8. (True of False?) An irreversible engine works in cycles. When the engine returns to its initial state, the entropy change of the engine is zero.

6. A uniform spherical planet has mass M and radius R. A very thin tunnel passes from one end of the planet, through the planet center, and then out from the other end of the planet. A particle of mass m fell into one end of the tunnel and then performed simple harmonic motion. Find the angular frequency of the simple harmonic motion.