So I read that the conservation of momentum is a result of: F1=-F2 <Newton's Third Law t1=t2 <Time in contact Therefore: F1*t1=-F2*t2 F=m(Δv/t) Ft=mΔv So we can conclude: m1Δv1=-m2Δv2 Therefore momentum is conserved. Now what force is this? Would it be the same normal force that exists when an object is sitting on a surface? I don't think that would make sense, because normal force simply counteracts other forces (such as gravity) when objects are in contact, yet an object moving in inertia wouldn't have any applied force, so it wouldn't be counteracting anything. So then, what is this force that opposes objects' motion as a collision occurs between masses?