Newton's first law of motion

Newton's first law of motion :

Every object in the state of uniform motion tends to

remain in that state of motion

unless an external force is applied to it.

Every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia. The key point here is that if there is no net force resulting from unbalanced forces acting on an object (if all the external forces cancel each other out), then the object will maintain a constant velocity. If that velocity is zero, then the object remains at rest. And if an additional external force is applied, the velocity will change because of the force.

Examples : The motion of a ball falling down through the air ; a model rocket being launched up into the atmosphere.