# Homework Help: Center of Mass. Confused on part b.

1. Oct 26, 2008

### Quickster357

In Fig. 9-39, three uniform thin rods, each of length L = 49 cm, form an inverted U. The vertical rods each have a mass of 14 g; the horizontal rod has a mass of 33 g. What are (a) the x coordinate and (b) the y coordinate of the system's center of mass? (Give your answer in cm)

http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c09/fig09_37.gif
Fig. 9-39
Problem 4.

a) 24.5cm

b) ? cm

(m1*x1+m2*x2+m3*x3)/(m1+m2+m3)

(33g*49cm+14g*0cm+14g*0cm)/(33g+14g+14g)

(33*49)/(33+14+14)=1617gcm/61g=26.5082cm <--incorrect

any help towards the correct answer will be extremely appreciated. thanks!

2. Oct 26, 2008

### cepheid

Staff Emeritus
Why are you considering all of the mass of the vertical rods to be concentrated on the ground (0 cm) when, in fact, their individual centres of mass are actually located in their centres (i.e. 49/2 cm above the ground)?

3. Oct 26, 2008

### Quickster357

because i never considered that when i should've, but i just did the adjustment for (24.5*14*2+33*49)/61 and got 37.7541 and it is still wrong unfortunately.

4. Oct 27, 2008

### cepheid

Staff Emeritus
What is the correct answer, and how do you know that it is correct?