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Motion and the Law of Inertia

by Ron Kurtus (28 October 2007)

Newton's first law of motion is often called the Law of Inertia. It states that matter remains in its state of motion and direction unless acted upon by a force. In other words, the law states 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 speed or 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 at an angle by some force. This law is important in defining how things behave around us.

Questions you may have include:

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

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

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Time = 3 min. 30 sec.

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 force of gravity. That force is just enough to make the Moon spin around the Earth. If gravity 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.

Answers to Readers' Questions

Don't let your personal inertia hold you back

Resources

The following resources provide information on this subject:

Websites

Physical Science Resources

Books

Top-rated books on Motion

Miscellaneous

Mini-quiz to check your understanding

1. If you see something move, what can you assume?

2. If moving objects are supposed to continue moving, why do things often slow down and stop?

3. If things are supposed to go in straight lines, why does a ball you throw curve downward?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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Motion and the Law of Inertia