Please Help Me with Physics?

Question

I would appreciate any helpful input on this physics problem. I am really confused on this one. Thanks for any help :)

A high-speed lifting mechanism suppports a 815 kg object with a steel cable 28.0 m long and 4.00 cm2 in cross-sectional area.

a) Determine the elongation of the cable (in mm).

b) By what additional amount does the cable increase in length if the object is accelerated upward at a rate of 2.7 m/s2 (in mm)?

c) What is the greatest mass that can be accelerated upward at 2.7 m/s2 if the stress in the cable is not to exceed the elastic limit of the cable, 2.2 x 10 to the 8th power Pa (in kg)?

Answer 1

The stress in the cable is m*g/A. The Young's modulus of steel is about 200*10^9 N/m^2 (pascal). Therefore the strain will be

m*g*/(A*200*10^9).

The elongation is strain*length.

For the second part, replace g above with (g+a), where a =2.7m/s^2

For the last, compute the mass from S = m*(g+a)/A, The elastic limit is Sm, so the max mass is m = Sm*A/(g+a)

Note: keep the units straight. Use kg, m, sec, N in all calcs then convert to the asked-for units last.

Answer 2

Sorry this isn't the answer, but it's always nice to know there's girls out there with a little bit of brains. Give me a year and I might be able to figure it out.