Newtons Laws

IGCSE PHYSICS

FORCES

Newton's Laws of

Motion

Mass & Inertia

When an object is not moving, we
need to apply a force to get it
moving. When an object is
already moving, we need to apply
a force to stop it from moving

The bigger the mass of the object,
the bigger the force we need

Masses have inertia - the
resistance / reluctance to start /
stop moving

Multiple Choice

Which sentence describes inertia?

• Inertia is the force that pulls objects toward the center of the Earth.
• Inertia is the ability of an object to float in water.
• Inertia is the resistance of any physical object to any change in its state of motion.
• Inertia is the speed at which an object moves.

Experiment 1

Experiment Set-up

Glass or container
Piece of card
pennies x 5

Example of inertia of rest

Newton's 1st Law of Motion

an object at rest, will continue to stay at rest
an object moving, will continue to move at a constant speed in
the same direction

If the forces on a mass are balanced (no resultant force), then:

Newton's 2nd Law of Motion

F = ma

1) An object of mass 50kg is moving with an acceleration of 2m/s^2.

a) What is the resultant force acting on the object?

b) If the object has a forward force of 90N, what are
the frictional forces acting on the object?

1) An object of mass 4kg has a 10N force applied. The
object moves at a constant velocity.

a) What is the force of friction?

The force is now increased to 15N.
b) What is the resultant force?

c) What is the acceleration?

Newton's 3rd Law of Motion

The action and reaction force are equal

and opposite

Force of your feet on the earth
moves the earth slightly backwards
Force of the earth on your feet,
moves you forwards