An overhead crane is lifting a crate of weight of 500 lb, as sketched in Figure P6-40a. The total weight of the crane cabin and the rigid horizontal beam is 2000 lb. Note that gravity acts on the system and that there are four identical columns which sup- port equal load. The cross-section of each column is square with dimensions shown in Figure P6-40b. As- sume in all cases that the horizontal beam and all four columns remain in static equilibrium. -20 ft. 20 ft. Cabin Beam 12 in. 20 ft. 500 Ib (a) (b) (a) Find the force and the corresponding stress in each of the columns when the crate is be- ing lifted with an acceleration of 10 ft/sec. (b) Find the force and the corresponding stress in each of the columns when the crate is be- ing lifted with a constant velocity of 1 ft/sec. (c) Suppose the columns are made of steel whose yield strength is 60,000 psi. Find the maximum acceleration of the crate without causing yielding of the columns. (d) Suppose the columns are made of steel whose modulus of elasticity is E = 29 x 105 psi. Assuming both ends of the columns are clamped, find the maximum acceleration of the crate without causing the columns to