Inertia, the property of matter that causes it to resist any change of its motion in either direction or speed. This property is accurately described by the first law of motion of the English scientist Sir Isaac Newton: An object at rest tends to remain at rest, and an object in motion tends to continue in motion in a straight line unless acted upon by an outside force. For example, passengers in an accelerating automobile feel the force of the seat against their backs overcoming their inertia so as to increase their velocity. As the car decelerates, the passengers tend to continue in motion and lurch forward. If the car turns a corner, then a package on the car seat will slide across the seat as the inertia of the package causes it to continue moving in a straight line.

Any body spinning on its axis, such as a flywheel, exhibits rotational inertia, a resistance to change of its rotational speed. To change the rate of rotation of an object by a certain amount, a relatively large force is required for an object with a large rotational inertia, and a relatively small force is required for an object with a small rotational inertia. Flywheels, which are attached to the crankshaft in automobile engines, have a large rotational inertia. The engine delivers power in surges; the large rotational inertia of the flywheel absorbs these surges and keeps the engine delivering power smoothly.

An object's inertia is determined by its mass. Newton's second law states that a force acting on an object is equal to the mass of the object multiplied by the

Any body spinning on its axis, such as a flywheel, exhibits rotational inertia, a resistance to change of its rotational speed. To change the rate of rotation of an object by a certain amount, a relatively large force is required for an object with a large rotational inertia, and a relatively small force is required for an object with a small rotational inertia. Flywheels, which are attached to the crankshaft in automobile engines, have a large rotational inertia. The engine delivers power in surges; the large rotational inertia of the flywheel absorbs these surges and keeps the engine delivering power smoothly.

An object's inertia is determined by its mass. Newton's second law states that a force acting on an object is equal to the mass of the object multiplied by the

**acceleration**the object undergoes. Thus, if a force causes an object to accelerate at a certain rate, then a stronger force must be applied to make a more massive object accelerate at the same rate; the more massive object has a larger amount of inertia that must be overcome. For example, if a bowling ball and a baseball are accelerated so that they end up rolling at the same speed, then a larger force must have been applied to the bowling ball, since it has more inertia.