# Homework Help: Two confusing questions

1. Aug 31, 2004

### rumaithya

Two confusing questions :(

Hello, I have two questions but i'm getting confused with them. Can someone help me please ?
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1) A structural I beam is made of steel. A view its cross-section and its dimensions are shown in: http://web.csuchico.edu/~ma260/physics/phys.jpg [Broken]
The density of the steel is 7.56 * 10^3 kg/m^3.

(a) What is the mass of a section 1.50 m long ?
(b) Assume that the atoms are predominantly iron, with atomic mass 55.9 u. How many atoms are in this section ?

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2) The picture in http://web.csuchico.edu/~ma260/physics/phys2.JPG [Broken] shows a frustrum of a cone. Of the following mensuration (gecometrical) expressions, which describes:
(a) The total circumference of the flat circular faces.
(b) The volume.
(c) The area of the curved surface ?
(i) pi(r1 + r2) [h^2 + (r1 - r2)^2]^1/2
(ii) 2pi(r1 + r2)
(iii) pi * h(r1^2 + r1r2 + r2^2)

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Thank you.

Last edited by a moderator: May 1, 2017
2. Aug 31, 2004

### Gokul43201

Staff Emeritus
What have you tried so far ?

1a) Before finding the mass what do you first need to find ? How would you do this ?
(b) What rule tells you about the number of atoms of a substance ?

Our policy is to help you once you show what you've tried or how you think you should go about it.

3. Aug 31, 2004

### rumaithya

hello, thanks for replaying.

1) (a) I know that o = m/v and the density is given but I couldn't find the volume from the diagram. thats a weird diagram. I dont see a measurement for depth either. All i know is to find the volume of each rectangle and add them. then solve for the mass.

(b) after getting the mass it's easy i know how to solve.

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2) I really have no idea about this question :(

4. Aug 31, 2004

### suffian

i think the trick for the second question is just to keep track of units. for the circumference you should end up with units of length (such as ft, m, cm, etc.), for the surface area lengthxlength, and volume lengthxlengthxlength.