Question: A car of mass 1250kg moving at 28m/s drives up a ramp onto the back of a moving lorry of mass 3500kg and speed 25.5m/s. The back of the lorry allows a braking distance of 5m. Show that this stunt is possible with a min force of about 600N. My solution: I get the answer by first working out the velocity of their combined mass using conservation of momentum, v=26.2 m/s. Then doing Initial KE = Final KE + Work Done which gives the force of just under 600N My question was, why can this not be done using the equations of constant acceleration. After working out that v=26.2m/s, why cant we use v^2 = u^2 + 2as to get 'a', and sub this into F=ma? I tried this and it gives a wrong answer.