1. The problem statement, all variables and given/known data A skier glides down a slope, and has at the bottom of the slope, at the beginning of a horizontal piece, a velocity of 30 m / s. He begins to brake and after 4 seconds later has a speed of 10 m / s. The friction between the skis and the snow is 400 N during braking. a. What is the acceleration of the skier during braking? b) What is the mass of the skier?? 2. Relevant equations F= m x a vf=af x t + v(i) 3. The attempt at a solution 10m/s= af x 4s + 30m/s af = -5m/s^2 Thats what i thought answer would be but answer should be +5m/s^2 Why is that ??? And for mass F= m x a m = F/a m = 400N / (-5) m=-80 kg Maybe I have to exchange value for v (initial ) and v (final )??