# Homework Help: Center of Mass problem

1. Apr 7, 2004

### 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!!

2. Apr 7, 2004

### 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

3. Apr 7, 2004

### psruler

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

4. Apr 8, 2004

### 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

5. Apr 8, 2004

### Chen

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?

6. Apr 8, 2004

### cookiemonster

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

cookiemonster

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