# Homework Help: This one has been annyoing me.

1. May 4, 2008

### SalivaKB

1. The problem statement, all variables and given/known data
Suppose you have 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 meter 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? Round down to the nearest metre, submit only a number (with no additional or extraneous information), and disregard any deformation of the bar or any other forces acting on the bar.

2. Relevant equations
P=m/V
Acircle= Pie (R)2

I seem to not remember the formula for this, such as the area for the regular pentagon. My son brought this up to me and I told him I will do my best but I hit a stump.