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Online Lesson Plan
"Finding" Horsepower

  Susan Kelly  


Subject Area:
Suggested Grade Level:
Grades 6-8
Learning Modality:
Multiple Intelligence:
Logical-mathematical & Naturalist
Bloom's Taxonomy:
What are these educational concepts? What are these educational concepts?

  Testing horsepower  


In this lesson, students will:

  • investigate horsepower as a unit of power;
  • calculate horsepower from a set of given data;
  • and look at history and discover that scientists and engineers of high achievement are considered to be among the most valued contributors to their culture.


StandardsThe engineer, James Watt, invented the term "horsepower." Watt lived from 1736 to 1819 and is famous for his work on improving the steam engine. Watt was working with ponies lifting coal from a mine and wanted to devise a way to describe the amount of power they exerted. A Watt as a unit is used to measure all power whether it is mechanical or electrical.

Power is the rate of doing work. It is a measure of how quickly a job can be done. You can calculate power by dividing the work done by the time it takes to do it. The unit is foot-lbs/second.

Horsepower is a unit of power. Power has units of energy per unit time (P = W/t), and is measured in watts. One watt is one joule of work done in one second. The joule measures energy in physics in meters/kilogram/sec. And one horsepower equals about 746 watts. If you took a one horsepower horse and put it on a treadmill, it could power a generator and produce a continuous 746 watts.

One horsepower equals measuring how much power a horse could deliver.

Although one horsepower equals 746 watts, according to current engineers, a more reasonable number for a horse working an eight-hour day is probably about 500 Watts. If Lance Armstrong (or another cyclist) could do 500 watts output power on a stationary bike, he would be considered strong as a horse! To help you understand the measurement, a bright light bulb draws about 100 watts, a microwave oven, 1000. A person uses food energy at an average rate of 100 watts.

The Resources


Resources from outside Wessels. Direct students to these Web sites that are produced by others outside of the Wessels Living History Farm. [Again, clicking these links will open a new window.]

The Process

If you want to know the horsepower of an engine, a dynamometer is used. It places a load on the engine and measures the amount of power the engine can produce against the load. The tractor testing labs at the University of Nebraska tractor test site determined the maximum horsepower of the engines below.

If you plot the horsepower versus the rpm values for the engine, you get a horsepower curve for the engine. Peak horsepower for an engine is when the rpm value available form the engine is at its maximum. Torque is a measure of turning force, or said another way, a measure of how much a force acting on an object causes that object to rotate.

Dynamometers actually measure torque (in pound-feet). To convert torque to horsepower, you multiply torque by rpm and divide by 5,252. The number 5,252 is the result of lumping several different conversion factors together.

Using the information in the chart below, calculate the horsepower for each of the six tractors. Use the maximum number of revolutions per minute.

Type RPM min. RPM max. Torque Horsepower "hp"
Allis 5650 1305 1850 123.6 lb-ft  
John Deere 5105 1402 1801 114 lb-f  
Massey Ferguson 243 1248 1789 133 lb-ft  
Massey Ferguson 263 1321 1790 161.8 lb-ft  
Massey Ferguson 2220 1498 1800 169 lb-ft  
Case IH DX48 1602 2206 96 lb-ft  

Learning Advice

Other tractor data may be obtained from TractorData.com.

Teachers may wish to assign other problems solving for work or power from their classroom textbook.

Teachers could easily add this information to a physical science unit on work/power/watts and efficiency.

Conclusion of the Lesson

For nearly 200 years, the horse was the only supply of power on the farm. Until tractors were developed and used, the workhorse supplied the power to pull farm implements over every acre of soil planted. If farmers had to rely on horsepower to produce food today, our nation would need 20 times the number of existing horses, and five times the number of farm workers!

Assessment Activity

Question 1: What is the difference between work and Power? Work and energy? Answers
Question 2: The words "work" and "power" are used in many ways in everyday language, but have very specific meanings in science. Write a sentence using work and power in a scientific way.  
Question 3: James does 2400 Joules of work climbing the stairs in 6 seconds. What is his power output? Answers
Question 4: How do machines make work easier? Answers
Question 5: Prepare a short report on the life of James Watt, Henry Ford, or Harry Ferguson. What was their family background? What life events led them to their success? Were they ever unsuccessful? What was their educational background? What was their contribution to science?  
Question 6: Torque is measured in "foot-pounds." Research other units that you may be unfamiliar with. Include their meaning and what science they represent.  
Question 7: Chart or graph the power to weight ratio (horsepower divided by the weight) of the following tractors:  

Tractor Type
Horsepower (hp)
Weight (lbs)
Years Built
Case IH DX48 25 3,700 (unavailable) (unavailable)
J.I. Case SC 19 4,200 1940-1955 $1,700
Case IH 9390 400 29,000 1997-1999 $160,000
Allis Chalmers 8070 150 13,750 1982-1985 $60,000
John Deere M 25 2,550 1947-1952 $1,075
John Deere 5320 50 4,550 2000-         (unavailable)
Ford 2N 24 3,070 1942-1947 $1,120
Ford 5900 50 5,760 1985-1993 $17,400
Ferguson TE20 224 2,760 1946-1956 (unavailable)

As you study the data from the horsepower chart, is there a correlation between the horsepower and the weight of the tractor? Is there a correlation between the weight and the cost of the tractor? Explain.

Can you estimate the cost for the missing data above based on the other information on the chart?

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