Newton’s First Law of Motion

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What do you think? If something is moving – planet, automobile, jet plane, etc, -- and if you want it to keep on moving in a straight line without changing its speed, what do you need to do to it? Keep pulling on it with a force, maybe? That is what people thought for a long time, but Isaac Newton, drawing on the work of Galileo and others, thought about it a little more precisely than anyone else had done and found a different answer. The answer he found was the first part of his laws of motion that explained how literally everything known up to that time moved. These laws also explained how most of what was discovered later moved.

Actually, the first part of Newton’s First Law of Motion is not very mysterious. It says that if something is at rest, you have to pull it to get it to move. More precisely: “An object at rest remains at rest unless acted on by a net outside force.”

So, if your automobile is at rest, and no force whatever acts upon it, then it remains at rest. However, it is very difficult, in fact impossible, to find an object that really has no forces on it. There is at least gravity. That is why Newton said something about a “net” force. If there is no “net” force, this means all of the forces balance. Here is an automobile at rest:

The forces balance, and as long as that is true, the car will remain at rest. If you produce a force to the right by means of using the engine, and if the brakes are on, then the forces might still balance. So the car will still remain at rest. That is what is happening in the picture below.

If you remove the force of the brake then the forces will be unbalanced. This will accelerate the car, which is discussed in the file on Newton’s Second Law of Motion. If you do accelerate the car, there will be a backwards force due to friction with the air and due to internal friction in the car. If the forward force on the engine is larger than this friction force, the car will accelerate. That is what unbalanced forces do – accelerate the object under discussion.

Forward force greater, forces not balanced, car accelerates

Forward force cut back so that it balances the backward friction force – car moves at uniform motion

You will presumably arrive, sooner or later, at the speed at which you want to drive. When that happens, the usual procedure is to let up on the gas. When you do that, you are really decreasing the forward force so that it exactly balances friction. Then the car moves at a constant speed in a straight line, and the forces are balanced. And that is the other part of Newton’s First Law. If an object is already in motion, and if all the forces balance, then the object will continue in motion at a constant speed in a straight line.

This type of motion, constant speed in a straight line, is called uniform motion. Accelerated motion is any motion that deviates from uniform motion. For a graphical look at this idea, go to the slide show on uniform vs. accelerated motion. Then come back here.

As you can see in the slide show, motion in a circle is accelerated motion, because it deviates from uniform motion. Motion in an ellipse or any other curve is also accelerated. That means the moon is accelerated in its motion around the earth, and the planets are accelerated in their motion around the sun.

When thinking about the moon, some people might think, “What keeps it up there?” Newton realized that the proper question is, “What keeps it down here – attached to the earth?” If there were no forces on the moon, it would move in uniform motion – at a constant speed in a straight line.

You would be very lucky to be in the generation that saw the moon go by.

Of course, there had to be a force on the moon to bend it into an orbit around the earth. To read about that force, go on to the discussion of Newton’s law of Universal Gravitation.