Consider a system of two blocks coupled by a massless string over a pulley. Block 1 is sliding over a table pulled by block 2 as it falls under gravity. The acceleration of the system can be derived by calculating the net force on the system. A free-body diagram for this system can be drawn by representing the two blocks with their outline. Here, the blocks are considered to be isolated. The external forces acting on them are identified and represented by vector arrows. The forces on block 1 are its weight, normal force, tension force due to the rope, and friction force opposing the tension force causing the motion. Similarly, the forces acting on block 2 are its weight and the tension force. Applying Newton's second law of motion to the blocks, an equation for the net force can be derived. Using the algebraic operation and rearranging the terms, the magnitude of the acceleration of the system can be obtained.