Lift Cable

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Frequently Asked Questions...

Physics homework on extention of a cable?

A large crane has a steel lifting cable of dimeter 36mm. The steel used has a Young modulus of 200 GPa. When the crane is used to lift 20kN, the unstretched cable length is 25.0 m. calculate the entension of the cable.

Please explain how you worked it out :)

Thank you all very much and you've really helped me a lot.

Best Answer...


The definition of strain:
epsilon = deltaL/L0

solved for deltaL (our objective):
deltaL = L0*epsilon

Definition of Young's Modulus:
E = sigma/epsilon

Update formula for deltaL
deltaL = L0*sigma/E

Definition of stress:
sigma = F/A_xsect

Cross sectional area of a circular cable:
A_xsect = pi*d^2/4

update formula for deltaL:
deltaL = 4*L0*F/(E*pi*d^2)

All information is within the RHS of this concluding equation. Convert all units to meters, regular Newtons, and regular pascals to plug in and get final answer.