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.

thermal energy is the answer