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Riding in the bed of a pick-up truck can be very dangerous. If the truck turns suddenly, the riders can be thrown from the truck bed. Why are the riders ejected from the bed?
The riders' inertia tends to keep them moving in the same straight-line motion while the truck turns beneath them. The walls of the bed of the pick-up truck are low so won't provide much centripetal force to hold the riders in the truck as it turns.