Question

a rock falls off a high cliff to the ground below. ignoring friction, which statement correctly compares the energy of the rock at different points of its fall? *hint: this is similar to the penny problem, we did in class. a) the total energy just before it falls is less than its total energy half - way down. b) the total energy just before it hits the ground is less than its total energy just before it falls. c) the total energy just before it hits the ground is greater than its total energy half - way down. d) the total energy just before it falls is equal to its total energy just before it hits the ground.

Explanation:

According to the law of conservation of energy, in the absence of non - conservative forces (like friction in this case), the total mechanical energy (sum of kinetic and potential energy) of a system remains constant. When the rock is just before falling, it has maximum potential energy and zero kinetic energy. As it falls, potential energy is converted into kinetic energy. Half - way down, it has both kinetic and potential energy. Just before hitting the ground, it has maximum kinetic energy and zero potential energy (assuming the ground level as the reference for potential energy). But in all these cases, the sum of kinetic and potential energy (total energy) remains the same.

Answer:

D. The total energy just before it falls is equal to its total energy just before it hits the ground.