Torque and center of mass question

Join the discussion
Registration is free. Ask a follow-up in this thread, or start your own.
4 replies · 3K views
ktpr2
Messages
189
Reaction score
0
We have a block of un-uniform density of mass m suspended by two massless wires, against gravity. The left wire makes an angle phi_1while the right angle makes an angle phi_2. The block has length L. They want me to find the center of mass.
THis is what I've done:
We have tension in the wires. Torque is force measured from a distance. In our case, the two wires create a torque about the center of mass. I find the forces,
F_1 = sin(phi_1)*m*g
F_2 = sin(phi_2)*m*g
And realize that the torques are
t_1 = F_1*x_c_m;
t_2 = F_2*(L - x_c_m)
since they equal another (net torque is zero), i set them t_1 = t_2 and solve for x. However the answer is incorrect and I am told to check my trignometry. My answer is
x = (L sin(phi_2)) / (sin(phi_1)+sin(phi_2))

whatever am i doing wrong?
 
Physics news on Phys.org
Where do you put your fulcrum?
 
my fulcrum is x_cm, or the center of mass of the block from the left side.
 
One question... are the wires attached to the ends of the block, you were not clear on that...
 
yes they are. the wire making an angle phi_1 is on the left side and the wire making an angle phi_2 is on the right side.