1. The problem statement, all variables and given/known data A block slides to a stop along a table top. Which is true? A. The work done by friction is equal to the force of friction times the distance that the block slid. B. The work done on the block is equal to the force of friction times the distance that the block slid. C. The work done on the table is equal to the force of friction times the distance that the block slid. D. The work done on the block is equal to the sum of its changes in kinetic and internal energies. Answer: D. Explanation (that I don't understand well): When considering the energy transfer due to friction, we must take into account internal energy change. The friction force times distance is equal to change in mechanical energy: fd = ΔKE + ΔPE, but this does not take into account the internal energy change of the block. Work by friction does not equal fd. 2. Relevant equations Fdcos(theta) = Work = mg(uk)d 3. The attempt at a solution My initial reaction was that this question is playing with positive and negative signs, so I was looking for the one with the proper sign. Work done by friction on the block is negative, and on the table is positive. Been trying to reason the answer for half an hour. Stuck and frustrated. Thanks.