Changing motions

The ice-skater

Imagine that you are watching an ice-skater. She stands motionless in the centre of the ice before pushing off and gliding out towards the edge of the rink. At first she is stationary and she ends up moving at more-or-less constant speed. Somewhere in between she must have speeded up (or accelerated) and it is this changing motion that we are interested in here. Everyday experience offers countless examples of changing motion, for example the sprinter accelerating out of their blocks; a football being kicked from the penalty spot; a train slowing down as it enters the station. We're sure you'll be able to add many to the list.

The basic condition for an object to be speeding up or slowing down is summarised in Newton's first law of motion:

If an object is either speeding up or slowing down, there must be a resultant force acting on it.

In other words, if the forces acting on an object do not all add to zero, then the object has a changing motion.

Forces do not add to zero  →  motion will change.