2.20–2.23: Circular motion and Newton's third law

Why does an object moving in a circle at constant speed have changing velocity?

Its direction changes continuously, and velocity includes direction.

What is centripetal force?

The resultant force acting towards the centre of a circular path.

Why is a resultant force needed for circular motion?

To continually change the direction of the object’s velocity.

In which direction does centripetal force act?

Towards the centre of the circle.

Does an object moving in a circle at constant speed have constant velocity?

No, because its direction is continuously changing.

State Newton’s third law.

When object A exerts a force on object B, object B exerts an equal-sized force in the opposite direction on object A.

What are the key features of a Newton’s third-law force pair?

The forces are equal in magnitude, opposite in direction, of the same type, and act on different objects.

Why do Newton’s third-law forces not cancel each other out?

They act on different objects.

Give an example of Newton’s third law in walking.

The foot pushes the ground backwards, and the ground pushes the foot forwards.

Give an example of Newton’s third law in a collision.

A moving car exerts a force on another car, and the second car exerts an equal and opposite force on the first.

How does Newton’s third law relate to momentum conservation in a collision?

Equal and opposite interaction forces produce equal and opposite momentum changes, so total momentum is conserved in an isolated system.

What is an equilibrium situation?

A situation in which the resultant force on an object is zero.

Can forces be balanced when an object is moving?

Yes. If the resultant force is zero, an object can move at constant velocity.