Newton's First Law of Motion

If there is no resultant force on an object, the object will remain at the same state of motion:

• An object at rest remains at rest.
• A moving object keeps moving in the same velocity.

Frequently asked example in IGCSE Physics:

• When a bus suddenly starts to move forwards from rest, passengers inside the bus tend to stay at rest due to inertia. This is why passengers jerk backwards when the bus starts moving forwards.
• When a bus moving forwards suddenly comes to a stop, passengers tend to keep moving forwards at the speed the bus was moving. Passengers will move jerk forwards when the bus stops.

Newton's Second Law of Motion

Newton's second law can be explained by the following equation:

F = ma

F = resultant force on an object (N)

m = mass of object (kg)

a = acceleration of object (m/s²)

A crate of mass 5 kg is pushed with 12 N force. The frictional force between the crate and the surface is 8 N. What is the acceleration of the crate?

Resultant force on crate = 12 - 8 = 4 N

 a = F / m = 4 / 5 = 0.8 m/s² 

Newton's Third Law of Motion

• For every action force, there is a reaction force between two interacting objects.
• If object 1 exerts a force on object 2 , object 2 also exerts a force on object 1.
• The size of the pair of forces is equal.
• The direction of the forces are opposite. 

Some examples about Newton's third law:

• When an object is placed on a flat surface, the object will push down on the surface by its weight. The surface will then push back the object by the same size but opposite force.
• When you throw a ball, the ball also pushes you back.
• When you fire a bullet from a gun, the bullet also exerts a backward force on the gun. This is why the gun recoils backwards when a bullet is fired.

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