# Homework Help: Internal energy

1. Apr 28, 2012

### ketz

A stone is falling under gravity in a vacuum. Is its internal energy increasing?

Well, since internal energy(Microscopic level) is defined as the energy associated with molecules(Sum of kinetic energy and potential energy of the molecules). The molecules inside the ball will not gain any K.E or P.E since it is not being affected. Hence, I think ΔU will be zero! Is it correct??

2. Apr 28, 2012

### Andrew Mason

Measurement of energy depends on the frame of reference in which the measurement is made. If the "internal energy" of a quantity of matter is defined as the potential and kinetic energy of molecules as measured in the frame of reference of the centre of mass of that quantity of matter, then, ignoring any tidal effects, you are correct.

If the stone is falling in the gravitational field that has a significant gradient - eg. close to a neutron star or black hole, the stone could stretch (one end accelerating faster than the other), in which case its internal energy could change.

AM