• Support PF! Buy your school textbooks, materials and every day products Here!

String with variable density

  • Thread starter xdrgnh
  • Start date
  • #1
413
0

Homework Statement


The linear mass density in a string is given by μ = μ0[1 + cos(x/R)] where R is a constant. If one averages this density over the large size L it becomes uniform: <μ> = μ0, where <…> means averaging. What is the minimum size L (in terms of R) such that the density can be considered uniform with an error less than 1% ?



Homework Equations





The Attempt at a Solution



So I intergrate with respect to dx over the range o to L then divide by L because I'm averaging and what I get is U+UR/L*sin(L/R). However this is my problem. The initial mass density makes no sense. When x=R*pi the density is zero. How can the density be zero on a freaking string. That makes no sense. Besides that I don't now what is meant by error. Should I equal the U+UR/L*sin(L/R) to .99U then solve?
 

Answers and Replies

  • #2
413
0
I appears in my title I messed up. I mean variable density.

[Moderator's note: thread title has been corrected by Redbelly98]
 
Last edited by a moderator:
  • #3
Redbelly98
Staff Emeritus
Science Advisor
Homework Helper
Insights Author
12,100
129

The Attempt at a Solution



So I intergrate with respect to dx over the range o to L then divide by L because I'm averaging and what I get is U+UR/L*sin(L/R). However this is my problem. The initial mass density makes no sense. When x=R*pi the density is zero. How can the density be zero on a freaking string. That makes no sense.

You're right that it can't be zero on a real string. Better to think of it as negligibly small compared to the average, and calling it zero is an approximation.

Besides that I don't now what is meant by error. Should I equal the U+UR/L*sin(L/R) to .99U then solve?
Yes. Ideally, it should be solved twice, using both 1.01U and 0.99U.
 
  • #4
20,133
4,208
Don't forget that the maximum and minimum values that sin(L/R) can take on are +1 and -1.

Chet
 

Related Threads on String with variable density

Replies
1
Views
97
Replies
3
Views
6K
Replies
1
Views
2K
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
4
Views
14K
Replies
2
Views
3K
  • Last Post
Replies
3
Views
1K
Replies
5
Views
4K
  • Poll
  • Last Post
Replies
19
Views
823
Top