A blocked is placed against the vertical front of the cart. What acceleration must the cart have so that block A does not fall? The coefficient of friction between the block and the cart is µs. _How would an observer on the cart describe the behavior of the block?

A block is placed against the vertical front of a cart as shown in the figure. The coefficient of static friction between the block and the cart is μ_s. What acceleration must the cart have in order that block A does not fall? 

A 3.1 kg block is placed on top of a 8.22 kg block. A horizontal force of F = 55.5 N is applied to the 8.22 kg block, and the 3.1 kg block is tied to the wall. The coefficient of kinetic friction between all moving surfaces is 0.196. There is friction both between the masses and between the 8.22 kg block and the ground. The acceleration of gravity is 9.8m/s^2. Determine the magnitude of the acceleration of the 8.22 kg block. Answer in units of m/s^2.

A 9.5-kg block is sliding down a wall with constant velocity. The coefficient of static friction between the block and the wall is μs = 0.70. The coefficient of kinetic friction between the block and the wall is μk = 0.40. What is the magnitude of the force (in N) pressing the block against the wall? Use g = 9.79 m/s2.