Newton’s First Law: Inertia Depends on Mass
Inertia: Forces on Objects at Rest
Newton’s First Law
External forces change the motion of an object. This is the basic point of Newton’s first law of motion, which is also called the Law of Inertia.
The Law of Inertia states that an object at rest remains at rest and an object in motion continues in motion with constant velocity unless the object experiences a net external force.
In this example, the force is acting only on the cardboard and not on the coin; therefore, the coin does not fly along with the cardboard—it stays at rest.
The first part of the law seems obvious: objects at rest continue at rest unless acted on by a net force. You have experienced this idea in your everyday life. Your computer will remain still on the table in front of you as long as nothing touches it. The net external forces are zero. This does not mean that there are no forces on it: the force of gravity is acting downward on your computer, but the upward forces from the table balance the force of gravity, so the computer remains at rest.