31_circular - Lesson 31: Circular Motion Think of spinning...

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Lesson 31: Circular Motion Think of spinning an object on the end of a string. How would you describe it to someone? The easiest thing to say is that it moves in the shape of a circle. This means you should be able to measure the radius of the circle, and from that calculate things like circumference . For the purpose of the lessons that follow, we will also assume that you keep spinning the bob at the same speed. You don't slow it down or speed it up. This means that the bob is rotating at the same speed the entire time. We can not say that the bob is moving at the same velocity the entire time, because at any instant, at any point, the bob is actually changing the direction it is moving. Illustration 1 shows a bob being twirled around clockwise on a string. At first, when it is at the top , the direction of the instantaneous velocity is to the right . The velocity of an object moving in a circle is the tangent line at that point. A moment later the bob is at the next position . Although it is traveling at the same speed (magnitude), the direction it is traveling has changed. It is now pointing down to the right . That means that the direction the bob is traveling is constantly changing! By definition, this means that the bob is accelerating (since acceleration is any change in velocity). If it wasn’t accelerating, it would be traveling in a straight line (according to Newton's First Law). We can get a visual idea of the acceleration as a change in velocity (since Δv is directly related to the acceleration of an object). Since we want Δv = v f v i , we will use the rules for vectors and show it as Δv = v f + - v i 7/22/2008 © studyphysics.ca The object that is attached to the end of the string is usually called a bob . Illustration 1: A bob moving
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This note was uploaded on 12/02/2011 for the course PHYSICS 235 taught by Professor Staff during the Fall '08 term at Rutgers.

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31_circular - Lesson 31: Circular Motion Think of spinning...

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