A body will continue in its present state of rest or, if it is in motion, will continue to move with uniform speed in a straight line unless it is acted upon by a force.

What Newton's first law of motion is saying in essence is that if you keep, for example, a pen on a table [the pen is not moving, so it is said to be at rest], it will continue to remain there for ever, unless someone or something [such as wind] applies a force on it to move it away from that spot on the table where you kept it.

In another hand, Newton's first law of motion is also saying that if an object, for example a ball, is thrown upward, it will continue to move upward for ever at uniform speed in a straight line.

However, in real life, things do not continue to move for ever when set in motion, they will at some point come to a stop. But this does not mean Newton's first law of motion is not obeyed, the law is actually perfectly obeyed if certain resistant forces do not act against the moving object to eventually bring it to a stop.

Therefore, the point to note here is that in the absence of certain forces that may act against a body in motion, the body will continue to move along a straight line, with the same speed with which it was set in motion.

Taking the example of the ball thrown upward, the ball will eventually fall back to the ground because of the forces of gravity and air resistant, making it look like Newton's first law of motion wasn't valid.

In actual fact, Newton's first law of motion is true in the absence of forces acting against the moving object. In such condition, the body will continue in its movement forever in a straight line.

Here are some experiences that prove Newton's first law of motion right:

 1. Proving the first part of the law. If you are in a car that is not moving and another one rammed into it from behind, this is what will happen:

Your car will be pushed suddenly forward, causing the seat you are in to move your body forward. But your head will tend to remain at its state of rest because it was not pushed forward with the body, and so it will jerk backward as the body moves forward.

This explains the first aspect of Newton's first law of motion; that an object will continue to remain in its state of rest unless a force acts on it.

The jerking backward of the head has been the reason for neck injuries during car accidents, and to prevent or reduce this, cars now have headrest to keep the head in proper position.

2. Proving the second part of the law. The second aspect of Newton's first law of motion, which is that an object set in motion will continue moving in a straight line, with uniform speed, can be shown to also be true with this experience:

You are in a moving car, and the brakes were suddenly applied. You will be jerked forward because your body wants to continue moving forward. If you are sitting in the front seat, you might crash into the windscreen unless you apply enough force to hold yourself back, or you have your safety belt on.

Newton's first law of motion is also known as the law of inertia. This is because inertia is the tendency of a body to remain in its state of rest or uniform motion.

See Newton' Second Law Of Motion