 We're going to look at Newton's first law in a different way from what we've done before. Up to now we've been looking at objects moving in a straight line and either speeding up or slowing down or maintaining the same velocity. Now we're going to look at objects that are changing direction. Now to review Newton's first law it says that an object continues in a state of rest or constant velocity unless acted upon by net external force. Well velocity is a vector and it has both magnitude and direction and so we can change the velocity of an object, either changing its magnitude which is its speed or changing its direction. In this demonstration we're going to change its direction. So I've got a string tied to a tennis ball here and I'm going to swing it in a circle. What I'm doing, I'm trying to swing it at about a constant speed but I'm changing its direction all the time because the direction of the ball, its velocity vector, is always tangent to its circular path. So that must require a net external force in order to do that. Where is that force being applied? Well it's being applied by my hand. I'm pulling on the string. It's a force of tension. Now to show you that there actually is a force of tension there, I've got a rubber band tied to the end of it and so I'm going to hold the rubber band instead of the string and you'll actually be able to see the rubber band stretch. And the fast drive swing you should see the band stretch more. We'll start out very slow, faster and faster. I'm feeling more tension, the band is stretching more and what else is happening? The ball is traveling faster. So I'm applying greater and greater tension force and which direction is that tension force acting? Well, I'm applying it with my hand through the string and so it is acting toward my finger or toward the center of the circular path. So the tension force acts along a radius and toward the center of the path and that is the force, the net external force which we call some tripletal force which makes the ball move in a circular path.