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Newton's Laws

01Newton's Laws

First law

This section covers Newton's first law and the relationship between balanced forces and constant velocity.

Motion without a resultant force

Newton's first law: an object remains at rest or continues moving with constant velocity unless a resultant force acts on it.

A resultant force is therefore required to change an object's motion. It may:

  • increase its speed;
  • decrease its speed;
  • change its direction of motion.

If all the forces acting on an object are balanced, the resultant force is zero. The object is then either stationary or moving with constant velocity.

driving forceresistive forceRW

For constant velocity, the horizontal forces balance:

∑Fx=0\sum F_x=0

The vertical forces must also balance if there is no vertical acceleration:

∑Fy=0\sum F_y=0

In the illustrated example, the forward driving force equals the backward resistive force, while the normal reaction balances the weight.

Worked example: A car moves at constant velocity. Its engine provides a driving force of 6.0 kN6.0\,\text{kN} and the only other horizontal force is friction. Find the frictional force.

Constant velocity means the resultant force is zero.

Therefore the frictional force must balance the driving force:

Ffriction=6.0 kNF_{\text{friction}}=6.0\,\text{kN}

It acts opposite to the direction of motion.

Exam Tip: Do not state that a moving object needs a resultant force to keep moving. Under Newton's first law, zero resultant force is consistent with constant velocity.

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