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Centripetal Force - Tennis Ball on a String

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Now we are going to look at Newton's first law in a different way than we've been doing. Up until now, we have been looking at objects moving in a straight line, either speeding up, slowing down, or staying at the same speed. Now we are going to look at objects that are changing direction. For review, Newton's first law says that an object remains at rest or constant velocity unless acted upon by a net external force. Well, velocity is a vector, and has both magnitude and direction. So we can change the velocity of an object, changing either its magnitude or direction. In this demonstration, we are going to change direction.

So I've got a string tied to a tennis ball, and I am going to swing it in a circle. What I am doing is trying to swing it at a constant speed, but I am constantly changing its direction all the time, because the direction of the ball, its velocity vector, is tangent to the circular path. So that must required a net external force in order to do that. Where is that force being applied? Well its being applied by my hand, I am pulling on the string. It is a force of tension.

To show you it's a force of tension, I have a rubber band tied to the end of it, and so I am going to hold the rubber band instead. You will be able to actually see the rubber band stretch, and the faster I swing the more the rubber band should stretch. Let's start out slow, and faster, and faster. I'm feeding more tension, the band is stretch more. And what else is happening, the ball is moving faster. So I am applying greater and greater tension force. What direction is that tension force going? Well, I am applying it with my hand, through the string, so it is acting toward my hand or the center of the path. So the tension force acts on the radius and towards the center of the path. And that is the force, that we call centripetal force, that causes the ball to move in a circular path.

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