# Homework Help: Waves: finding the tension of a cord

1. Oct 24, 2009

### nowtrams

1. A telephone cord is 3.95 m long and has a mass of 0.180 kg. A transverse pulse is produced by plucking one end of the taut cord. The pulse makes four trips down and back along the cord in 0.790 s. What is the tension in the cord?

d=3.95m, m=0.18kg, 4t=0.79s

2.
v=(T/u)^0.5 --- v2=T/u --- T=v2*u

v=d/t

u=m/d

3.
4t = 0.79s
t = 0.79s/4 = 0.1975s

v = d/t = 3.95m/0.1975s = 20.0m/s

u = m/d = 0.18kg/3.95m = 0.0455696kg/m

T = v2*u = (20.0m/s)2*0.04557kg/m = 400m2/s2*0.04557kg/m = 18.2278481kg*m/s2
T = 18.2278N

this is from an electronic assignment and when I submitted this answer it said my answer "differs from the correct answer by 10% to 100%."
also, when I first attempted this problem I accidentally divided v2 by u instead of multiplying (which gave me 8777.778) and the response I got for that was that my answer differed from the correct answer by "orders of magnitude," which to me sounds like that answer was off by some multiple of 10.

2. Oct 25, 2009

### rl.bhat

Hi nowtrams, welcome to PF.
Four trips down and back means 4d or 8d?

3. Oct 26, 2009

### nowtrams

that's a good question, that hadn't occurred to me. I'll give that a try

4. Oct 26, 2009

### nowtrams

That was all my problem was, thanks!