Figure 4 Although the horse-drawn plow and the gasoline- powered engine get their power from different sources, both are capable of doing work at a rate of about four horsepower.
James Watt and Horsepower
Besides the watt, another common unit of power is the horsepower. One horsepower (hp) is equal to about 746 watts. The horsepower was first defined by Scottish scientist James Watt (1736–1819). Watt was looking for a way to compare the power outputs of steam engines he had designed. Horses were a logical choice for comparison as they were the most commonly used source of power in the 1700s. Watt did not want to exaggerate the power of his steam engines. Thus, after many experiments, he defined the horsepower based on the power output of a very strong horse. Figure 4 shows a comparison of two equivalent power sources.
Section 14.1 Assessment
Reviewing Concepts
What conditions must exist in order for a force to do work on an object?- What formula relates work and power?
- How much work is done when a vertical force acts on an object moving horizontally?
Critical Thinking
Applying Concepts A desk exerts an upward force to support a computer resting on it. Does this force do work? Explain.
Predicting Two cars have the same weight, but one of the cars has an engine that provides twice the power of the other. Which car can make it to the top of a mountain pass first? Which car does more work to reach the pass?
Comparing and Contrasting You carry two heavy bags of groceries upstairs to your kitchen. Will you do more work on the bags if you carry them up one at a time? Explain.
Math Practice
How much work does a 25-newton force do to lift a potted plant from the floor to a shelf 1.5 meters high?
You lift a large bag of flour from the floor to a 1-meter-high counter, doing 100 joules of work in 2 seconds. How much power do you use to lift the bag of flour?
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