Relative Motion

     All motion is relative.  This means that even though a car moves at 50m/s, if another car is also going 50m/s then relative to each other the cars are really not moving.  However to the ground it is going 50m/s.  But what happens when one car is going 5 m/s but at the same time, a car in the opposite direction is going 10m/s?  This means that when the cars pass each other, relative to each other they are actually seeing the other car go 15m/s in the negative.  This can apply in the real world when we watch baseball.  

      If the pitcher throws the ball at 42m/s and the batter swings his bat at 38m/s then when the bat meets the ball, they are actually meeting at 80m/s.  This is because the ball is going 42m/s in the opposite direction in which the bat is being swung.  This results in an 80m/s collision.