A particle experiences a potential energy U as a function of position x as shown in the diagram above.
At which position is the particle in a state of unstable equilibrium?(A) A
(B) B
(C) C
(D) D
(E) EFor these, you either know what to do with the diagram or you don’t. Here you, of course, remember that
equilibrium is represented on an energy-position diagram by a horizontal slope and that unstable
equilibrium requires the energy-position diagram to be at a maximum. Thus, the answer is C.
Three Things You Can Do with a Graph
You could see so, so many graphs on the AP exam... . It’s often difficult to remember which graph
means what. But if you know your equations, you can usually figure out how to interpret any graph you
are faced with. Why? Because there are pretty much ONLY three things you can do with a graph:
Take the slope.
- Find the area under the graph.
- Read off an axis.
For example, an AP Physics C exam question described an experiment in which a solenoid was
stretched to vary the number of turns per length, n . At constant current, the magnetic field inside was
plotted as a function of n ; the question asked for an experimental value of the permeability of free
space μ 0 . Chances are that you’ve never done this experiment and that you’ve never seen this
particular graph. But you do remember your equations: the magnetic field of a solenoid is B = μ 0 nI .Solving for μ 0 , .