Force and motion

A book pushed across a table soon stops. A bicycle slows when you stop pedalling. These everyday experiences can make it seem that motion needs a constant supply of effort.

We will begin with Newton’s account of motion and gravity. It gives a useful description of falling objects and orbits. Later lessons will show how Einstein’s theory connects these motions with measurements of distance and time.

The surroundings are what slow them down. Friction acts between the book and the table. The bicycle loses speed through rolling resistance and air resistance. Reduce that resistance and an object travels farther after the same initial push.

What a force changes

An object’s velocity includes both its speed and its direction. A change in either is an acceleration.

A hand pushing a puck and a string pulling a ball both exert forces. Several forces can act at once, and their combined effect is called the net force. If they balance, the object’s velocity does not change.

This is Newton’s first law: with no net force, an object stays at rest or continues in a straight line at constant speed.

Motion depends on the observer

A puck sliding past you appears stationary to someone moving alongside it. Both descriptions are correct. Each measures motion relative to a different observer.

At the everyday speeds shown here, observers moving steadily relative to one another agree on how the puck’s velocity changes. They can disagree about its speed while agreeing on its acceleration. Later, we will look at what changes when an observer accelerates too.

Turning is also acceleration

A push along the puck’s direction of travel speeds it up. A push against that direction slows it down. A force kept at right angles to its motion turns it without changing its speed.

Friction is one way of changing motion, but it does more than slow things down. It opposes slipping between surfaces. The friction between your shoes and the ground helps propel you forwards when you walk.

For circular motion, a net force must keep turning the object towards the centre. Remove that force and it continues along the direction it had at that instant: the tangent to the circle.

The Moon also follows a curved path. The next lesson looks at how gravity produces that motion without a supporting surface or a string.