Explain why a roller coaster's second hill cannot be taller than the first hill.

The reason behind the second hill in the roller coaster not taller than the first hill is the frictional force acting on the roller coaster.

Explanation:

The height of the first hill in the roller coaster is taller with the decrease in the subsequent hills as the frictional forces reduces the kinetic energy.

Coming down the first tallest hill, the speed and kinetic energy both increases as the motion is in line with gravity, but due to conservation of energy, the potential energy decreases. And as the potential energy decreases, the height of hill has to be decreased in order to complete the motion.

There will not be enough momentum from the first hill to cross another hill if he same or larger size because of the way potential energy and kinetic energy works it will not be able go as high as it could go on he fist hill.