# Work in a circular motion

1. Oct 12, 2004

### DavidWi

Ok, right now in my college physics class, we're going over work in a circular motion. I see how gravity does the amount of work that the thing falls in a circular motion. However, my teacher told us that the tension of the string does no work. Gravity makes the ball go down, so gravity does taht work. What causes the work that makes the ball go to the side. the only work I can see in this example is gravity working down, but if that were true, then we wouldn't have the ball going left and right. Could anyone help me and tell me how the ball on the string can go left and right if the only work on it is gravity?

2. Oct 12, 2004

Work is only done when an object moves in the direction of the Force. In other words, if the string length remains constant (and taut) it can't do any work.

3. Oct 12, 2004

### kuenmao

The ball goes left and right due to the initial speed. Gravity keeps it in a circular orbit.
Suppose we have a spaceship. When the spaceship takes off, the rockets do work and push it into orbit, giving it an initial speed. Then, suppose that the spaceship throws out a satellite. The satellite just moves in uniform circular motion. Since gravity is always towards the centre of the Earth, and the spaceship always moves along it's circular orbit, gravity does no work.

Your teacher is referring to the part where the object experiences uniform circular motion and is no longer accelerating.

4. Oct 12, 2004

### omin

I assume you are holding the string in your hand and swinging the ball over your head so that the ball is traveling on the horizontal axis to normal human veiw and is making a circular path perpendicular to the center of the earth.

According to the physics book I'm reading, gravity isn't exerted horizontally upon the ball. It may only exert a force upon the ball vertically.

When ball is not in circular motion, it will hang on the end of the string according to the first law, at rest. To get the ball into circular motion, you must do work upon it. At a constant rotation speed in air there is friction. The ball will try to slow, so a constant force is necessary to keep it's circular speed constant, which means work is being applied to the ball being swung in air.

The force doesn't skip the string, going from the your hand strangely to the ball. Force is exerted through the string, therefore work is done on the ball.

But, I'm not to the tension chapter of physics. So, I don't know what is meant by tension. Maybe the 'tension' of the string does not do work. If it doesn't, then 'what' of the string does do work is what I would ask my instructor.

I'd be carefull to, sometime my book says to ignore other forces to isolate a specific thing. This explanation would change if air resistance is taken out of the picture.

Last edited: Oct 12, 2004