1. The problem statement, all variables and given/known data A 2000kg car is moving at 50km/h. What is the force required to stop it? 2. Relevant equations KE=1/2m(v^2) p=m*v 3. The attempt at a solution 1/2(2000kg)*(50km/h)^2 = 1000kg*2500km^2/h^2 = 2,500,000 km^2/h^2 converting from hours to seconds and km to m (to get joules) 2,500,000 km^2/h^2 = 2,500,000,000 m^2/h^2 2,500,000,000 m^2/h^2 / 60 / 60 = 694444.4... J My answer may or may not be right. (I'm actually not so concerned with the math). Rather, I have a fundamental problem understanding the difference between KE and momentum. Why am I using the formula for KE to compute the energy to stop a moving mass and not the formula for momentum? If not here, when is the formula for momentum used? I am extremely confused on whether to use KE or momentum to handle collisions. I've come across an analogy that KE is the work, and momentum is the transportation for that work. The equal and opposite reaction (Newton's third law), is this based off of momentum or KE? I really would love a simple place to start for a comparison of these two terms. Thanks for any help.