Module 6—Work and Energy
Lesson Summary
In this lesson you explored the following questions:
- What is power?
- What is the relationship between power and work?
- How is efficiency determined?
Power is the rate of doing work. It is the ratio of the work done to the time interval in which it is done. In the case of the chairlift, the power does work on the riders, increasing their mechanical energy during the time they are moving up the hill. The power is also consumed doing work against non-conservative force, such as friction. More power is required to do more work. Increasing the power on a chairlift will cause it to move at a faster rate, moving more mass up the hill in less time.
Efficiency is the ratio of the output (useful work) to the input (total energy used). For a chairlift, the useful work is equal to the gain in potential energy of the riders once they reach the top of the lift. The total energy used is greater than this since some of the energy is consumed overcoming non-conservative forces such as friction. Minimizing non-conservative forces will lead to greater efficiency.
Lesson Glossary
efficiency: the ratio of the output (useful work) to the input (total energy used)
Expressed as an equation, it is
Since the input and output occur in the same time interval, efficiency can be determined using either power or energy.
power: the rate of doing work