# A mass whirled in a horizontal plane

1. Sep 9, 2007

### Vijay Bhatnagar

A mass is tied to a string and whirled in a horizontal plane. The tension in the string provides the centripetal force required for circular motion of the mass. But which force balances the weight mg of the mass?

2. Sep 9, 2007

### Staff: Mentor

Is the string perfectly horizontal?

3. Sep 9, 2007

### Vijay Bhatnagar

Yes, the string is perfectly horizontal. If it were not so, the vertical component of the tension would support the weight and the horizontal component provide the necessary centripetal force. However, when the string is horizontal there is no vertical component of the tension. As a result the mass will tend to fall down. Then, do we conclude that it is not possible to whirl the mass in a perfectly horizontal plane? But practically it does look possible to do so.

4. Sep 9, 2007

### Staff: Mentor

Realize that just because the mass travels in a perfectly horizontal plane, that doesn't mean that the string is horizontal. (In fact, it can't be!)

5. Sep 9, 2007

### Staff: Mentor

Get a friend to whirl an object around on a string over his head, and watch carefully. Start with the object orbiting rather slowly, then faster and faster. Does the string ever become perfectly horizontal?

6. Sep 9, 2007

### lugita15

It should not even be possible that the mass travels in a horizontal plane. It has a force mg downwards but no upward mg force.

7. Sep 9, 2007

### Staff: Mentor

It's easy to get the mass to travel in a horizontal plane. But the string can't be perfectly horizontal; if it were, there'd be no balancing upward force on the mass. (Perhaps you're saying the same thing?)

8. Sep 9, 2007

### Gokul43201

Staff Emeritus
The upward force comes from the tension in the string.

9. Sep 9, 2007

### lugita15

Yes, I am saying the same thing.

10. Sep 11, 2007

### Shooting Star

Suppose you have a massless string and a mass m tied in the middle, then would you able to stretch the string perfectly straight by pulling it from both sides? No, that would take infinite force, because there's no verically upward force to balance the weight mg. In real life, that's why wires and cables sag down in the middle.

Similarly, the whirling string has to make an angle, say 'x', with the vertical in the downward direction. It lies on a cone, whose semi-vertical angle is 'x'. If 'T' is the tension in the string, then mg=T*cos(x). The centipetal force is T*sin(x). The mass m travels in a horizontal plane, but not the string.