Motion and the Law of Inertia

by Ron Kurtus (revised 6 August 2014)

Inertia is a property of space and matter such that an object has a tendency to remain in its state of motion unless acted upon by a force. This is stated in Newton's First Law of Motionwhich is often called the Law of Inertia.

What the law states is that if an object is motionless, it will stay motionless unless acted upon by some force. Likewise, if an object is moving at a constant velocity, it will continue at that speed unless acted upon by some force along the line of motion. And finally, if an object is moving, it will move in a straight line unless acted upon by some force at an angle .

This law is important in defining how things around us behave.

Questions you may have include:

• Why do objects remain motionless?
• Why do things continue moving?
• Why do objects go in straight lines?

Object stays motionless

The Law of Inertia states that objects that are not moving will remain motionless, unless you apply a force on them. This law of nature makes sure things will stay where you put them. It requires that you provide a push or pull to get something moving.

A trick using inertia

There is the parlor trick that you may have seen where a person quickly pulls a tablecloth from under a setting of heavy dishes or some other objects. If it is does correctly, the objects remain in place on the table after the tablecloth is pulled from under them. This trick works because the inertia of the heavy objects tends to keep them in place. By quickly pulling the tablecloth, the force of friction is easily overcome. If the tablecloth was pulled slowly, the friction would be greater than the inertia, and the dishes would follow along.

(Note: The tablecloth must be pulled down at the edge, otherwise the dishes or objects may fly upward.)

Object will continue motion

Once you start an object moving, it will keep moving unless you apply a force in the opposite direction to slow it down. Typically, the force of friction will slow things down. But in outer space, where friction is almost zero, an object will move at its given velocity forever unless acted upon by some external force.

If you push a moving object in the direction of motion, it will accelerate to a new velocity. Once you stop pushing, the object will continue at the velocity it had once you stopped pushing.

Things move in straight lines

The Law of Inertia states that moving objects go in a straight line. You must apply a force on an object to make it go in a circular motion. For example, when you spin an object around on a string, you are applying a force on that object from the string to make it go around. Once you let the string (or the force) go, the object will fly off in a straight line.

Likewise, the Moon is attracted to the Earth by the gravitational force. That force is just enough to make the Moon spin around the Earth. If gravitation would stop, the Moon would fly off in a straight line into outer space.

Summary

The Law of Inertia states that matter remains in its state of motion and direction unless acted upon by a force. Motionless objects will stay motionless unless acted upon by some force. Moving objects continue at a constant speed unless acted upon by some force. Also, moving objects will move in a straight line unless acted upon at an angle by some force

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