With limited knowledge of physics, my friends and I have been discussing this question and cannot come to an agreement. If a truck is driving on a highway (at 60 mph lets say), and a car is on top of the truck, how fast does the car need to be moving to not fall off the back or to drive off the front? Does it need to be driving the speed of the truck? Or does it just need to be travelling fast enough to break through the wind (and how fast would this be)? Can it stay parked, or will it fall off?