Part 02 · Forces and Newton's laws · Card 5 of 8
Newton's third law
When object A pushes on object B, B pushes back on A with an equal force in the opposite direction.
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Analogy
Push a wall while on roller skates and you roll backwards.
In practice
A rocket pushes gas down and backwards, and the gas pushes the rocket up and forwards.
Practice
Flashcards
- What is Newton's third law?When A pushes B, B pushes A with an equal force in the opposite direction.
Questions people ask
If forces come in equal pairs, why does anything move?
The two forces in a pair act on different objects, so they cannot cancel each other. A horse pulls a cart and the cart pulls back on the horse equally. The cart still moves because the horse also pushes the ground, and the ground pushes the horse forward.
Is the weight of a book and the table's push on it a third law pair?
No. They happen to be equal and opposite when the book is still, but they act on the same object. A true pair acts on two different objects. The real pair for the book's weight is the book pulling the Earth upwards with an equal force.