# Thread: Physics Question. I really need help with this!

1. ## Physics Question. I really need help with this!

There is a very long steel bar with a regular pentagonal cross-section where each side of the pentagon is 2 cm in length. This bar will be suspended vertically over a very deep chasm. At the bottom end of the steel bar, there is a spherical steel weight with a diameter of 50 cm. Going up the bar, at every 3 metre increment, there is a spherical steel weight with a diameter of 30cm. If the density of the steel is exactly 7.8 g/cm3, the steel can withstand a tensile stress of 400 MPa, and gravity is 9.81m/s2, what is the maximum possible length, in metres, that the bar can extend without breaking?

Can anyone help me out?

2. Originally Posted by silvernode
There is a very long steel bar with a regular pentagonal cross-section where each side of the pentagon is 2 cm in length. This bar will be suspended vertically over a very deep chasm. At the bottom end of the steel bar, there is a spherical steel weight with a diameter of 50 cm. Going up the bar, at every 3 metre increment, there is a spherical steel weight with a diameter of 30cm. If the density of the steel is exactly 7.8 g/cm3, the steel can withstand a tensile stress of 400 MPa, and gravity is 9.81m/s2, what is the maximum possible length, in metres, that the bar can extend without breaking?

Can anyone help me out?
Didn't you post this under another username here? If so, please don't double post.

Anyway, I'm not going to type it all out again. I have answered this on PHF. (For those of you who are not silvernode, it's under "Urgent Homework Help.")

-Dan