Newton's Laws of Motion

Newton's First Law (inertia)

An object will remain at rest or continue with constant velocity unless acted on by an unbalanced force

For a body to be at rest, there must be a force balancing the weight acting in the opposite direction (reaction force)

E.g. When an ice hockey puck is stationary on the ice. The force of the weight of the puck is balanced by the upward force from the ice, so it is at rest. But when a player hits the puck, an unbalanced external force is applied so it accelerations.

Newton's Second Law (acceleration)

The acceleration (for a body/object of constant mass) is proportional to, and in the same direction, as the unbalanced force applied to it

F = ma

• E.g. To give a soccer ball the same acceleration as a table tennis ball, the player must apply a larger force because the soccer ball has a greater mass than the table tennis ball.

Newton's Third Law (reaction)

When one body or object applies a force to another, the second body or object will apply a force equal in size but opposite in direction to the body or object.

• Two forces are on two different bodies or objects (not on the same body/object)

• The forces on the objects are exactly the same size

• The effects of those forces may be different if the bodies/objects are of different masses (2nd law)

• Forces happen at exactly the same time

• E.g. the blocks will push back into the sprinter with the same force, but in the opposite direction (forwards and upwards). As the blocks are connected to the ground (which has a much larger mass than the athlete), the ground will move backwards, but the athlete will move forwards and upwards out of the blockers as soon as they raise their arms from the ground