Question

The mobile in the figure (Figure 1) is in equlibrium. Object B has mass of 0.605kg.

Determine the masses of object A, C, and D (neglect the weights of the crossbars)

Answer 1

For equilibrium of the crossbar to which C and D are attached,
mD * L5 = mC * L6
mD * 17.50 = mC * 5
mC = 3.50 * mD ----------(1)

For equilibrium of the crossbar to which B is attached,
(mC + mD) * L3 = mB * L4
(mC + mD) * 15 = 0.735 * 5
mC + mD = 0.245 -----------------(2)

From equations (1) and (2),
3.50 mD + mD = 0.245
4.50 mD = 0.245
mD = 0.245/4.50 = 0.0544 kg
From equations (1), mC = 3.50 * 0.0544 = 0.191 kg

For the equilibrium of the crossbar to which A is attached,
mA * L1 = (mB + mC + mD) * L2
mA * 30 = (0.735 + 0.191 + 0.0544) * 7.50
mA = 0.245 kg

Ans: mass of A = 0.245 kg, mass of C = 0.191 kg, mass of D = 0.0544 kg