# Tennis ball Centripetal force question

1. Oct 18, 2004

### saltrock

A tennis ball connected to a long piece of string is swang around in a horizontal circle above the head of the pupil.
the pupil feels that there is a tension in the string and argues that for equilibrium there must be an outward "centrifugal force acting on the ball.critisize this arguement and explain why there is a tension in the string?

2. Oct 18, 2004

### saltrock

>when the ball accelerates in a circle,the mass tries to act inwards,as a result there is a tension in the string
>Also there is a reaction force acting outside(I dunno where the reaction force comes from).
>Due to the difference between the two forces there is a net centripetal force acting inside.
>It is due to this inward pulling cebtripetal force the ball is in equilibrium,not that it is due to centrifugal foce.

THIS IS WHAT I THINK IS RIGHT BUT I DUNNO WHAT I NEED TO WRITE SO AS TO GET 5 MARKS.CAN YOU PLEASE HELP ME ANSWER THIS QUESTION.THANKS IN ADVANCE TO THOSE WHO CAN HELP ME OUT.

3. Oct 18, 2004

### ponjavic

I just learned about the centripetal force a few days ago but I would not say that there is a force acting outwards. Naturally since the ball is stuck to the pupil with the string it has to continue moving in a circular motion. In order for the ball to maintain it's "speed" while changing its angle the net force has to be directed towards the center. Since the gravitational force is acting downwards this force can't be sufficient for this net force all the time. Therefore it has to be the string that pulls the ball constantly. Any thoughts?

4. Oct 18, 2004

### Pseudo Statistic

The Tension should be due to centripetal force...

5. Oct 18, 2004

### Staff: Mentor

Just about every statement you made is incorrect. The most important thing to realize is that the ball is not in equilibrium! It is accelerating towards the center of the circle. So there must be a net force on the ball towards the center. Gravity acts down, so something else must be contributing a force on the ball. The string, of course. It's the tension in the string (the horizontal component) that pulls the ball inward, keeping it going around in a circle.

6. Oct 19, 2004

### saltrock

Where does the tension in the string come from?

7. Oct 19, 2004

### ponjavic

When the ball is moving in a circular motion the string pulls it towards the center, this is the centripetal force. In order for the ball to change its vector (changing angle/speed, this case angle) it has to be affected by a force, in this case centripetal therefore inwards from the string. The string wont release the ball so it pulls it inwards.
Also there cannot be an equilibrium in that case both the speed and angle of the ball's velocity vector would remain constant thus it would not move circular.

Is that helping at all? :P

8. Oct 19, 2004

### Staff: Mentor

The ball "wants" to continue in a straight line motion, not go in a circle. But, since the string is attached, it cannot. The string pulls on the ball, forcing it to go in a circle. If the ball is going too fast, or is too massive, then the string will not be able to create enough tension before it snaps. But as long as the string is strong enough, and you are able to grip it tight enough, the ball will be forced into a circular path.

The tension in the string is its response to being pulled at both ends.

9. Oct 19, 2004

### spacetime

circular motion and equillibrium

An object is not in equillibrium if it is in circular motion. That is, it is accelerating. That is, its velocity is changing, which is true since it is always changing directions.

All this means there's a force acting on it, making it move in a circle. And that is the tension. That is the explanation for tension.

spacetime
www.geocities.com/physics_all/index.html