This is going to be a very elementary question, but I am having a hard time wrapping my head around it for whatever reason. In static equilibrium, like a book on a table, I have no problem seeing that the force exerted on the table by the book is equal and opposite to the force exerted on the book by the table. But when things are being accelerated I am having a hard time grasping how the force pairs are equal. For example, if a hand is holding up a mass that is too heavy for the person to hold, then the mass and the hand will both be accelerated downwards. The forces acting on the mass would be the force of gravity, acting down, and the pushing force of the hand, acting up. The force of gravity is greater than the pushing force so there is a net force downwards. How is it that the hand exerts an equal and opposite force on the mass? The forces on the hand would be the weight of the mass, acting downwards, the force of gravity on the hand, acting downwards, and the pushing force of the arm on the hand, acting upwards. The force of the mass on the hand is equal to the weight of the mass (mg), and the force of the hand on the mass is equal to the pushing force of the arm. But if these were equal, then the pushing force of the arm would equal the weight, or force of gravity, on the mass, an then there would be no net force downwards on the mass. What am I getting confused?