# Unit 8: Limits on Efficiency

In the previous unit we examined the mechanical efficiency of the body, defined as the ratio of useful work done to loss of chemical potential energy. We also investigated the the fuel efficiency of a person on a bicycle in units of miles per candy bar (MPB). In this unit we will examine limits on those efficiencies. For example, we will examine how drag forces affect fuel efficiency and why the body and car engines could never be 100% mechanically efficient, even if friction and drag could be eliminated. We will also examine how the body and many safety systems work to limit the elastic efficiency of collisions to aid in preventing injuries. The associated lab will guide us through an analysis of  forces on the body during a vehicle collision and how crumple zones are designed to reduce those forces. The learner outcomes for this unit are listed below, and below that are some related key terms to watch out for as you complete the chapter.

### Learner Objectives

1. Calculate the drag force on objects and calculate the rate of energy dissipation caused by the drag force. 
2. Explain how energy dissipation during a collision can be maximized in order to reduce impact forces and prevent injuries. 
3. Compare and contrast thermal energy, temperature, and heat. 
4. Calculate maximum possible efficiencies for heat engines and explain how the Second Law of Thermodynamics limits that efficiency .

Key Terms and Concepts

Temperature

Second Law of Thermodynamics

Heat Engines

Carnot Efficiency 