A block of mass 10kg is pulled along a straight horizontal road by a constant horizontal force of magnitude 70N in the direction of the road. The block moves in a straight line passing through two points A and B on the road, where AB = 50m. The block is modelled as a particle and the road is modelled as a rough plane. The coefficient of friction between the block and the road is 4/7. a) Calculate the work done against friction in moving the block from A to B. The block passes through A with a speed of 2ms^-1. b) Find the speed of the block at B. for a) I got: 4/7*10g = F = 56 (taking g=9.8), 56*50 = 2800J, which is correct for b) Work done against friction = Change in KE so 2800 = 5v^2 - 20 and going on to get v to be 23.6, which is incorrect. I asked a friend and he said change in KE = Work done by resultant force, and finding the resultant force to be 14N: 14*50 = 5v^2 - 20 v = 12, which is correct however when my teacher marked this he commented to use the work energy principle, and I don't understand how I can use the work energy principle here? I understand the work energy principle states the the change in total energy = work done on the particle, but the work done on the particle is 2800J, and the change in total energy is 5v^2 - 20, as I done before giving me the wrong answer. How can I use the work energy principle here?