 In this demonstration we're 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 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 a rest. Or once I get it started moving it very quickly comes to a stop. So again it seems like it's trying to remain a 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 its velocity or to cause it to decrease in velocity. And that force of course we're talking about is friction. This brick has a lot of friction acting on it. Friction acts opposite 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 this 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 that 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 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 will force 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's a lot of friction. If I give the glider a push it very quickly comes to a stop. Now 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 actually brought it to a stop. Now there is a little bit of friction even with the air coming out. And if the track is tilted slightly that would also either accelerate or decelerate the cart. But I think you can see that without by reducing the friction as much as possible the cart continues its motion.