Young's Modulus and Hooke's Law

In summary, the conversation discusses the calculation of work done in stretching a wire using Hooke's Law. The work formula is W = (k∆L)∆L, where k represents the average force, and ∆L is the amount of stretching. The value of k can be calculated using the Young's Modulus equation, which is k = YA/L. Taking the average force into account, the final work equation becomes W = (YA/2L) x (∆L)^2. Calculus can also be used to calculate the work done.
  • #1
mlostrac
83
0

Homework Statement



A wire of length L, Young's modulus Y, and cross-sectional area A is stretched
elastically by an amount ∆L. The restoring force is given by Hooke's Law as
k∆L.

b. show that the work done in stretching the wire must be: W = (YA/2L) x (∆L)^2.

The Attempt at a Solution



Not sure what work equation I use and how to put it into the equation. I tried
W = F x ∆L but my answer ended up being W = (∆L)^2 x YA x L
 
Last edited:
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  • #2
The restoring force F is not constant over the stretched distance, it varies linearly from 0 to the maximum value of k(delta L). You can use calculus to calculate the work done, or just use the average force in your work formula. What's the value of k in terms of A, E, and L?
 
  • #3
1)From the Young's Modulus equation: ∆L = (1/Y) (F/A) x L = (1/Y) (k∆L)/A x L

So I isolated for k and get, k = YA/L

2) I then isolated for k from the work equation: W = (k∆L)∆L

...which gave me k = W/(∆L^2)

So when I make the 2 equations equal to each other, I see that I'm almost there, but where does the "2" in 2L come from? That's all I am missing
 
  • #4
You missed my tip: the work is not k(delta L)^2, because the force , k(delta x), is not constant. Use the average force.
 
  • #5
Hmmmm...I don't know why I can't think what the average force is? Is it F = k∆L/2 ? Stumped
 
  • #6
mlostrac said:
Hmmmm...I don't know why I can't think what the average force is? Is it F = k∆L/2 ? Stumped
Yes, if the force varies from 0 as it just starts to stretch, to k(deltaL) when it is stretched to deltaL, then the average force is the (sum of the min force plus maxforce)/2. Or you can use the calculus for the work done, the integral of F(dx), where F=kx.
 
  • #7
Awesome, it all worked out and I got the right answer. Thank you so much!
 

What is Young's Modulus?

Young's Modulus, also known as the elastic modulus, is a measure of the stiffness of a material. It is a constant that relates the amount of stress applied to a material to the resulting strain.

What is Hooke's Law?

Hooke's Law is a principle in physics that describes the relationship between the force applied to an elastic material and the resulting deformation. It states that the strain of a material is directly proportional to the stress applied to it, as long as the material remains within its elastic limit.

What is the formula for calculating Young's Modulus?

The formula for calculating Young's Modulus is E = σ / ε, where E is the Young's Modulus, σ is the stress applied to the material, and ε is the resulting strain.

What is the significance of Young's Modulus?

Young's Modulus is an important property of materials as it helps engineers and scientists understand the behavior of materials under different types and levels of stress. It is also used to determine the strength and stiffness of materials and is essential in the design and construction of structures.

What factors can affect the accuracy of Hooke's Law?

There are several factors that can affect the accuracy of Hooke's Law, including the material's elastic limit, temperature, and the presence of impurities or defects in the material. Additionally, Hooke's Law is only valid for materials that exhibit linear elasticity, meaning that the stress-strain relationship remains constant throughout the material's deformation.

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