Center of Mass problem

[psruler]

Hi, here is the problem:

Imagine three spherical masses all fixed on the x-axis: mass 5M at the origin, mass 2M at x =+12.0 units, and mass M at x = -8.0 units, all connected by a rigid rod whose mass can be ignored. At what position along the x-axis is the center of mass of the system located?

is this the correct equation? if so, i don't know where to plug in the numbers.
x = (m1x1 + m2x2)/ m1+m2

Thanks!!

[cookiemonster]

Yes.

Take a guess. You got 2 sets of values in the given and two sets of x and m in your equation. What makes sense?

cookiemonster

[psruler]

i got 2units as the answer. is that correct?

[cookiemonster]

Oops.

You got 3 sets of values and 3 pairs of x and m. Your formula should be

\bar{x} = \frac{m_1x_1 + m_2x_2 + m_3x_3}{m_1 + m_2 + m_2}

Sorry. I didn't catch that third value the first time.

2 "units" is not the correct answer. You should be getting something in units of meters.

cookiemonster

[Chen]

2 "units" is not the correct answer. You should be getting something in units of meters.
Actually the answer is 2 "units". Where does it say we are working within the SI system, or any other unit system for that matter?

[cookiemonster]

I could have sworn I read "meters" in that post last night! I must have been tired. =\

cookiemonster