Question

While you're driving behind a truck that is carrying dirt, you notice that a big rock is about to fall from the back of the truck. Suppose at this point, both you and the truck are traveling at 60 [mi/h] and you are trailing the truck only by 15 [ft]. Given your proximity to the truck in front of you, where would the rock land if fell from the back of the truck?

Answer 1

The rock maintains its horizontal velocity as the truck, which is 60 mi/h, when it falls. It will land on the road directly beneath where it fell from the truck, barring external forces.

Step-by-step explanation:

When a rock falls from the back of a moving truck, because it is initially moving at the same velocity as the truck, it will continue to move horizontally forward even as it falls due to gravity. Therefore, if the truck is moving at 60 mi/h and you are trailing the truck by just 15 ft, when the rock falls, it would land on the road roughly beneath where it fell off the truck, assuming no air resistance or other horizontal forces are acting upon it.

This is because the rock retains the horizontal component of its velocity, which is the same as the truck's velocity, at the moment it separates from the truck. The rock does not suddenly come to rest in the horizontal direction when it falls; so relative to the ground, its initial velocity is the same as the truck's, and it will hit the ground behind the truck due to its forward motion and the time it takes to fall.