Force has units of Newtons. N = kg*m*s^-2. It's not the same thing. You also can't use the units he gave you. You need to convert them all (thanks america).
I was using conservation of energy and more specifically Kinetic Energy = 1/2*m*v^2. Like someone said above it's a problematic assumption since you are going from movement to a dead halt. It doesn't take into consideration anything else, which makes it a very rough estimate at best.
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