Inertia

Definition of Inertia

Inertia is the property of an object that resists any change in its state of motion. This means an object will resist changes whether it is at rest or moving at a constant speed in a straight line. Inertia is directly related to the mass of the object 64the more mass an object has, the greater its inertia.

In simple terms, inertia is why you feel pushed back into your seat when a car accelerates or why a stationary object does not move unless a force acts on it. For instance, a 10 kg object resists changes to its motion more than a 1 kg object due to its greater mass.

Newton's First Law

Newton's First Law of Motion states that an object will remain at rest or continue to move at a constant velocity unless acted upon by an unbalanced force. This is often called the law of inertia.

This means:

• If no forces are acting on an object, or if all forces balance out, the objects motion will not change.
• An unbalanced force is required to change an object's speed or direction.

Inertia explains this behaviour because it is the object's resistance to any change in motion. The greater the inertia (mass), the harder it is to change the motion.

For example, if a hockey ball is sliding on ice, it will keep sliding at the same speed and direction unless friction or another force acts on it to slow it down or change its direction.

Mass and Inertia

Mass is a measure of how much matter an object contains and also measures its inertia. The larger the mass of an object, the greater its inertia, meaning it is harder to change its motion.

For example, it is much harder to push a heavy car to start moving than a light bicycle because the car has more mass and therefore more inertia.

This is why mass is sometimes called a measure of inertia. It tells us how resistant an object is to changes in its motion.

For instance, a small ball and a large ball rolling at the same speed will both keep moving unless a force acts on them, but the large ball (with more mass) will be harder to stop because of its greater inertia.