Asmall sphere is hung by a string from the ceiling of a van. when the van is stationary, the sphere hangs vertically. however, when the van accelerates, the sphere swings backward so that the string makes an angle of θ with respect to the vertical. (a) derive an expression for the magnitude a of the acceleration of the van in terms of the angle θ and the magnitude g of the acceleration due to gravity. (b) find the acceleration of the van when θ = 10.0°. (c) what is the angle θ when the van moves with a constant velocity?