When is thermal equilibrium achieved between two identical objects?
a. when one object is colder than the other
b. when one objects is hotter than the other
c. when both objects have the same temperature
d. when the particles in one object have more kinetic energy

If a balloon is taken outside on a very cold day, what will occur?

Consider a system with two masses that are moving away from each other. why will the kinetic energy differ if the frame of reference is a stationary observer or one of the masses?

Two identical stars with mass m orbit around their center of mass. each orbit is circular and has radius r, so that the two stars are always on opposite sides of the circle. (a) find the gravitational force of one star on the other. (b) find the orbital speed of each star and the period of the orbit. (c) how much energy would be required to separate the two stars to infinity?