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A three-child operation
It took three children to operate this merry-go-round. One child sat on each end and the third child pushed it around and around as fast as possible.
Oops
We quickly learned that if we sat on the plank between the handle and the end of the plank and tried to hold ontothe handle, we would slide off the end of the plank when the rotation of the plank reached a certain speed. Therefore, we learned to sit with our legs across thehandle, with the handle pressing against the back insides of our knees. By doing this, we could survive without sliding off the plank no matter how fast the third child caused theplank to rotate.
Why am I telling you this?
Although I didn't understand it at the time, when the plank was rotating around its spindle at a relatively high rate of speed, the handle exerted a force on the inside of both knees. Inother words, the handle exerted a force on each leg in the direction of the center of the plank. This is a force that we will call a centripetal force .
Centripetal force
According to The Physics Classroom , any object moving in a circle (or along a circular path) experiences a centripetal force. That is, there is some physical forcepushing or pulling the object towards the center of the circle. This is the centripetal force requirement.
The word centripetal is merely an adjective used to describe the direction of the force. We are not introducing a newtype of force but rather describing the direction of the net force acting upon the object that moves in the circle. Whatever the object, if it movesin a circle, there is some force acting upon it to cause it to deviate from its straight-line path, accelerate inwards and move along a circular path.
A centripetal force on my body
The effect of this centripetal force was to cause my body to accelerate in the direction of the force, which was directly toward the spindle at the center of the plank.The spindle was at the center of the circle around which I was experiencing circular motion (although probably not uniform circular motion).
Consider tying a weight, such as a full bottle of soda, to the end of a string. Then swing the bottle around and around above your head in anapproximate circle. You will feel a force pulling your hand toward the periphery of the circle, so you will exert a force on the string to keep the bottle fromflying away. The force that you exert on the string will be exerted on the bottle at the other end of the string and that force will be directed along thestring towards the center of the circle. That is the force that we will refer to as centripetal (center seeking) force, and that force will act on the mass ofthe bottle to cause the bottle to accelerate in the direction of the center of the circle.
What happens if the string breaks?
If the string breaks, there will no longer be a centripetal force acting on the bottle and it will fly off into space along a line that is tangential to itslocation on the circle at the instant the string breaks. It won't continue moving in a circle. Instead, it will move in a straight line until the resistance ofthe air and force of gravity bring it to a sudden stop on the ground. (Actually, it will move along a parabolic arc under the force of gravity until it strikesthe ground.)
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