Newton's 3 Laws of Motion

Newtons 1st Law

If no Net Force Acts Upon an Object it Will:

• Remain Stationary or
• Keep Moving:
• a) at the Same Speed and
• b) in the Same Direction

This does not often happen as most objects experience the forces of friction and weight.

Isolated objects in space will keep moving with the same speed.

Newton's 2nd Law

F=ma Tells Us

• Massive (heavy) objects are more difficult to start moving quickly
• You need more force to make an object gain speed faster
• Also you need more force to stop massive objects...
• ...or change their direction

Weight (W) is a Force

• So like other forces W= m x a
• Also W= m x g (gravitational field strength)
• So m x a = m x g and so a=g.
• The acceleration of falling only depends on g
• It does not depend upon mass

Momentum = mass x velocity

• Momentum tells us the impact objects have in collisions
• Newtons second law says:
• Force is the rate of change of momentum
• So Force x time is change in momentum

Car safety features causes you to change speed slower so there is less force

Newton's 3rd Law

Reaction Force

• If A causes a force on B:
• B causes a force on A
• This reaction force is the same size...
• ...but in the opposite direction

The gravitational pull caused by the moon on Earth (causing tides) is equal to the Earth's pull on the moon.

When you hit an object, you can feel the reaction force of the object on your body.

Balanced Forces

• Equal and opposite forces can act on the same object
• The net force is zero so the motion does not change
• this is different to action - reaction forces
• Balanced forces are different forces acting on the same object
• Action-Reaction are the same force on different objects

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