The Math Forum

Ask Dr. Math - Questions and Answers from our Archives
Associated Topics || Dr. Math Home || Search Dr. Math

Massless Cable

Date: 05/06/2003 at 17:04:53
From: Jeremy
Subject: Massless cable.

Why is it that cables are always considered to be massless in a 

My guess is that a cable can support weights around a wheel at both 
ends, so the mass of the cable is negligible or not important.

Date: 05/07/2003 at 11:47:34
From: Doctor Dotty
Subject: Re: Massless cable.

Hi Jeremy,

Thanks for the question.

When applying maths to the real world, we can often make our 
calculations easier by simplifying our description of the 
situation.  For that reason, mathematicians and physicists 
often ignore such factors as air resistance, the size of the 
tip of a fulcrum, the gravitational effect of Pluto, etc.

Let's say that you have a van being pulled by a rope:

    500 N             _ _ _ _  
     /_______________| 5 tons \_
     \               |_ _ _ _ _ _|
                       O     O

The 5 ton m ass is probably rounded to the nearest ton. 
The 500 Newton force is probably rounded to the nearest 
50 Newtons. We would take the acceleration due to gravity 
to be 9.8m/s^2, which is rounded to one decimal place. 

Let's say the van is about to move.

If you had to work out, for example, the coefficient of friction 
between the van and the ground, the answer would only be correct to 
a couple of decimal places because of this. The mass of the rope is 
so small when compared to the 5-ton van, that it would not affect 
this rounded answer at all.

Does that make sense? 

If I can help any more with this problem or any other, please write 

- Doctor Dotty, The Math Forum 
Associated Topics:
High School Physics/Chemistry

Search the Dr. Math Library:

Find items containing (put spaces between keywords):
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

Math Forum Home || Math Library || Quick Reference || Math Forum Search

Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.