Reversible process

1 A table is moved forwards on a gently upward sloping floor and then allowed to move back down the slope to its original position. One castor is locked. Heat is generated in the stressed castor. As the table is returned, more heat is generated and the castor squeals. The table has returned to its original position, but not to its original condition. One castor is now warm. The displacement was not reversible.

2 The castor is released. The table is gently and quietly displaced up the slope and returned to the original position, on perfectly free-running, oiled, castors. In the absence of friction, and neglecting turbulence in the air, the displacements are now reversible.

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