And then they collide... Just for laughs, what happens to the other two zombies we left of the lake? Well, we assume that they collide and move as a simgle mass. If you remember, the zombie chasing you was moving at 2 m/s. Simple enough, we treat this an a perfectly inelastic collision. Since the two zombies are about the same mass, the equation we use to figure out the speed that they move at would be: V1i * M1 + V2i * M2 = V1f * M1 + V2f * M2 So, considering that the second zombie (M2) is still, V2i is equal to zero. Also, we can consider the two zombies have equal mass, so M1 = M2. Then we get the equation: (3m/s) * M = Vf* (M + M) Or further simplified as (3 m/s) * M = Vf * (2 * M) Deviding both sides by mass: Vf = 1.5 m/s, in the direction that the first zombie was sliding along the lake.