Newton’s First Law:

If the resultant force acting on an object is zero and:

  • the object is stationary, the object remains stationary
  • the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.

So, when a vehicle travels at a steady speed the resistive forces balance the driving force.

So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.

Newton’s First Law Flowchart

The tendency of objects to continue in their state of rest or of uniform motion is called inertia.


The forces acting on the car are balanced.

There is zero resultant force.

Newton’s Second Law:

The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.

resultant force = mass × acceleration

Inertial mass is a measure of how difficult it is to change the

Inertial mass is defined as the ratio of force over acceleration.

resultant force = mass × acceleration

1.2 = mass x 2.4

mass = 1.2 / 2.4

The mass of the trolley is 0.5 kg

The boat will accelerate

to the right

because there is a resultant force in that direction

Newton’s Third Law:

Newton’s 3rd Law

Whenever two objects interact, the forces they exert on each other are equal and opposite.

The two forces must be of the same type acting on each individual object.

  • the forces of the bike on the trailer and the trailer on the bike are equal in size
  • and opposite in direction