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Newton's First Law

In the past few weeks we discussed many aspects of motion. We did not concern ourselves with the reason why something moves, but only looked at the object once it was moving. In the next few chapters we will find out what makes objects move, what is the underlying reason for making something go.

All of this is related to forces. It is a little more difficult to define what forces are. Right now it is enough to know that forces are interactions between two objects – always to forces. The force is exerted by something on something else. One object exerts the force and there has to be another object onto which the force is exerted on.

The most important and most difficult aspect of his chapter is to understand the connection between forces and motion. It is not a simple cause and effect relationship as Aristotle thought of it 2000 years ago. He thought that the natural state of an object is to be at rest and that something (we now call “a force”) is necessary to keep it moving. He thought that for an object to keep moving a force is necessary.

But forces do not simply cause motion. If you think about it I am sure you can come up with an example of something that is moving without force acting on it. The best example, although you have no first-hand experience with it, is outer space. A spaceship in outer space would simply continue to go if it is moving already, there is nothing to stop it or slow it down. There is no force acting on it and the spaceship is moving. Therefore there is no connection between force and motion.

What needs to be done to the spaceship to stop it? It could slam into something or the thrusters could be applied to slow it down as well. The thruster example is a little more complicated because it is not quite obvious what the two objects are that are interacting. (It is not the spaceship and the air, as there is no air in outer space. It is in fact the spaceship and the gas that gets pushed out the back.) The same is true for launching the ship – it could use it thrusters or it could be slingshot or any other way, as long as a force is applied to it, that makes it go.

 

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Summing this up, we realize that a force is necessary to start or stop a spaceship but not to keep it going. Forces don’t cause motion they cause the change in motion.

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Galileo whom we talked about when we discussed free fall was the first to investigate the effect on forces. He did this when he studied what happen to a ball rolling down an incline. He figured out, something that you might at first find trivial, that is that the incline causes the ball to go faster, to accelerate. The steeper the incline the more acceleration there will be, not the faster the ball rolls. A ball can attain any speed if you make the incline long enough, so a short steep incline might result in a smaller final speed than a long shallow one. The angle of the incline causes the acceleration not the fast speed. What makes this so difficult is that you might think that acceleration is always linked to high speeds, but even a small acceleration might lead to a very large speed; it will just take much longer.

Galileo then realized that if there is no incline that this simply means there is no acceleration not necessarily no speed. The bal could be moving at a speed but with no incline there would be no acceleration, therefore the speed would be constant. He was one step away from formulating the fact that an object’s natural state of motion (no incline) is not at rest, but constant velocity.

Newton, working independently of Galileo, first formulated this law and it is therefore called Newton’s first law: An object’s natural state of motion, in the absence of unbalanced forces, is constant velocity. Unbalanced forces would be the push of a hand, or a slingshot. Balanced forces are those counteracting each other when for instance a book rests on a table. Gravity pulls the book down and the table pushes it up.

He then went further to investigate the effects of balanced forces; what if there is unbalanced force acting on an object? This object will then accelerate or change its motion. This is not what Aristotle said almost 2000 years before Newton. But the fact that it took so long for somebody to figure this out already hints at the difficulty of this concept: Forces don’t cause motion, forces change motion. This is one of the most difficult concepts to really buy into. Sure, you could memorize it, but when faced with a problem where this concepts is part of the analysis it takes more then having it memorized to get the problem right.

 

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