# Homework Help: Qn on ratational motion.

1. Oct 16, 2005

### Wen

a thin rod is fixed on one end and rotate in a vertical motion. Its period of oscillation is pi s .
It is now held in the horizontal position and let go. An ant happen to cling in to the rod at distance 1 m from the pivot. Find the contact forcce the rod exert on the ants when it swing to the lowest position. Express the mag. of the force in terms of the ant's body weight.

When it reaches the equilibrium position, the aceleration is 0. hence the net force acting on ant in the horizontal direction should be 0. Only friction and the weight of the ant act on it . Is this way of doing correct , if not , please explain to me how?

2. Oct 16, 2005

### andrevdh

Wen,
1. Is the rod rotating like the spoke of a bicycle wheel while the wheel is held upright?
2. Have you done simple harmonic motion?

3. Oct 16, 2005

### Wen

yes to both .

4. Oct 16, 2005

### andrevdh

What forces are the ant experiencing at the bottom and in which directions are they?

5. Oct 16, 2005

### Wen

the friction and its weight because the aceleration of both the rod and ant is 0

6. Oct 16, 2005

### andrevdh

Forget about the rod, just think about the ant. The projected motion of the ant on the x-axis gives a zero acceleration for the ant in the x-direction, but we also need to consider the projection of the ant on the y-axis in order to describe it's motion fully, that is it also has an acceleration in the y-direction as it revolves around the pivot. In what direction will the friction force (called the contact force in your problem) be?

Last edited: Nov 29, 2006