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Home Minds On Circular and Satellite Motion Mission CG4 Centripetal Force Requirement Help Help 9

Mission CG4 Centripetal Force Requirement

The Question

A bucket of water is held by a rope and twirled in a vertical circle. The tension force acting upon the bucket would decrease to zero if ____.

Physics Rules

Centripetal Force Requirement:
Circular motion requires an inward force. To travel along the curved path of a circle, there must be a force directed centripetally. Any object or thing could supply the force as long as it is directed toward the center of the circular path.

Hands On Time

Give it a try (but not inside)! If you have a plastic bucket and a rope or heavy string, then you might give this one a try. Fill the bucket with an inch or two of water (more is not better), secure the rope or string to the bucket's handle and get ready to whirl. Go outside and find plenty of air space free from trees, windows, cars, streets, power lines and curious neighbors. With a quick burst of energy and a good grip on the string, begin spinning the bucket in a circle. Compare the tension in the string when the bucket is at the top and the bottom of the circular turn. Then gradually slow the bucket down over the course of several circles; observe the tightness of the string as the bucket approaches the top of the circle at this slower speed. What do you observe?

Think About It

Circular motion requires an inward net force. At all points along the circle, the net force must be directed toward the center. The tension force is always inwards and gravity is always downwards. The downwards gravity force is towards the center of the circle when the bucket is at the top of the loop and away from the center of the circle when the bucket is at the bottom of the loop. At the top of the loop, both tension and gravity are directed towards the center. Together, these two forces supply provide sufficient force to sustain the acceleration experienced by the bucket at this location. Gravity can sustain an acceleration of 9.8 m/s/s. Whatever additional acceleration that there is must be sustained by the tension force. If the acceleration decreases to 9.8 m/s/s, then there will no longer be a tension force when the bucket is at the top of the loop. The bucket and the water would reach a free fall state in which the only force is gravity.

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