Why Is My Calculated Torque Different from the Textbook's Answer?

[bigsaucy]

Hello all, I've been wrestling with this question for awhile, please help!

There is a 2.0kg, uniform, horizontal rod measuring a total of 100cm. Toward the left end of the rod there is a point 25cm from the left end and 75cm from the right end. The question is to find the gravitation torque about the point.

I used the formula T = rF and got 14.7 N . m

the book says the answer is -4.9 N . m

im so confused, please help!

 

[da_nang]

You need to divide the rod into separate masses and calculate the sum of their torques.

 

[bigsaucy]

hello

i considered separating the masses but i didnt know of any feasible means of doing so

 

[gneill]

hello

i considered separating the masses but i didnt know of any feasible means of doing so

You mean you can't determine 1/4 and 3/4 of 2.0kg?

A more expedient route to the torque when the rod is of uniform density is to simply use the distance of the center of mass of the rod (as a whole) from the point about which the torque is to be found.

 

[bigsaucy]

i'm so terribly confused =[

 

[gneill]

Draw a diagram of the rod. Pencil in the rod's center of mass. Pencil in the location of the pivot point (point about which you are trying to find the torque due to gravity). What's the distance between the two points?