at a construction site a crane raises two boxes on a plank, one box weighs 10 kg and the other weighs 20 kg and are the same size.

a) if the coefficient of static friction is 0.4 how would i calculate the angle at which the boxes begin to slide if the plank is tilted?

b) if the coefficient of kinetic friction is 0.3, how would i calculate how fast the the boxes will accelerate along the plank once they start to slide?