Lisa Marie Summers, age 12, of  Monroe, La., for her question:

WHAT IS INERTIA?

Inertia is a property of all matter. It makes an object that is not moving continue motionless unless some force puts it into motion.

Inertia also makes a moving object continue to move at a constant speed and in the same direction unless some outside force changes the object's motion. Only such a force can make a moving object slow down, speed up, stop or turn. One force that commonly slows or stops a moving object is friction with other objects.

The force necessary to change an object's motion depends on the mass of the object. Mass may be defined as the amount of matter in an object.

The greater an object's mass, the harder it is to put the object into motion or to change its direction or speed.