Physical Principles That Apply to Social Science

I can think of two:

• Le Chatelier's Principle: Any change in status quo prompts an opposing reaction in the responding system. (I was reminded of this by a comment by BostonBridget at Instapundit.)
• The Thomsen–Berthelot Principle: The reaction most likely to occur is the one that produces the most heat.

The Law of Conservation of Momentum, on the other hand, does not apply. If the stock market went up yesterday, that does not mean it will go up today. If crime increased in the 1960–1990 period, that does not mean it will continue to increase. The failure of predictions based on it may have made attempts at “social physics” look naive.