# Mechanical forces

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1. Oct 7, 2014

i'm not really a phycisist and to be really sincere i usually cannot answear my questions in class... so yeah, a question i have for a long time now is, wich force anulates the gravity preventing a rock tied on a rope spinning over my head to fall and just stay fixed in his horizontal axis, the centrifugal force and the centripetal are both horizontal in this case... what happens?

2. Oct 7, 2014

### Staff: Mentor

Tension in the rope. The rope is not horizontal.

3. Oct 7, 2014

what do you mean? (i'm really sorry if it is a dumb question.). The rope in this case is spinning with the rock in the horizontal axis over my head.

4. Oct 7, 2014

Like in the case of a olive being spin inside a cup turned upside down... why it don't fall?

5. Oct 8, 2014

### jbriggs444

A rock spinning in a horizontal plane is spinning around a vertical axis. I think that this is what you mean. In order to spin around that axis it must be subject to a horizontal net force. The force from gravity is downward. The force from the rope is toward the center and slightly upward. The rope is not exactly horizontal. The upward component of force from the rope and the downward force from gravity are equal in magnitude.

6. Oct 8, 2014

### Staff: Mentor

No, it isn't. Get a relatively lightweight rope and a decently massive weight. Have someone who is shorter than you spin it at a moderate speed, and look at it from the side. You will see that the rope is not horizontal.

7. Oct 8, 2014