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Name: Peter
Status: student
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How and why does the weight of a pendulum (tension in the cable) change in relation to the movement of its bob?


The tension in the cable is only equal to the ball’s weight when the bob is not moving. The bob is affected by two forces. The weight of the bob pulls straight downward. The cable pulls toward the pivot. Together they produce the motion of the pendulum.

Consider spinning a rock on a string over your head. To keep the rock on its circular path, you have to pull inward. The faster the rock moves, the harder you must pull. Length of the string also has an effect. This same thing happens when a pendulum bob swings. It is moving very fast at the bottom. Speed drops to zero at the top of the swing. The tension is greatest at the bottom, where speed is greatest. If the tension matched the bob’s weight, this would be enough to keep the bob from curving downward, to keep it on a straight path. To make the bob curve upward requires even greater pull. As speed and direction change, so does the tension.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Dear Peter,

The tension in the cable changes due to the centripetal force needed to move the bob in a circular path. The centripetal force varies both in magnitude as the speed of the bob changes (F = mvv/r) and direction (perpendicular to the velocity of the bob and parallel to the cable. The gravitational force of the bob(mg) is, of course, always vertically down. To find the total tension in the cable, you must vectorially add these two forces.

Best, Richard J. Plano...

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