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Newton's First Law: The Influence of Friction

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In this demonstration we are looking at Newton's first law. Newton's first law states that: An object continues in its state of rest or uniform velocity unless acted on by a net external force. Now, this may seem counterintuitive to you. If I take a brick, I have to push on it to get it moving in the first place. So it would seem like its natural state of motion is to be at rest. Or once I get it started moving it very quickly comes to a stop. So, again, it seems like it is trying to remain at rest rather than in a state of constant velocity. But what we want to do today is look at this in terms of forces which are acting on it to either resist an increase in velocity or to cause it to decrease in velocity. And that force, of course, we are talking about is friction.

This brick has a lot of friction acting on it. Friction acts opposite of the direction of motion. So if I push the brick and it moves this way the friction of the table on the brick will act the opposite way on it. And it actually serves as an unbalanced force which causes it to decrease velocity and eventually come to a stop. Let's do that one more time.

So to show that friction really is the culprit in this case what we need to do is reduce the friction and see what the results are. I can do that by using a cart with wheels on it and put the brick on the cart. Now when I give it a push...first of all it was easier to get it started in the first place but it also went further. Now it did slow down; take another look at that but not nearly as much or as quickly as it did when the brick was directly on the table.

Now I can reduce the friction even more by using a cart that has special low friction wheels. Now I'll put the brick on it, I'll have to put it sideways on this small cart, give it a push (it doesn't take much of one) and it just seems to keep going with the velocity I originally gave it. So here friction is almost zero and the object continues once I push the object and it reaches a particular velocity then it remains at that velocity or very nearly so. Because I have removed the friction which would be an unbalanced force acting on it this way.

Let's extend our argument further...as far as we can go. In front of me is an air track. It has a row of holes (actually four rows of holes) along the track and when I turn on the air supply it forces the air out. And so this glider will ride on a cushion of air much as a puck would ride on an air hockey table. Now without the air coming out you can see that there is a lot of friction. If I give the glider a push it very quickly comes to a stop. I'm going to turn the air on and we'll see what happens. So you can see that the glider traveled a lot further and in fact didn't come to a stop until I brought it to a stop. Now there is a little bit of friction even with the air coming out and if the track were tilted slightly that would also either accelerate or decelerate the cart. But I think you can see by reducing the friction as much as possible the cart continues its motion.

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