Newton's First Law
Forces and Motion
When the resultant force is zero
Physics Narrative
for 11-14
Zero resultant force predicts a constant speed
When describing the conditions for constant speed the key points are:
- All forces acting on an object add to zero: the resultant force is zero.
- Retarding forces are equal to the driving forces.
Often the retarding forces are the result of friction. It is difficult for us to imagine a world without such frictional forces.
- Friction is a blanket term for the forces that act on moving objects and on objects which we are trying to move.
- When we reduce friction (for example with oil, a cushion of air or a streamlined shape), we find it much easier to make things move or to keep them moving.
- Frictional forces only show up when something is moving (drag or slip) or when a driving force would otherwise accelerate it (grip).
