http://img18.imageshack.us/img18/3320/lalalacars.png [Broken] First of all sorry for the bad picture. But I hope you get the idea. I think we have all seen this situation in some movies. So basically a smaller car is leaning on a bigger truck with 2 wheels, while the other 2 wheels of the car are on the road. Now lets say the truck and the car are both driving at 90 km/h. The 2 wheels of the car that are on the road are going at the same speed as the wheels on the truck. But the road is moving at 90 km/h relative to the car, while the the truck is moving at 0 km/h relative to the car. How do the wheels act? How come the car keeps going at the same speed, like the 2 wheels were still on the ground?