# Homework Help: Monkey physics

1. Mar 14, 2005

### UrbanXrisis

The monkey climbs the rope trying to get the banana seen http://home.earthlink.net/~urban-xrisis/clip002.jpg [Broken]

I need to find the net torque and the total angular momentum about the pully axis.

I am absolutly stumped but I'll give it a shot. So the monkey is moving upwards that means it has to sustain and move its own body. That means it has to apply a force of Mg to hold onto the rope and Mg to move up the rope. The total force is 2mg. Total torque would be the force of the monkey (2Mgr) plus the banana (Mg) which equals 3Mgr right?

For the angular momentum, that is the integral of torque. Does that mean angular momentum is $$\frac{3Mgr^2}{2}$$

any help?

Last edited by a moderator: May 1, 2017
2. Mar 14, 2005

### MathStudent

from the description in the picture, it seems as if the monkey is just clinging, and not actually moving with respect to the rope. So this means that its just the monkeys weight that adds to the tension of the rope.

Torque is a vector quantity, so it matters which direction the torque tries to rotate the pulley. By convention, a torque that tries to rotate an object clockwise about a pivot is considered negative, (just like a clockwise angle rotation is negative), and converseley a torque that tries to rotate the object counter-clockwise is positive.

3. Mar 14, 2005

### Staff: Mentor

This is incorrect.

If the monkey climbs at a constant speed, what must be the tension in the rope?

Note that you are asked to find the net torque about the pulley. Hint: Assume the tension is the same throughout the rope.

4. Mar 14, 2005

### UrbanXrisis

there is Mgr and -Mgr so that means that the net torque is zero?

5. Mar 14, 2005

### Staff: Mentor

That is correct.

6. Mar 14, 2005

### UrbanXrisis

then that means there is no angular momentum right?

7. Mar 14, 2005

### Staff: Mentor

No torque means no change in angular momentum. But, assuming things start from rest, then the net angular momentum will remain zero.

8. Mar 14, 2005

### UrbanXrisis

so the monkey can climb up the rope without the pully rotating because of opposite torques makes the net torque zero and therefore not change in angular velocity. Then that means the monkey can reach the bananas right?

9. Mar 14, 2005

### UrbanXrisis

10. Mar 15, 2005

### Staff: Mentor

No. The rotation of the pulley, which is assumed to be very light, will not affect the angular momentum of the system. But the monkey climbing the rope surely will. If the monkey raises himself at speed v, what must happen to the bananas so that the net angular momentum of the system remains unchanged?

11. Mar 15, 2005

### Staff: Mentor

No. Any additional force that the monkey exerts is transmitted, via the light rope, to the bananas. As I said before, the tension in the rope is the same throughout.