Help with physics problem

Homework giving you a headache? Math gives you a migraine? Can't quite figure out how to do something in photoshop? Never fear, the other members of CAA share their expertise in this forum.

Help with physics problem

Postby Nate » Thu Sep 22, 2005 2:02 pm

A 65-kg petty thief wants to escape from a third-story jail window. Unfortunately, a makeshift rope made of sheets tied together can support a mass of only 57 kg. How might the thief use this "rope" to escape? Give a quantitative answer.

Okay, I'm COMPLETELY lost on this one. I have the following theories:

1. Eat less and work out until he loses 8 kg.
2. Use the rope to strangle the guards.
3. Trade the rope to a lighter thief for a stronger one that he can use.

Unfortunately, none of these have a quantitative answer. I'm THINKING this has something to do with force, since that's the chapter this problem is in, but I'm lost on how to go about this. Any help?
Image

Ezekiel 23:20
User avatar
Nate
 
Posts: 10725
Joined: Thu Sep 02, 2004 12:00 pm
Location: Oh right, like anyone actually cares.

Postby Technomancer » Thu Sep 22, 2005 2:27 pm

It's a bit of a vague question, because it doesn't tell you how much material you have to work with, or what some of the necessary dimesions are. However, if the thief had two equivalent ropes, he could anchor them to two separate points on the wall/window or whatever. Given a 65 kg weight, you will need to find the necessary angle between the ropes so that no one rope supports more than 57 kg (you should really be working in Newtons here not kg!). In other words imagine a 'V' shape, where the two ends are anchored and the thief is at the bottom of the 'V'.
The scientific method," Thomas Henry Huxley once wrote, "is nothing but the normal working of the human mind." That is to say, when the mind is working; that is to say further, when it is engaged in corrrecting its mistakes. Taking this point of view, we may conclude that science is not physics, biology, or chemistry—is not even a "subject"—but a moral imperative drawn from a larger narrative whose purpose is to give perspective, balance, and humility to learning.

Neil Postman
(The End of Education)

Anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that my ignorance is just as good as your knowledge

Isaac Aasimov
User avatar
Technomancer
 
Posts: 2379
Joined: Fri Jun 13, 2003 11:47 am
Location: Tralfamadore

Postby SP1 » Thu Sep 22, 2005 2:53 pm

Slide down the rope, but don't stop. The thief can supply 57 kg x 9.8 m/ss or about 559 N of "braking" force to counteract his weight of 65 x 9.8 = 637 N. The result is a net 78 N of force or 1.2 m/ss on his 65 kg body. I suspect that his velocity when he hits the groung will be acceptable.

Or, teleport the whole prison to the moon, where he weighs less, but the rope has the same breaking strength. Or, double up the rope (use two lengths, but that is a lot of sheets). :thumb:
User avatar
SP1
 
Posts: 861
Joined: Wed Aug 17, 2005 6:30 am
Location: Kentucky


Return to Tutorials

Who is online

Users browsing this forum: No registered users and 160 guests