Newton's Second Law
Forces and Motion

Forces acting and changing motion

Physics Narrative for 11-14 Supporting Physics Teaching

Forces for speeding up and slowing down

Consider an object moving on a horizontal surface, maybe a pencil case that you are pushing across the table. For such examples the essential forces to consider are the forward driving force (your push) and the retarding force acting in the opposite direction (due to friction between the pencil case and the table). Let's look closely at these two forces:

  • If the driving force is greater, the object will speed up.
  • If the retarding force is greater, the object will slow down.
  • If the forces are the same size, then the object continues at a constant speed (it's in equilibrium).

It is the driving force which starts the object moving. As long as the driving force is greater than the retarding force, the object will speed up. If the driving force is removed, the retarding force will continue to act on the object until it eventually comes to rest (the pencil case soon comes to a halt if you stop pushing it).

The cyclist

The forces acting on a cyclist moving at constant speed add to zero. As soon as the cyclist stops pedalling, the driving force disappears. The frictional forces (drag and slip forces) continue to act and the effect of these retarding horizontal forces will be to slow down the cyclist.

Resultant force  →  change in motion.

Newton's Second Law
is expressed by the relation F=ma
can be used to derive Kepler's First Law
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