University of Arkansas - AgriScience Project

AGRISCIENCE EXERCISE

PHYSICAL SCIENCES IN AGRICULTURE

Key Concept: Electricity and Electronic Systems

Sub-Concept: Electromagnets and Electromotors

Agricultural Application: Electric motors are essential for many aspects of modern agricultural production and processing.

Exercise: Determining Mechanical Work and Power for an Electric Motor

Applied Principle(s): Work and power in mechanical systems

Goals:

  1. Calculate the mechanical work done by an electric motor.
  2. Calculate the mechanical power developed by an electric motor.
  3. Calculate the horsepower output of an electric motor.


Materials:





References: Olive, C.T. and Olive, T.P. (1984). Fundamentals of Applied Physics, Third Edition. Albany, NY: Delmar Publishers, Inc.

Procedures for Conducting the Activity:

1. Divide the class into pairs of students, and provide each with a data sheet and the necessary materials for this exercise.

2. Instruct the students to complete the activity as directed on their data sheets. You may wish to monitor their progress as they work; however, it is suggested that the students be left to follow the instructions and complete the activity on their own.

3. Once all groups have completed the exercise, discuss the answers to the discussion questions as a class. Be sure to make note of the practical agricultural applications of the principles demonstrated.



AGRISCIENCE EXERCISE

- Determining Mechanical Work & Power for an Electric Motor -

Student Data/Instruction Sheet

1. Gather the following materials:





2. Fasten the motor securely to the edge of the table using the clamp. NOTE: The motor should be situated so the pulley extends over the edge of the table.

3. Connect the circuit as illustrated in the schematic below. Leave the switch in the "off" position.



















4.Attach one end of the string to the pulley on the motor shaft. Attach the other end of the string to the cup.

5. Pour a quantity of sand into the cup. Close the switch and see if the motor is capable of lifting this load. By trial and error, determine the maximum load the motor can lift. Determine the exact weight of this load and record in the appropriate cell of the Data Chart.

6. Place the cup on the floor and measure the exact vertical distance from the top of the cup to the bottom of the motor pulley. Record this in the appropriate cell of the Data Chart.

7. Switch the motor "on" and determine the exact amount of time required to lift the load. Record this in the appropriate cell of the Data Chart.

8. Reduce the weight of the load by exactly one-fourth. Record this new weight in the appropriate cell of the Data Chart. Determine the amount of time required to lift this load. Record this in the Data Chart, as well.

9. Reduce the weight of the load so that it is exactly one-half the weight of the original load. Record this new weight in the appropriate cell of the Data Chart. Determine the amount of time required to lift this load. Record this information in the Data Chart.

10. Use the Data Chart to calculate the work, power, and horsepower of each trial. Record your answers in the appropriate cells.

DATA CHART

Trial No. Weight

(lbs.)

 Distance

(ft.)

 Time

(min.)

 Work

(ft.-lbs.)

 Power(ft.-lbs./min.) Horse-

power

1
2
3

Note: Remember to convert your measurements to the appropriate units before making calculations.



11. Answer the following questions about this activity. Be prepared to discuss your answers with the class.

a. During which trial was the most work accomplished? Why?













b. Which trial required the least time to lift the load? Why?











c. During which trial was the most power (and horsepower) developed?











d. What is the relationship between power (or horsepower), speed, distance, and time?











Between power (or horsepower), work, and time?





















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