Physics homework help

Question

AHHH I've been at this question for literally a DAY and I still can't figure it out. PLEASE HELP!!

A 0.119 kg meterstick is supported at its 33 cm mark by a string attached to the ceiling. A 0.695 kg mass hangs vertically from the 4.08 cm mark. A mass is attached somewhere on the meterstick to keep it horizontal and in both rotational and translational equilibrium. The force applied by the string attaching the meter stick to the ceiling is 21.9 N.

a) Find the value of the unknown mass. The acceleration of gravity is 9.81 m/s^2. Answer in units of kg.

b) Find the point where the mass attaches to the stick. Answer in units of cm.

Answer 1

Hi Chris F.,

The total pull down by all the masses (and the meterstick) must equal the pull of the suspension quoted. That allows you to calculate the unknown mass. That's the "non-rotational piece".

Additionally, the meterstick exerts a torque around its string attachment point (calcel a symmetrical piece and add up the remaining piece, it acts as if at its piecewise center of mass). You have the torque of the one mass (mass and position) and you know you must balance the torque, and you have the mass of the "other" mass. So all you have to do, is take the torque it has to have (which cancels the sum of the meterstick and the other mass's), and divide by its mass ????

-- Cheers, --Mr. d.