# Volume of Object

1. Sep 7, 2016

### devo24

Hello,
I'm very new to physics & I've been stuck on this question, which seems pretty straight forward, for so long that I had to ask you guys :(.

- d, (the diameter of the hole) is 0.184 cm
- D (the diameter of the cylinder) is 0.916 cm
- h (the height of the hole) is 2.33 cm
- H (the height of the cylinder) is 13.6 cm

Using these numbers, find the volume of the object. Round to 3 significant figures.
___________________________

The formula for volume of a block = L * W * H
The formula for volume of a cylinder is = πr^2h

I'm confused as to how to use these given formulas. Thanks for taking the time to read, sorry to bother you guys about this.

2. Sep 7, 2016

### Staff: Mentor

What is the description of the object?

What have you tried so far?

Note: do not remove the homework template. Fill-in the required information, in particular the attempt at a solution.

3. Sep 7, 2016

### devo24

The object is a cylinder.

I tried to fill in the formulas but it makes no sense to me. In the formula L * W * H, I only plugged in 13.6cm for Height. I don't see any a choice for Length or Width which is throwing me off completely.

I apologize for removing homework template, won't happen again next time.

4. Sep 7, 2016

### Staff: Mentor

It can't be just a cylinder if there is also the description of a hole, no?

You wrote yourself that this is the equation for the volume of a block. Do you have a block?

5. Sep 7, 2016

### devo24

Yeah, there's a hole on top of the cylinder. Nope, there's no block. I'm just providing the formulas my teacher gave me to solve this problem. A cylinder with a hole on the top. The height & diameter of the hole is in my original post.

Forgot to mention she also gave us
density of an object = mass / volume

6. Sep 7, 2016

### Ray Vickson

Where do you see a block? I don't---I see two cylinders: a big one, and a smaller one (removed from the big one).

7. Sep 7, 2016

### devo24

I agree, there's no block. I'm going to use the formula v = πr^2h.

I'm getting π(don't know how to compute radius)^2(13.6). I gotta fill out the radius but I don't know how to compute it.

Am I on the right track?

8. Sep 7, 2016

### RUber

Do you know how diameter is related to radius?

9. Sep 7, 2016

### devo24

I don't :(. I'm very new to all of this.

Was I right about being on the right track on my previous post?

10. Sep 7, 2016

### RUber

Yes. It appears you have a solid cylinder that is missing a smaller cylinder from its center.
So to find the volume of the object, you should need to compute both volumes.
A radius is defined to be 1/2 the diameter of a circle.

11. Sep 7, 2016

### devo24

Oh ok.
For the holes, I'm gonna do π(.092)^2(2.33)

For the cylinder, I'm gonna do π(.458)^2(13.6)

If I'm correct, do I add the two products together to get the final answer?

Last edited: Sep 7, 2016
12. Sep 7, 2016

### Staff: Mentor

Does that make sense? If you remove part of a cylinder, does the resulting cylinder have a greater volume than the one without a hole in it?

13. Sep 7, 2016

### devo24

It doesn't make sense, you're right. I'm lost as hell, I've tried everything I could. I know this is a very simple question but I just can't seem to figure it out.

14. Sep 7, 2016

### Staff: Mentor

If you remove some of the volume of the larger cylinder, will the volume of what remains be a) larger, b) smaller, c) the same?

15. Sep 8, 2016

### RUber

@devo24 Hopefully after a night of rest this makes sense. Draw a picture and imagine what you are actually calculating. The volume for the larger cylinder is for a solid cylinder. The smaller volume is the volume of empty space within that larger cylinder (which is not part of the total volume of the solid).
Your equations are correct. You have done all the work, now put it together.

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