# Homework Help: Impossible(?) question

1. Feb 13, 2014

### Glemming

A bunch of bananas hangs from the end of a rope that passes over a light, frictionless pulley. A monkey of mass equal to the mass of bananas hangs from the other end of the rope. The monkey and the bananas are initially balanced and at rest. The rope is infinite, so the monkey can't just drag the bananas over the pulley. Is it possible for the monkey to somehow get to the bananas?

If the monkey climbs up with the speed v, the bananas will go up at the same speed v. So that's not an option.
If he climbs down, the same will happen.

If the monkey tries to swing back and forth, he will create a bigger tension in the rope, which will eventually drag the bananas higher and away from him.

Is it a solution to this problem at all?

2. Feb 13, 2014

### azizlwl

What if the monkey is accelerating upward.

3. Feb 13, 2014

### Glemming

If the monkey accelerates, it will create a bigger and bigger tension in the rope, which again will drag the bananas higher and higher.

4. Feb 13, 2014

Yes.If the bananas are in range of the hand

5. Feb 13, 2014

Wait,if the rope is infinite,How can a bunch of bananas hang from the end of the rope?

6. Feb 13, 2014

### Glemming

Haha. The length between the bananas and the pulley is infinite, as well as the length between the monkey and the pulley. This is not a practicalnproblem, but a theoretical one.

7. Feb 13, 2014

### PhysicoRaj

Are the monkey's hands infinitely long?

8. Feb 13, 2014

### BvU

Don't climb up. Climb down!

9. Feb 13, 2014

### PhysicoRaj

The monkey thinks.. "Bananas are sour.." and the monkey doesn't want to get them.

10. Feb 13, 2014

What will that yield?
since the rope is infinite,how long you climb down,the banana won't come down or up.The banana is at infinite distance from the pulley,monkey too!This question in OP does not make any sense

11. Feb 13, 2014

### phinds

NOTHING that the monkey does will have any effect on the rope that the bananas are attached to. Since whatever force is applied to the monkey's side of the rope can only propagate along the rope at the speed of sound in the rope, and since the rope is infinite, that force will never even reach the pulley, much less be translated back down to the bananas. SO, the monkey can just climb the rope until he reaches the bananas.

If you are going to apply magic to the terms of your problem (infinite rope), you have to live with the results.

12. Feb 13, 2014

### Staff: Mentor

Monkeys are alive. They can urinate, defecate, spit, pull out some hair and drop it... easy to change their weight a bit.

So I submit that the monkey takes a pee and subsequently begins to rise and the bananas to lower.

13. Feb 13, 2014

### nil1996

That's an interesting solution

14. Feb 13, 2014

### voko

The question is not specific enough. How far aside are the two ends of the rope? Are the bananas and the monkey at the same level? If not, which one is higher? How far away is the ground?

Can we accept that the speed of sound in the rope is infinite, to counter the remark by phinds?

15. Feb 13, 2014

### PhysicoRaj

Ha Ha Ha Ha Ha..:rofl:
But..
EDIT: voko has pointed out this..

16. Feb 13, 2014

### PhysicoRaj

Since the ropes are infinitely long, no matter what the size of the pulley is, the monkey and the bananas will be touching each other.. why? Because of mutual gravitation!

17. Feb 13, 2014

### Staff: Mentor

Yes, well I take it that question arises from a brainteaser scenario where certain realities are waved. So, light inextensible ropes, massless frictionless pulleys, infinitely long infinitely rigid materials are available, etc. Not a practical physics problem really.

18. Feb 13, 2014

### voko

Because the bananas and the monkey are said to hang, we have to assume there is an external field of gravity that is far stronger than the gravity between the bananas and the monkey.

19. Feb 13, 2014

:rofl::rofl:

I think the OP would need to elaborate it a little.OP,is this a question you found in a text book?

20. Feb 13, 2014

### PhysicoRaj

So only goodness knows if the monkey really gets those. Whatever be it, I have only one thing to say, "It's a fantastic homework question, probably given to solve in the vacations."

21. Feb 13, 2014

### Glemming

No, not in the textbook. The professor gave it to us as a brainteaser :)

22. Feb 13, 2014

### BvU

Well, that worked... all these monkeys had to scratch their heads on how to deal with $\infty$