text: The Schiehallion experiment attempted to calculate the mean value of one example of this quantity by measuring a nearby mountain's effect on a pendulum.
full_question: The Schiehallion experiment attempted to calculate the mean value of one example of this quantity by measuring a nearby mountain's effect on a pendulum. The time derivative of an operator named for this quantity is proportional to the commutator of the operator with the Hamiltonian, according to von Neumann. The expectation value of any observable of a system is equal to the trace of the product of the observable's operator with a matrix named for this quantity. For an electric field's energy, this quantity is given as vacuum permeability times electric field squared over two; for an ideal gas, it equals molar mass times pressure divided by the gas constant and temperature. For 10 points, name this value that can be measured using Archimedes' principle, which equals mass divided by volume.
first_sentence: The Schiehallion experiment attempted to calculate the mean value of one example of this quantity by measuring a nearby mountain's effect on a pendulum.
raw_answer: mass density [or density of the Earth; or density matrix; or density operator; or energy density]