A ball with a weight of 2 N is attached to the end of a cord of length 2 m. The

Question

A ball with a weight of 2 N is attached to the end of a cord of length 2 m. The ball is whirled in a
vertical circle counterclockwise. The tension in the cord at the top of the circle is 7 N and at the
bottom it is 15 N. (Do not assume that the speed of the ball is the same at these points.)
a. Three students discuss the net forces on the ball at the top:

Deante: The net force on the ball at the top position is 7 N since the net
force is the same as the tension.
Edgar: The net force on the ball at the top position is 9 N. Both the tension
and the weight are acting downward so you have to add them.
Fiona: No, you are both wrong. You need to figure out the centripetal force
(mv^2/r ) and include it in the net force.
Which, if any, of these students do you agree with?
Deante: _____ Edgar: _____ Fiona: _____ None of them: _____
Explain your reasoning.

b. Now the students discuss the net force at the bottom.
Deante: The net force on the ball at the bottom position is 15 N since the net
force is the same as the tension.
Edgar: The net force on the ball at the bottom position is 17 N, since you
need to add the weight of 2 N to the tension of 15 N.
Fiona: The net force on the ball at the bottom position is 13 N. I agree that
you need to take into account both the weight and the tension but
they are in different directions so they will subtract.
Which, if any, of these students do you agree with?
Deante: _____ Edgar: _____ Fiona: _____ None of them: _____
Explain your reasoning

Explanation / Answer

1) centripetal force is not a force but a sum of other forces

only two forces are acting 7N tension and 2N gravity bothd own

so Edgar is right

2)

Now tension is up and gravity is down so Fiona is right

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