Newton’s First Law states that

An object  will remain at rest or in uniform motion  unless an unbalanced force acts upon it.

Seems obvious but deserves a little analysis.

For example these beer bottles have the force of gravity pulling them down but they don’t move because the table top is pushing up at the same time, with an exactly equal and opposite force. The overall force is zero.

Reaction to a weight on a table

Even though this truck is being pushed hard, it is not moving because friction is acting. The forces are balanced.

The freewheeling cyclist will only stop if friction brings her to a halt, which, on Earth, it always will.

Freewheeling cyclist

An astronaut or any other object in deep space, a long way from any large masses and gravitational fields will experience no forces. Once moving the object will move at the same speed for ever in a straight line, unless another force (such as gravity) comes in to play.

However if an unbalanced force acts, as with this rocket where the thrust of the rocket is far greater than friction, the object accelerates.

This leads on to Newton’s Second Law.

Download a pdf on all the notes for Newton's Laws here  Newton's Laws of Motion

Related pages are:

Newton's second Law

Newton's Third Law

Law of Momentum

Impulse and Momentum