An object in motion stays in motion, in a , unless acted on by an external force.

An object in motion stays in motion, in a uniform motion, unless acted on by an external unbalanced force.

Explanation:

This statement is also known as Newton's First Law of motion (also called "Law of inertia").

It states that:

"When an object is in motion at constant velocity (or at rest), the object tends to remain in its state of motion at constant velocity (or at rest), unless there is a net external unbalanced force acting on it".

The action of an external unbalanced force would change the state of motion of the object; in particular, it will cause an acceleration in the object, according to Newton's Second Law of motion:

where

F is the net force on the object

m is the mass of the object

a is its acceleration

Therefore, we can see that if the external force acting on the object is zero:

F = 0

Then the acceleration of the object is also zero:

a = 0

And so, the object will continue its motion with constant velocity (or it will remain at rest, if it was at rest).

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explanation: