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Scientists Say: Inertia

All objects have inertia. This is the natural tendency of objects to resist changes in their motion. Objects that are not moving tend to stay that way. Objects in motion tend to keep moving at the same speed and in the same direction. Overcoming an object’s inertia to change its motion requires applying a force.

For instance, a soccer ball laying on the ground will stay there until someone applies a force to it — say, by kicking it. A kicked ball would sail through the air forever, if not for the forces of gravity and air resistance dragging it down.

These rules of inertia make up physicist Isaac Newton’s first law of motion: An object at rest will stay at rest; an object in motion will stay in motion, until a force is applied. (The second law states that an object’s change in motion depends on its mass and the force applied. The third law says that whenever one object exerts a force on another, the second object applies an equal and opposite force back.)

The more massive an object is, the more it resists changes in its motion. That is, the more inertia it has. For example, it takes a lot more force to get a train rolling than a bicycle. That’s because a train has far more mass than a bicycle. It also takes a lot more force to stop a train in its tracks.

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