Newton's First Law
Forces and Motion

When the resultant force is zero

Physics Narrative for 11-14 Supporting Physics Teaching

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).
Newton's First Law
formalises Inertia
includes the quantity Force
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