Illustration 7.1: Choice of System
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The animation represents a ball sliding on a curved wire (position is given in meters, time is given in seconds, and energy on the bar graph is given in joules) subject to the forces of gravity, the normal force, and friction. Note that the wire does not depict the potential energy function of the ball (see Illustration 7.3 for the Illustration on Potential Energy Diagrams). There are also three bar graphs that accompany the animation. They represent the kinetic energy (orange), gravitational potential energy (blue), and the energy dissipated due to friction (red). The two animations represent two different systems in which to analyze the motion via energy. Restart.
First play Animation 1. Note that in this animation there is no potential energy due to gravity and no energy dissipated due to friction. How can this be? Well, in this case we have chosen the system to be just the ball. Animation 1: show system. As a consequence, the system is not isolated because the ball experiences an external force due to gravity in addition to the external dissipative force of friction. Gravity does positive and then negative work on the ball, changing the ball's kinetic energy. In addition, the force of friction dissipates energy by doing a negative amount of work on the ball.
Now play Animation 2. What is going on here? What is the system now? Here there is potential energy due to gravity as well as energy dissipated due to friction. The system includes Earth and the room, and therefore the total energy must include gravitational potential energy and the frictional energy. Animation 2: show system. Given that we have defined a system that includes Earth and the room, the total energy (found by adding up all three bar graphs) should stay constant.