Expert: Ahmed Salami - 6/1/2004

Question
-------------------------
Followup To
Question -
i m preparing for written test of M.S

 i want to ask u some formualas or related topics
i hope that u will guide me .

1.A thin circular plate of radius units has a circular hole of radius of one unit at its center . what formula is used for calculating the mass of the plate if mass per unit area is  Ps=3r for    1<=r<=2.  

Answer
Hi,
I am assuming you mean that the mass per unit area is 3r.
The area of a circular plate of radius r without any hole in it is #r^2 where # is pi. Its mass would therefore be (#r^2)(3r) = 3#r^3
The area of the circular plate of radius one unit cut off is
#. Its mass is then (#)(3) = 3#
The area of a thin circular plate of radius  r units which has a circular hole of radius of one unit at its center is
3#r^3 - 3#
= 3#(r^3 - 1)

I hope this is what you need. If not , get back in touch and explain better.