Newton's laws

Inertia, Newton's second law and action–reaction, applied to sport movements.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Law of inertia: what is it?
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A body stays at rest or in uniform motion in a straight line unless a net force acts on it.
A hockey puck slides a long way on ice.
A swimmer flip-turns and pushes against the wall. Why does he move off?
- The wall exerts an equal and opposite force on him
- His incoming speed is kept by inertia
- Buoyancy sends him back forwards
- The water he displaces pushes on his back
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The wall exerts an equal and opposite force on him
This is action–reaction: he pushes the wall backwards, the wall pushes him forwards with a force of the same size.
Newton's second law: what is it?
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The sum of forces equals mass times acceleration: ΣF = m × a.
At equal force, a lighter athlete accelerates more.
Why does a moving cyclist keep going when he stops pedalling?
- Because a forward force stays stored in the bike
- By inertia, slowed gradually by air and friction
- Because the freewheel keeps driving him
- Because his mass creates a forward push
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By inertia, slowed gradually by air and friction
Without a net force his speed would hold: on the flat, only air and friction bring it down.