(a) A concrete block of mass 2 kg is placed on a rough road surface and is pulled by a string as shown in Figure Q2a. (i) (ii) 2 kg Figure Q2a The block starts to move at constant speed when F is increased to 2 N. What is the friction between the block and the surface? (3 marks) Find the acceleration of the block when F is increased to 4 N. (5 marks) (iii) The string is broken when the block is moving. Find the subsequent acceleration of the block. (5 marks)