Newton's Three Laws — And What Each One Actually Explains

Force and the Idea of Inertia

A force is any push or pull that can change an object's state of rest or motion, its direction, or its shape. SI unit: the newton (N).

Newton's First Law: an object continues in its state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

This is also called the Law of Inertia. Inertia is the tendency of a body to resist a change in its state of motion, and mass is the measure of inertia — a heavier object is harder to start or stop.

Three familiar examples:

• ›A bus brakes suddenly and you lurch forward — your body keeps moving
• ›The bus starts suddenly and you jerk backward — your body was at rest
• ›Dust flies off a carpet when beaten — the carpet moves, the dust stays put

Balanced vs unbalanced: balanced forces cancel and cause no acceleration. Only an unbalanced force changes motion.

1. A passenger standing in a bus falls forward when the bus stops suddenly. This is due to:

2. The SI unit of force is:

3. A force of 30 N acts on a body of mass 6 kg. The acceleration produced is:

4. Rocket propulsion is based on which principle?

5. Which of the following remains unchanged when a body is taken from the Earth to the Moon?

6. A cricketer moves his hands backward while catching a ball in order to:

7. A gun of mass 5 kg fires a bullet of mass 25 g with a velocity of 200 m/s. The recoil velocity of the gun is:

8. Which statement about action and reaction forces is correct?