# Homework Help: Length of a Steel Wire (Standing Waves)

1. Mar 16, 2015

### smashd

Question: A copper wire and steel wire with identical diameters are placed under identical tensions. The frequency of the third resonant mode for the copper wire is found to be the same as the frequency of the fourth resonant mode for the steel wire. If the length of the copper wire is 3.44 m and the density of steel is 7.85 g/cc, find the length of the steel wire.

Known Values: $~ ~ ~ ~ ~ L_c = 3.44 ~ m ~ ~ ~ ~ ~ , ~ ~ ~ ~ ~ ρ_s = 7.85 ~ g/cc$

Relevant Equations: $~ ~ ~ ~ ~ ƒ_n = \frac{v}{λ_n} ~ ~ ~ ~ ~ , ~ ~ ~ ~ ~ λ_n = \frac{2L}{n} ~ ~ ~ ~ ~ ~ , ~ ~ ~ ~ v = \sqrt{\frac{T}{μ}}$

Attempt at a Solution:

$$ƒ_{3-c} = ƒ_{4-s}$$
$$\frac{v_c}{λ_{3-c}} = \frac{v_s}{λ_{4-s}}$$
$$\frac{\sqrt{\frac{T}{μ_c}}}{λ_{3-c}} = \frac{\sqrt{\frac{T}{μ_s}}}{λ_{4-s}}$$
$$μ_c = \frac{m_c}{L_c} ~ ~ ~ , ~ ~ ~ ρ_c = \frac{m_c}{V_c} ~ ~ ~ ⇒ ~ ~ ~ m_c = ρ_c ~ V_c = ρ_c ~ π ~ r^2 ~ L_c$$
$$μ_c = ρ_c ~ π ~ r^2$$​
$$μ_c = \frac{m_s}{L_s} ~ ~ ~ , ~ ~ ~ ρ_s = \frac{m_s}{V_s} ~ ~ ~ ⇒ ~ ~ ~ m_s = ρ_s ~ V_s = ρ_s ~ π ~ r^2 ~ L_s$$
$$μ_s = ρ_s ~ π ~ r^2$$​
$$\frac{\sqrt{\frac{T}{ρ_c ~ π ~ r^2}}}{\frac{2}{3} ~ L_c} = \frac{\sqrt{\frac{T}{ρ_s ~ π ~ r^2}}}{\frac{1}{2} ~ L_s}$$
$$\frac{\frac{T}{ρ_c ~ π ~ r^2}}{\frac{4}{9} ~ L_{c}^2} = \frac{\frac{T}{ρ_s ~ π ~ r^2}}{\frac{1}{4} ~ L_{s}^2}$$
$$\frac{T}{\frac{4}{9} ~ ρ_c ~ π ~ r^2 ~ L_{c}^2} = \frac{T}{\frac{1}{4} ~ ρ_s ~ π ~ r^2 ~ L_{s}^2}$$
$$\frac{4}{9} ~ ρ_c ~ L_{c}^2 = \frac{1}{4} ~ ρ_s ~ L_{s}^2$$
$$∴ ~ L_{s} = \frac{4}{3} ~ L_{c} ~ \sqrt{\frac{ρ_c}{ρ_s}}$$

My problem is that I don't know what $ρ_c$ is. Any ideas how to solve this? Am I even going about this the right way?

Last edited: Mar 16, 2015
2. Mar 16, 2015

### smashd

Meh... The density of copper was something that's just supposed to be looked up. I thought I was supposed to solve for it another way. It's 8.96 g/cc.

Never mind, problem solved!