Two cars with different masses travel at the same speed down a hill toward a stop sign
What will happen when both cars apply brakes at the same time to stop?

See the explanation below.

Explanation:

As cars have different mass, their potential energy is different when they are at the top of the hill. The car with the greatest mass will have more potential energy.

By the principle of energy conservation, we know that potential energy will become kinetic energy. That is to say at the highest point of the hill we will have the maximum potential energy, this energy will be equal to the kinetic energy in the lower part of the hill, where both cars brake.

From the last expression we can see that the car with greater mass will have more kinetic energy, therefore it will need a greater force to be able to stop it. If both cars have the same braking force the car with the highest mass, it will need more braking distance.

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.