Newton's First Law

Defintion

A body at rest remains at rest and a body in motion remains in uniform motion,
in a straight line motion, unless acted on or compelled to change by an external unbalanced force.

It known that forces are necessary to cause anything to move if it is originally at rest.

Newton called inertia, the property of a particle that allows it to maintain a constant
state of motion or rest inertia. Newton's First Law is sometimes called "the Law of
Inertia". All objects have inertia.

So bjects in nature tend to:
• Stay at rest, or
• Stay in straight-line motion,
• Unless a force acts on them and changes their status.

Examples

• A steel bar that slides on the shop floor soon comes to rest because of its interaction with the floor.
• The same bar would slide much farther on ice before stopping.
• If a hockey puck is slid on ice, it will stop due to ice friction.
• If a ball is thown in space, it would not stop because there is no friction, gravity or air resistance to stop it.

• See below illustration regarding the two scenarios when the forces are balanced:

Question: Why do passengers in a train seem to lean forward when it slows down and then backward when it picks up? Tip: Read Newton's first law.