A cart weighing 200 N can roll without friction along a horizontal path . The cart carries a block weighing 20 N . The coefficient of friction between the block and the cart is `0.25` and g= `10 m//s^(2)` . Find the force of friction between the block and cart and their acceleration when a force of (a) 2N is applied to the block , (b) 20 N is applied to the block .