Newton's Laws Of Motion

This lesson covers: 

  1. Understanding Newton's first law
  2. Quantifying force, mass, and acceleration using Newton's second law of motion
  3. Exploring Newton's third law of motion and the principle of action and reaction forces

Newton's first law

Diagram illustrating Newton's first law with an apple on a table showing reaction and weight forces.

An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.


Key points:

  • A stationary object will stay still until a resultant force acts upon it.
  • An object in motion continues to move at a constant speed and direction unless a resultant force changes its speed or direction.


Newton's second law

Newton’s second law states that the rate of change of momentum is directly proportional to the net force experienced.


F = tΔP


Where:

  • F = net force in newtons (N)
  • ΔP = change in momentum (kg m s−1)
  • t = time in seconds


Newton’s second law can also be expressed as:

F=ma


Where:

  • F = resultant force in newtons (N)
  • m = mass of the object in kilograms (kg)
  • a = acceleration in meters per second squared (m s-2)

Worked example: Calculating the force acting on an accelerating object

A car of mass 1,000 kg accelerates at 2 m s-2. Find the net force.


Step 1: Formula

F = m x a


Step 2: substitution and correct evaluation

F = 1,000 x 2 = 2,000 N

Newton's third law

Newton’s third law states that for every action, there is an equal and opposite reaction.

Illustration showing Newton's third law with a hand pushing against a wall, indicating action and reaction forces.

Interaction of Forces:

When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

  • The forces are always of the same type (e.g., both gravitational )
  • The paired forces act on different objects.