Energy, Machines, and Motion
Part One: Energy
Lesson 7: The Force Exerted by a Motor
Students design an experiment to determine which combination of variables will produce the maximum force from a small electric motor. Students will experiment with the number of batteries, the arrangement of those batteries and where the string is attached to the motor.
Focus Question: What are the operating conditions that produce the maximum force from a motor?
3. Lesson Set-up and Management
EALR 2: Inquiry
Core Content: Questioning and Investigating
6-8 INQC Collecting, analyzing, and displaying data are essential aspects of all investigations.
6-8 INCD For an experiment to be valid, all (controlled) variables must be kept the same whenever possible, except for the manipulated (independent) variable being tested, and the responding (dependent) variable being measured and recorded. If a variable cannot be controlled, it must be reported and accounted for.
6-8 INCF It is important to distinguish between the results of a particular investigation and general conclusions drawn from these results.
EALR 4: Physical Science
Core Content: Balanced and Unbalanced Forces
6-8 PS1C Unbalanced forces will cause changes in the speed or direction of an object’s motion.
- Students observe that motors exert forces that can be measured.
- Students learn that the performance of a motor depends on the arrangement of the batteries being supplied to the motor.
- Students learn that an experiment has both an independent/manipulated variable and a dependent/responding variable.
3. Lesson Set-Up and Management
Materials:
- Set up one pegboard system prior to doing the investigation (see figure 7.1 in the student guide).
- Check the motors to see if they are working.
- Check pulleys to see if they fit easily on the motor, if they are
too tight they won’t fit on the motor’s shaft, too loose they can fall
off. Have a spare pulley ready in case a group is having difficulty.
- Use cardboard separators so the string can wind up on the pulley or the nail without slipping as the motor is engaged. This will work on the same principle as a sewing bobbin.
- Make sure batteries are in proper alignment if a group reports
that their system is not working.
Students may have had some experience with series and parallel circuits in elementary school. You may want to review these circuits with them. Series circuits will produce more power, while the parallel circuits share power that allows the circuit to run longer. Consider also having students brainstorm where you would use these types of circuits. Consider setting up both circuits for students to use as a reference. In the reference displays point out to students the alignment of the negative and positive terminals. - When you are finished with the lesson at the end of your last class leave the knife switch, clamp and motor attached to the peg board for Lesson 8.
- You may want to consider marking peg boards with group numbers so students are using the same system through the next several lessons.
Student Management:
- Students are asked to determine which arrangement of batteries and placement of the string to the pulley will yield the greatest force. It is very difficult for students to control all of these changes. Spend time as a class coming up with a data table for students to collect their results. This will help students when they are asked to write a procedure. It will also help keep students focused on the arrangement that yields the most power. Lesson Seven Data Sheet.
- Consider having jobs for your small groups. One student can
collect data, one student can keep track of the data collection, one
student can be in charge of adding washers, one student can wind the
string, the last student will engage the system with the knife switch.
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Students should recognize that when the washers are hanging at the end of the string with the motor not running that the forces in that system are balanced. When the motor is engaged the string winds around the motor’s shaft causing unbalanced forces moving the string. The force in this system is a mechanical force coming from the motor.
- Students are still developing the idea of energy transformation. You will want students to identify any transformations they see in this system. You will not see this in the lesson format, but would be a good place to continue to build this concept.
- The pacing for Lesson 7 is three class periods. It will take one day to develop a data table and experimental design, one day to test, and the third day to share results and follow the procedures to test the force of the motor.
- Use questions from student guide #5 in the procedures for period one
as a guide for your students to write their experimental design:
- Do you think the number of batteries will affect the motor’s performance? How many batteries should you try?
- Will the way the batteries are connected (series or parallel) affect the motor’s performance?
- Will the placement of the string on the nail or the plastic pulley affect the motor’s performance?
- Writing the procedure can be a long process for some in this investigation.
- Consider developing a class data table that could be placed in everyone’s notebook rather than writing out the entire experiment.
- Much of the emphasis of this lesson is in the experimental design. You will want to continue working with your students on the types of forces they find in this system along with any energy transformations that are occurring.
- In Lesson 8 students will need to know how much force is exerted by
their motors. They will collect this information at the end of Lesson 7.
Consider setting up one station where students can gather this
information. Groups can come back to a station that is already set up,
insert their motor and test the force.
Writing Support:
- Develop student data sheet. The picture of the chart is a student
example of the type of data that will need to be collected.
- Make a Claim and Evidence Chart: You could use this type of chart to elicit the force of each variable. This could be used to collect the information students will need for their reflection questions.
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Claim: |
Evidence: |
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Example from student notebook…. |
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Two batteries can lift more than three batteries. |
The greatest number of washers lifted by three batteries was eight, but two batteries could only lift five washers. |
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The batteries in series can lift more washers |
The greatest number of washers lifted in series was 8 washers, whereas the parallel circuit could only lift 2 washers. |
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The nail could lift more washers than the pulley. |
With three batteries in series the nail could lift 8 washers while the pulley under the same conditions could only lift 4 washers. |
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After students have reflected on each variable they could write a summary of the investigation along with a conclusion based on the data they have collected.
Reading Support:
- After students have completed the investigation the reading selection “Motors—Getting Smaller Every Day” describes that motors are very large or very small. Different applications of these motors are described in this reading selection.
Math Support:
- Having students test for the amount of force when you have parallel or series circuits, two or three batteries and lifted by the nail or the pulley presents a challenge of how many different combinations there are to test.
- Students can be challenged to create a data table to collect data for different conditions outlined in the lesson.
- Students can apply averaging skills when collecting trial data within their group and an average for each group in the class.
- Students continue using a spring scale to measure the force of the motor that exerts the greatest force.