Introduction: Traditionally the study of energy in the mechanics portion of introductory physics has focused on idealized systems where thermal energy plays a negligible role (ball on ramps, pendula, masses on springs, Atwood's machines, etc...) These systems are pretty far removed from the energy systems that capture student interest in the "real world" or in the life sciences. Once you introduce living beings into the system for energy analysis it is difficult to keep everything idealized and thermal energy becomes an essential player. This activity provides learners with a chance to model the energy of a human being when they are lifting or lowering a bowling ball at constant speed. We have found that this analysis leads to conclusions that are both engaging and surprising for the majority of students.
Part 1. Raising something heavy
Practice lifting up something heavy (a textbook is fine) at a constant speed. In the analysis below, we are only concerned with when the heavy thing is moving: not when it starts or stops moving. Construct a series of energy pie charts showing the system near the bottom, middle, and top of this motion. The person lifting the heavy thing must be part of the system for the energy pie charts.
Use your energy pie charts to answer the following questions:
• How can you tell from the energy pie charts that the heavy thing is moving at constant speed?
• How can you tell from the energy pie charts that the heavy thing is moving upward?
• How can you tell from the energy pie charts that it takes effort on the part of the person to lift the heavy thing?
• According to your energy pie charts, is the total increase in gravitational energy of the system greater than, less than, or equal to the decrease in chemical energy of the person?
Part 2. Lowering the heavy thing
