger puller. The scales show a force between the two pulls. But Scale A and Scale B still read the same as each other.

The two forces in a pair act on different objects. One acts on Scale A. The other acts on Scale B. That is why they do not cancel each other out.

Force

A push or a pull on an object.

Newton (N)

The unit we use to measure force. A small apple weighs about 1 N.

Spring scale

A tool that measures force. A bigger pull stretches the spring more, so the marker moves farther.

Action and reaction forces

A pair of forces between two objects. They are equal in size and opposite in direction.