Explanation of Motion and the Law of Inertia by Ron Kurtus - Succeed in Understanding Physics. Key words: physical science, Newton, direction, motionless, force, speed, velocity, straight line, School for Champions. Copyright © Restrictions
Motion and the Law of Inertia
by Ron Kurtus (revised 22 August 2013)
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.
Useful tool: Metric-English Conversion
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.
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.
Don't let your personal inertia hold you back
Resources and references
Newton's First Law of Motion - Physics Classroom
Newton's laws of motion - Wikipedia
Questions and comments
Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.
Click on a button to bookmark or share this page through Twitter, Facebook, email, or other services:
Students and researchers
The Web address of this page is:
Please include it as a link on your website or as a reference in your report, document, or thesis.
Where are you now?
Motion and the Law of Inertia