Young's modulus of spider thread

In summary, the spider's thread has a fractional increase in length of 1.799x10^-^3 due to the spider's weight. If a person of 76kg hangs vertically from the same nylon thread, the radius of the thread must have a fractional increase of 1.799x10^-^3 to match.
  • #1
faoltaem
31
0

Homework Statement



Q 1. A spider with a mass of 0.26 g hangs vertically by one of its threads. The thread has a Young’s modulus of 4.7 x 10[tex]^9[/tex] N/m2 and a radius of 9.8 x 10[tex]^- ^6[/tex] m.
a) What is the fractional increase in the thread’s length caused by the spider?
b) Suppose a 76 kg person hangs vertically from a nylon rope (Young ’s modulus of 3.7 x 10[tex]^9[/tex] N/m2). What radius must the rope have if its fractional increase in length is to be the same as that of the spider’s thread?

Homework Equations



[tex]F = Y (\frac{A_o}{L_o}) \Delta L [/tex]

The Attempt at a Solution



This is the only equation that i know for this. I realize that the area that i need to solve this problem using this equation is the cross sectional area so, is it possible to work this out with this information as i don't have the cross sectional area and i can't use [tex]A = \pi r^2[/tex] because the radius that I've been given is for the length of the string itself.
 
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  • #2
faoltaem said:

Homework Statement



Q 1. A spider with a mass of 0.26 g hangs vertically by one of its threads. The thread has a Young’s modulus of 4.7 x 10[tex]^9[/tex] N/m2 and a radius of 9.8 x 10[tex]^- ^6[/tex] m.
a) What is the fractional increase in the thread’s length caused by the spider?
b) Suppose a 76 kg person hangs vertically from a nylon rope (Young ’s modulus of 3.7 x 10[tex]^9[/tex] N/m2). What radius must the rope have if its fractional increase in length is to be the same as that of the spider’s thread?

Homework Equations



[tex]F = Y (\frac{A_o}{L_o}) \Delta L [/tex]

The Attempt at a Solution



This is the only equation that i know for this. I realize that the area that i need to solve this problem using this equation is the cross sectional area so, is it possible to work this out with this information as i don't have the cross sectional area and i can't use [tex]A = \pi r^2[/tex] because the radius that I've been given is for the length of the string itself.
The spider is hanging vertically by a single thread. The radiius of the thread is given, and from this, you can determine the cross sectional area and solve for the length change using the equation you have correctly noted. I think you might be assuming that the given radius is the radius of the web? This is not the case.
 
  • #3
Why can't you use the radius you are given?
Young's modulus is just stress/strain.
You know the stress is just force (weight of spider ) / area and you are solving for strain.
Just use the initial diameter of the thread.
 
  • #4
Yeah, i was taking the radius as the radius of the web.
so I've done this:
nb: s=spider p=person

a) What is the fractional increase in the thread’s length caused by the spider?
[tex]Y_s[/tex] = 4.7x10[tex]^9[/tex] N/m[tex]^2[/tex]
m = 0.26g = 2.6x10[tex]^-^4[/tex] kg
[tex]r_s[/tex] = 9.8x10[tex]^-^6[/tex] m

[tex]F = Y(\frac{A_o}{L_o})\Delta L[/tex] => [tex]mg = Y_s(\frac{\pi r_s^2}{L_o})\Delta L[/tex] =>[tex]\frac{\Delta L}{L_o} = \frac{mg}{Y_s \pi r_s^2}[/tex]

[tex]\frac{\Delta L}{L_o} = \frac{2.6\times10^4 \times 9.81}{4.7\times10^9 \times \pi \times (9.8\times10^-^6)^2}[/tex]
[tex]\Delta L = 1.799\times10^-^3 Lo[/tex]

b) Suppose a 76 kg person hangs vertically from a nylon rope (Young ’s modulus of 3.7 x 10 N/m2). What radius must the rope have if its fractional increase in length is to be the same as that of the spider’s thread?
[tex]Y_s[/tex] = 3.7x10[tex]^9[/tex] N/m[tex]^2[/tex]
m = 76kg
[tex]\Delta L_p = \Delta L_s = 1.799\times^-^3 L_o[/tex]

assume: [tex]Lo_p = Lo_s[/tex]

[tex]r^2 = \frac{mgL_o}{Y_p \Delta L \pi}[/tex]

[tex]r^2 = \frac{76\times9.81\times L_o}{3.7\times10^9 \times 1.799\times10^-^3 \times L_o \times \pi}[/tex] = 3.567x10[tex]^-^5[/tex]

r = [tex]\sqrt{3.457\times10^-^5}[/tex] = 5.97x10[tex]^-^3[/tex] m



Is this correct? Is it possible to find [tex]L_o[/tex] in part (a)? And is it right to assume that [tex]Lo_p = Lo_s[/tex]
 
  • #5
faoltaem said:
Is this correct? Is it possible to find [tex]L_o[/tex] in part (a)? And is it right to assume that [tex]Lo_p = Lo_s[/tex]
I didn't check all your numbers, but your solution method and approach looks good to me. You are asked to find a fractional or percentage increase in length, which is the 'strain' value that mgb_phys alluded to above. You cannot find the Lo value without additional data, but it makes no difference what it is when you are just looking at fractional increases. Also, you need not assume that Lo_p = Lo_s, the result for the fractional increase (strain) is the same regardless of length.
 
  • #6
thanks for all your help
 

What is Young's modulus of spider thread?

The Young's modulus of spider thread, also known as the tensile modulus, is a measure of the stiffness or elasticity of a material. It is the ratio of stress (force per unit area) to strain (change in length per unit length) when a material is stretched or compressed.

Why is the Young's modulus of spider thread important?

The Young's modulus of spider thread is important because it gives us insight into the strength and flexibility of spider silk. This information can be useful in various applications, such as developing new materials for medicine, textiles, or engineering.

How does the Young's modulus of spider thread compare to other materials?

The Young's modulus of spider thread is incredibly high, with some species of spiders having a modulus that is stronger than steel. It is also more elastic than most synthetic materials, making it a unique and valuable material to study and potentially mimic in engineering designs.

What factors affect the Young's modulus of spider thread?

The Young's modulus of spider thread can be affected by a variety of factors, including the species of spider, the environment in which the spider lives, and the properties of the silk itself, such as its composition and structure.

How is the Young's modulus of spider thread measured?

The Young's modulus of spider thread is typically measured using a specialized instrument called a tensile testing machine. This machine pulls on a sample of the spider silk until it breaks, while measuring the force and elongation of the material. The Young's modulus can then be calculated using this data.

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