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Newton's Laws of Motion

13 min read

Newton's first law: inertia

First law
An object stays at rest, or keeps moving at a constant velocity in a straight line, unless a resultant force acts on it.

The tendency of an object to resist a change in its motion is called inertia. The bigger the mass, the bigger the inertia. When a bus stops suddenly, passengers lurch forward because their bodies tend to keep moving.

Newton's second law: F = ma

Second law
The acceleration of an object is directly proportional to the resultant force on it and inversely proportional to its mass.
Force (N) = mass (kg) × acceleration (m/s²). One newton gives a 1 kg mass an acceleration of 1 m/s².
  • resultant force (N)
  • mass (kg)
  • acceleration (m/s²)
Forces on a blockA block resting on a table and being pulled to the right. Weight acts downwards, the normal reaction from the table acts upwards, the pull acts to the right and friction acts to the left.blockR normal reactionW weightP pullfriction F
A force diagram of a block pulled along a table. If the pull is bigger than friction, the resultant force is to the right and the block accelerates.
Try itPush a trolley

Change the force and the mass. A bigger force gives a bigger acceleration; a bigger mass gives a smaller one.

Worked exampleFind the force

A 1 200 kg car accelerates at 2.5 m/s². What resultant force acts on it?

  1. Use F = ma.

Answer: The resultant force is 3 000 N.

Newton's third law

Third law
If body A exerts a force on body B, then B exerts an equal and opposite force on A.
ActionReaction
A rocket pushes gas backwardsgas pushes the rocket forwards
You push the ground backwards when walkingthe ground pushes you forwards
A swimmer pushes water backwater pushes the swimmer forward
Action and reactionTwo boxes touching. Box A pushes box B to the right with a force, and at the same moment box B pushes box A to the left with an equal force. The two forces act on different objects.ABA pushes B →← B pushes AEqual in size, opposite in direction,acting on different objects

Momentum

Momentum = mass × velocity (kg m/s). In a collision with no outside forces the total momentum is conserved.
Check yourself A 5 kg object has a resultant force of 20 N. What is its acceleration?
Check yourself Why does a passenger lurch forward when a bus brakes?

Key terms

Inertia
Resistance to a change in motion.
Resultant force
The single force with the same effect as all the forces.
Momentum
Mass times velocity.

Summary

  • First law: no resultant force means no change in velocity.
  • Second law: F = ma.
  • Third law: forces come in equal and opposite pairs on different objects.
  • Momentum p = mv is conserved in collisions.
Motion in a Straight Line Temperature