# Homework Help: Volume of a cylinder

1. Nov 1, 2013

### Eagle's Wings

1. The problem statement, all variables and given/known data

a hollow cylindrical copper pipe is 1.5m long and has an outside diameter of 35cm and an inside diameter of 2.5 cm how much does it weight?

2. Relevant equations

ρ = M/V = 8.9E3
w = Mg

3. The attempt at a solution

okay, so I know that I can just solve for M because I already have ρ but how do I find volume? do I just do πr^2h but what do I do with the inner and outer diameters?

2. Nov 1, 2013

### Staff: Mentor

Are you sure about the dimensions of your cylinder? As you gave them, you have a nearly solid chunk of copper with a narrow hole in the middle.

The "hole" is also a cylinder, so you can find its volume and subtract that from the larger volume.

3. Nov 4, 2013

### contrivance

Assuming that OP meant 3.5cm, and 2.5cm respectively;

Use your equation for the Volume of Cylinder:
$$V = \pi r^2 h$$
So like Mark said - find the volume of the cylinder and subtract the volume of the inner 'cylinder' and then you will have the V to find your weight with. (converting cm to m):

$$V_{outer} = \pi (0.035m)^2 (1.5m)$$
$$V_{inner} = \pi (0.025m)^2 (1.5m)$$
$$V_{outer} - V_{inner} = V_{pipe}$$

(in cubic meters)

Now you can find the Mass.