# Homework Help: Mechanics problem - centre of mass

1. Jan 18, 2009

### thehammer

1. The problem statement, all variables and given/known data
Three uniform rods, each of length L = 22cm, form an inverted U. The vertical rods each have a mass of 14g; the horizontal rod has a mass of 42g. What are (a) the x coordinate and (b) the y coordinate of the system's centre of mass?

2. Relevant equations

x(com) = 1/M $$\int$$ x dm
y(com) = 1/M $$\int$$ y dm

dm/M = dx/L
dm/M = dy/L

3. The attempt at a solution

I used the result of the integration done to find that the centre of mass of a uniform rod in terms of its length: x(com) = L/2.

x(com) = 0.22 m / 2 = 0.11 m

However, I've no idea about what the y coordinate of the centre of mass would be. I am completely confused and I cannot find any examples of such a situation in my book. I have a mechanics examination tomorrow and need to clear up any misunderstood concepts so I would greatly appreciate any help.

2. Jan 18, 2009

### LowlyPion

Try viewing it from the side.

Pretend you have a 42g object sitting on a see-saw that weighs 28g and is 22 cm long, and they have asked you to find the balance point with no weight at the other end.