potential energy
Summary
Potential energy is energy stored in a system by the position or arrangement of its parts, such as height in a gravitational field or a stretched spring. Only changes in U matter, and when only conservative forces act, a loss of potential energy equals a gain in kinetic energy.
Detail
Meaning: Potential energy (U) is energy stored by position or arrangement within a system. It goes with conservative forces, whose work is path-independent and equals −ΔU. The zero level is chosen freely, so only changes in U matter. Forms include gravitational, elastic (½kx², with k the spring constant and x the stretch or compression), and electrostatic (kq₁q₂/r, with q the charges and r their separation).
Equation: Ug = mgh, where Ug is gravitational potential energy (J), m is mass (kg), g is 9.8 m/s² near Earth's surface, and h is height above the chosen reference level (m).
On the MCAT: Ramps, pendulums, roller coasters and springs. When only conservative forces act, Ki + Ui = Kf + Uf, so a mass dropping a height h from rest reaches v = √(2gh). On a graph of U versus position, force is the negative slope and stable equilibrium sits at a minimum.
Common confusion: Potential energy is not electric potential. Potential is energy per unit charge (V = U/q, in volts). Also, friction has no potential energy; it converts mechanical energy to thermal energy, so conservation of mechanical energy fails unless that loss is counted.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.