Light
Newton's First Law, also known as the law of inertia, states that an object at rest will stay at rest and an object in motion will stay in motion with a constant velocity unless acted upon by an external force.
The tendency of an object to resist changes in its motion.
A push or pull that can cause an object to accelerate or change its state of motion.
When all forces acting on an object are balanced, resulting in no net force and thus no acceleration.