# Linearly changing density

1. Mar 11, 2014

### rmfw

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

I need to find the Kinetic energy of a bar rotating about its center of mass.

I know the bar as length 3b and it's center of mass is located at 2b, the bar density changes linearly along it's length.

2. Relevant equations

T=1/2 W^2 I

3. The attempt at a solution

So I was trying to find I for this setup, which requires me to find the equation for density (λ).

I know the density changes linearly along it's length so it must be similar to an equation of the type: λ = k x , with k being a constant and x being position.

Now to find k I did the following equations:

∫λ dx = ∫ k x dx = m/2 (limits of integration are from 0 to 2b)

∫λ dx = ∫ k x dx = m/2 (limits of integration are from 2b to 3b)

The problem is that I get a false statement this way, making it impossible to find the equation for density.

This is very basic stuff but it's giving me a headache since I need to move forward on the problem but I can't due to this niche, help? thanks

2. Mar 11, 2014

### TSny

In general, it is not true that half the mass will be on one side of the CM and the other half on the other side of the CM.

See here, near the bottom of the page, for finding the CM of a continuous distribution.

3. Mar 11, 2014

### TSny

Note, "varying linearly" could be interpreted more generally as saying that $λ = a + kx$ where $a$ is some constant. But, you should be able to use the integral formula for $x_{cm}$ and the fact that $x_{cm} = 2b$ to show $a = 0$.

4. Mar 11, 2014

### Staff: Mentor

What is your general equation for the center of mass if the linear density λ(x) varies with with x? First state your equation for the total mass in terms of λ(x).

Chet