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## tfguss 3 years ago I'm not sure where to start. The drawing shows two crates that are connected by a steel wire that passes over a pulley. The unstretched length of the wire is 2.0 m, and its cross-sectional area is 1.6 *10^-5 [m^2]. The pulley is frictionless and massless. When the crates are accelerating, determine the change in length of the wire. Ignore the mass of the wire.

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1. tfguss

Attached is the image. Please help!

2. hosein

your goal is tension ?

3. tfguss

Yes

4. tfguss

Young's modulus of steel is 200gPa I think- how do I factor in the weight? I tried the usual deformation formula using 9.81*(m1+m2) as the force in newtons, but this does not yield the answer. I'm confused.

5. banditelol

what is the cross sectional area means?

6. tfguss

That is the area across the end of the wire, aka you cut the wire in half and its the area of the circle at the end of the wire.

7. banditelol

oooo.. I think you should just search the tension by using the (sigma)F=ma for both crates, and just use the modulus young equation to search for the change of length

8. banditelol

|dw:1329064147162:dw| in which, Y=modulus young F= the tension(in this case I think 2 times the rope's tension) A = cross sectional area l = the original length (delta) l = the change of length

9. hosein

do you know relation of atwood acceleration & tesion ? use from that

10. hosein

\[T=2m _{1}m _{2}g/m _{1}+m _{2}\] for more see this: http://en.wikipedia.org/wiki/Atwood_machine

11. hosein

did you get it?

12. tfguss

Yes! Thank you!

13. hosein

your wellcome

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