Question

A ball with a weight of 2 N is attached to the end of a cord of length 2m . The ball is whirl in a vertical circle counterclockwise. The tension in the cord at the top of the circle is 7N, and at the bottom it is 15 N. (The speed of the ball is not the same in all 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: _____

Answer 1

Answer:

we agree with

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.

Explanation:

Weight of the ball is given as

$W = 2N$

so we have

$m = \frac{W}{g}$

$m = 0.204 kg$

now tension force at the top is given as

$T_{top} = 7 N$

$T_{bottom} = 15 N$

Now at the top position by force equation we can say that ball will have two downwards forces

1) Tension force

2) Weight of the ball

so net force on the ball is given as

$F_{net} = T + W$

$F_{net} = 7 + 2 = 9 N$

So we agree with

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.