Newton’s third law: forces come in pairs — Physics, 11–13 years
When one object pushes or pulls another, the second pushes or pulls back just as strongly. The two forces act on different objects, so they do not simply cancel.
Every push has a partner
Forces always come as an interaction between two objects. If your hand pushes a wall, your hand pushes the wall and the wall pushes your hand back with equal strength in the opposite direction. The forces are a pair, but each acts on a different object.
Why does a push come back?
Without this idea, it is hard to explain how walking, swimming or launching a rocket works. Your foot pushes the ground backwards, and the ground pushes you forwards. Newton developed the rule to describe this same pattern in every push or pull.
A skateboard push
A rider pushes the ground backwards with a force of 40 N. Step 1: the ground pushes the rider forwards with 40 N. Step 2: the directions are opposite. Step 3: the forces act on different objects, so the rider can accelerate instead of the forces cancelling on the rider.
The forces do not cancel
A common mistake is saying that equal opposite forces must stop everything. That sounds reasonable because equal numbers with opposite signs add to zero. The missing question is: zero on which object? A pair acts on two objects; only forces on the same object are combined.
Walking and swimming
Walking works because your shoe pushes the ground backwards and friction lets the ground push you forwards. A swimmer does something similar by pushing water backwards with hands and feet. This idea also helps explain propellers, rowing and rocket engines.
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