Tutorial Section 5.4 - The zero-point of potential energy doesn't matter 1. A 5kg ball, shown in the picture, is dropped from =7 mmal herghl 01 ball rest at an initial height of 10m above surface level. The ' bottom of a hole is 15m below surface level. What is the l ,f I : 5U ace eve ball's speed at surface level? w bottom of a hole 2. Now consider the same problem without numbers. The ball in the picture (mass m) is dropped from the initial location shown. Write its speed at surface level (v5) in terms of the change in potential energy APE of the system. 3. Imagine two students analyze the problem shown. Student 1 says h=0 at surface level. Student 2 says h=0 at the lowest point (at the bottom of the hole). Which choice, if either, is correct? Explain. 4. For the two choices of potential energy above, compare the change in potential energy between the initial and nal locations. |:l Checkpoint. Please continue working. 5. Find the error in the students' thinking: Student 1: "The ball has potential energy of zero at surface level. That means that when it reaches the surface level, it has no potential energy and only kinetic energy." Student 2: \"The ball has potential energy of zero at the bottom of the hole. That means that when it reaches surface level, it has both potential energy and kinetic energy.\" Student 1: \"That can't be true. The ball-earth system has a xed amount of energy since no external forces act (until the ball hits something, of course). If, at surface level, all the energy is kinetic in my system -- and is split between potential and kinetic in your system, they will have different speeds at surface level. But we can measure the speed! Both of us can't be right."