Need some help with physics...?

Question

A boulder that weighs 200 N is posed on the edge of a 100-meter cliff. What is its gravitational potential energy? Draw a diagram showing how its potential energy and kinetic energy change as it falls to 50 m, 20 m, and 10 m. I have this as a question in 9th grade Physics, don't get how to draw the diagram, get everything else.

Answer 1

There is no standard way to diagram energy, so you can get creative. Think how the kinetic energy relates to the boulder's velocity at each point. How would you represent different velocities, visually? You've got it.

Answer 2

My guess is that they want a plot of kinetic and potential energy vs distance above the bottom of the cliff. Create a graph with energy as the vertical axis and distance above the bottom of the cliff as the horizontal axis. You say you understand the first part, so you can plot the value of potential energy at the 50m, 20m, and 10m points. There are two ways to get the kinetic energy: the easy way and a more direct, but more complicated way. I think the question wants you to do the direct way, which means finding the velocity at the three distances, and compute the KE = .5*m*v^2. The velocity is √[2*g*s], where s is the distance the boulder falls. The easiy way is to note that the kinetic energy at any point will be equal to the loss in potential energy; so at 20m, the kinetic energy is the potential energy equivalent of a drop of (100-20)m. I think this last result is supposed to become obvious from looking at the chart you computed using the velocity method.