Laws of Motion: Inertia, Force and Weight

Newton's First Law and the Nature of Inertia

Newton's first law of motion is also known as the law of inertia. It establishes a fundamental rule about how objects behave when left alone: an object at rest remains at rest unless acted upon by a net external force, and an object in motion continues at constant velocity unless acted upon by a net external force. In simple terms, things keep doing what they are already doing unless an unbalanced force interferes with them.

Inertia is the natural tendency of an object to resist changes in its state of motion. Mass serves as the quantitative measure of an object's inertia, meaning that objects with greater mass possess greater inertia. It is vital to recognize that inertia depends solely on mass; it does not depend on an object's velocity, speed, or the local gravity acting upon it. A heavy boulder has the same resistance to changing its motion whether it is stationary, rolling quickly, or resting in outer space.

When the net force acting on an object is zero, the object is in mechanical equilibrium. Mechanical equilibrium occurs in two distinct states. Static equilibrium occurs when an object is at rest and the net force is zero. Dynamic equilibrium occurs when an object moves at a constant speed in a straight line and the net force is zero. In both conditions, because the net force is zero, there is no change in motion.

To evaluate the forces on an item, physicists use a free-body diagram. A free-body diagram displays all the individual external forces acting on a single object. By sketching these forces, you can determine whether they balance out or leave an unbalanced net force.