Explanation:

Total physical product (TPP) -- Quantity of output (Y) that is produced from a firm's fixed inputs and a specified level of variable inputs (X).
 
Marginal physical product (MPP) is the change in the level of output due to a change in the level of variable input; restated, the MPP is the change in TPP for each unit of change in quantity of variable input.
 
MPP = (TPP2 - TPP1)/(X2 - X1)