A 4.0-kg block is on an incline plane of 30 degrees angle with respect to horizontal.

a)What is the minimum coefficient of static friction (_s)_min is need to prevent it from sliding down?

b) When the surface _s is 0.30 {which is less than the (_s)_{min ,} this means the block is going to slide down the incline} , to prevent the block sliding, a horizontal force F is applied to the block. Please find the magnitude of this force F.

c) When a 20 N uphill force is applied to the block, the block starts to move up along the incline. Find the acceleration of the block.

The coefficient of kinetic friction is 0.25.