Newton’s third law — Physical education, 14–17 years
Forces always come in pairs between two interacting objects. Physical education, 14–17 years.
The two-way push
When one object pushes or pulls another, the second object pushes or pulls back at the same time. The two forces are equal in size and opposite in direction, but they act on different objects.
Why forces come in pairs
A force is not something an object owns by itself; it describes an interaction. Newton needed a rule that connected both sides of every interaction, so motion could be explained consistently whether a person pushes a wall or a planet pulls a moon.
A swimmer pushes water
A swimmer pushes 30 N of water backwards with their hands. Step 1: identify the interaction—hands and water. Step 2: the water pushes the swimmer forwards with 30 N. Step 3: these forces do not cancel, because one acts on water and the other on the swimmer.
The equal-force trap
A common thought is: “If the forces are equal, nothing can move.” That sounds reasonable if both forces act on one object, as opposite forces can cancel. Here they belong to a pair on two objects, so the swimmer can move while the water moves the other way.
Walking and rockets
Walking works because your shoe pushes the ground backwards and the ground pushes you forwards. A rocket does the same idea in space: it pushes hot gases backwards, and the gases push the rocket forwards. It does not need air to push against.
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