Calculate the force the professor exerts on the cart in Figure 4.10 using data from the previous example if needed. 

Strategy

If we now define the system of interest to be the cart plus equipment (System 2 in Figure 4.10), then the net external force on System 2 is the force the professor exerts on the cart minus friction. The force she exerts on the cart, F_prof, is an external force acting on System 2. F_prof was internal to System 1, but it is external to System 2 and will enter Newton's second law for System 2.