The problem is simple, its a matter of finding a car's initial velocity after locking up the wheels while braking. Here is what I know: The braking distance (skid marks) go for 20 feet, the final velocity is 50m/s, the mass of the vehicle is 2000kg, and the coefficient of friction of the tire and asphalt is 0.6. What is the initial velocity? Here is how I did it first: 1) force friction = 0.6 * 2000 * 9.8 = 11760N 2) m * a = 2000 * a = -force friction + ( 2000 * -8.5 ) 2000 * a = -11760 - 17000 a = 14.38 m/s^2 3) Then I go on to solve for the intial velocity after getting the deceleration. In step 2, I used -8.5 for a typical deceleration rate of a vehicle when it slams on the brakes. What I'm told though, is that I don't need to add this force for braking and I only need to put in the friction force (0.6 * normal). So I'd just have "m * a = -force friction" and the deceleration would be -5.88 m/s^2. It seems to me though, that a vehicle would deceleration more quickly if the brakes were applied, more so than just slowed down by the force of rolling friction the road applies. So can anyone clear this up for me? Something to do with internal/external forces?