Brief Explanation of Newton's First Law of Motion: Science Help

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Terms of Newton’s First Law of Motion

Let’s begin by defining Newton’s First Law of Motion.

Newtons’ First Law of Motion: An object will remain at rest or in uniform motion in a straight line unless acted on by an external unbalanced force.

Let’s take a look at a couple of terms in this law.

  • Uniform motion in a straight line: An object in the situation has a constant velocity. An object at rest has a constant velocity of zero. Basically what this means is that if these objects are left alone, they will continue to move at the same speed and direction as they are right now (or lack of).

  • External force: This is any force from outside the object. For instance, a moving car will stop if it hits another moving car, but not if you step on the floorboard because stepping on the floorboard is an internal force.

  • Unbalanced forces: These are forces that are not equal and cause motion. If the forces were balanced, there would be no motion.

  • At rest: The object is not moving at all (or seemingly).

When you put these terms together and translate them, it basically means that an object which is not moving will remain still until something comes in contact with it and has the force to move it. By the same rule, a moving object will keep going in a straight line until something comes in contact with it and has the force to impact its movement.


Balanced forces: As mentioned above, balanced forces are ones that do not create motion. One example is tug of war. As one side pulls on the other, there is sometimes no motion at all. But there are forces are work. Those forces are balanced and so even though there is force, there is no motion.

Unbalanced forces: These are the forces that cause movement. In the example above, the forces were balanced and so there was no motion. However, if one team pulled harder than the other, there would be motion.

Objects at rest: If unbalanced force causes motion, then you are probably wondering how to describe the motion that occurs from and object at rest being moved. For example you may push a piano and it may not move. That is because the force of gravity uses the weight of the piano as a type of force that keeps balance unless an outside force or external force creates an imbalance. The harder you push, the more of a chance you have of creating an unbalanced force. If you push hard enough you will create an unbalanced force and the piano will move.

This review should have helped you to understand Newton’s First Law of Motion. However, feel free to increase your understanding by practicing this law of motion in the world around you. If you need more help with understanding the various elements of motion, feel free to look over these study guides on motion.