1. The problem statement, all variables and given/known data At an elastic bar we give work because of hydrostatic pressure P and applied tension force F at the axis length that has length l. 2. Relevant equations 1) Give an expression for dU. 2) With the help of legendre transformations find the thermodynamic equations and the differentials that have as regular variables the (S,V,F),(S,P,F) , (T,P,F) 3. The attempt at a solution The problem i have is that i can't find a way to insert F into dU equation dU = TdS - PdV I attach the file because i can't write the expressions in this board. Thanks!