1A) Look at the Figure in the left margin. A block of 5 kg is connected to a block of 7 kg by means of a 3 kg rope. An vertically upward pulling force F of magnitude 192 N is exerted on the block of 5 kg. 5 hy 7h TIL Since the rope is un-stretchable, the 2 blocks (and the rope) all move "in sync" (= have the same acceleration - in other terms, the system is undeformable). This is the absolutely necessary condition for a valid choice of system-of-interest: each part of the system has to have the same acceleration, otherwise we can (obviously) not write down Newton's second law Fnet on system = msystem system We will see later how to deal with deformable systems that have parts moving in different ways. Although there are only 3 physical objects, we can distinguish many more "systems-of-interest". Practice making different FBDs for the following valid systems of interest. Use the definition of tension from the lectures: tension is a force On the rope! Note the tension at the top/bottom of the rope top/bottom or Fby 5 kg or 7 kg block on rope, and note their companions as "Ttop/bottom" or "Fby 5 kg or 7 kg block on rope". 1) Make the FBD for the whole system (= 2 blocks + rope). 2) Make the FBD for the 5 kg block. 3) Make the FBD for the rope. 4) Make the FBD for the 7 kg block. These were most likely the systems-of-interest that came to your mind. But there is more... 5) Make the FBD for the bottom half of the rope. Note that there is a tension force BY the top half of the rope On the bottom half of the rope. Call it Tmidpoint 6) Make the FBD for the top half of the rope. Note that there is a tension force BY the bottom half of the rope ON the top half of the rope. How do you note it? 7) Make the FBD for the 7 kg block and the bottom half of the rope. 8) Make the FBD for the 5 kg block and the top half of the rope. That's 8 systems of interest already for these 3 physical objects. But there are quite a few more valid choices (remember the criterion for validity: parts having the same acceleration) for a system-of- interest, for instance: the 5 kg and the 7 kg block//the 5 kg block and the bottom half the rope//the 5 kg block and the bottom half of the rope and the 7 kg block//etc.