Carolyn Steinkamp, age 16, of Glendale, Ariz., for her question:

CAN YOU EXPLAIN INERTIA?

Inertia is 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 either is acted upon by an outside force.

You can experience an excellent test of inertia in the family automobile. Passengers in an accelerating car feel the force of the seat against their back overcoming their inertia so as to increase their velocity. As the auto decelerates, the passengers tend to continue in motion and lurch forward. If the car turns a corner, 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. A force must act upon the wheel to slow or speed up its rotation. The matter in the wheel is constrained to move along a curved path by the molecular forces that held the wheel together.

The speed of the material along the curved path cannot be changed, however, without overcoming its inertia, and this gives rise to rotational inertia. The inertia of spinning objects results in other phenomena, notably the gyroscopic effect.

By common experience, inertia is generally related to mass, the amount of matter in a body. A greater force is needed to accelerate a baseball than to accelerate a Ping Pong ball.

Albert Einstein theorized that gravitational forces and inertial forces are identical and that it is impossible to distinguish between them. This equivalence principle is the basis of Einstein's theory of general relativity. According to this theory, local properties of inertia and gravitation are the result of the entire mass of matter in the universe.

The greater an object's mass, the harder it is to put the object into motion or to change its direction or speed. For example, a locomotive has more mass than an automobile. Therefore, it is harder to stop a moving locomotive than to stop an automobile traveling at the same speed.

Because of the relationship between inertia and mass, physicists usually define mass as a measure of inertia rather than as a measure of matter.

The difficulty involved in changing the direction or speed of an object also depends on how quickly the change is made. It is harder to slow down, speed up or turn a moving object suddenly than to make the same change gradually. A car has more difficulty holding the road on a curve at high speed than at a low speed.

Physicists use the term "acceleration" to describe the rate of change in an object's direction or speed.