Step 1

What You Need

• DC geared motor and attached string spool1
• C Clamp
• Battery holder
• 2 D cells
• Alligator clip leads
• Switch
• Multi-meter
• Stopwatch
• Spring scale
• Meter stick
• Weight

1 The modified servo motor (gear motor) needs to have a thread spool attached. If you don't have this done, click the next link for some hints.

Video. Modified Geared Motor

Step 2

Set up the System

Using the image below as a guide 

• Attach the geared motor to a tabletop using a clamp (or another method of securing it).
• Attach the weight to the string and unwind it so that it doesn't quite touch the floor.

Figure. Diagram of component (note that the battery pack is not shown)

Step 3

Set up the Electrical Component

• Next connect the motor leads to the battery box and switch. Make sure the batteries are removed before you do this. Use "alligator" clip leads where necessary.
• Set the Multi-meter to the 20 volt DC range with the probes in the correct position and connect it to the points as shown in the block diagram. Make sure you follow the wire color convention (red – positive, black – negative).

Figure. Block diagram of the circuit setup for voltage measurement

Step 4

Measure the Voltage

Before you get started, use the next link to open and print a measurement entry table.

• Measurement Table

Complete the following 

• Place the 2 D cells in the battery box. Make sure the negative end of each cell (flat end) is touching the spring.
• Turn on the switch. (If the object goes down, wind the string in the opposite direction).
• As the object is lifted, read the voltage and record in the table.

Step 5

Re-configure the Circuit

The Multi-meter needs to be re-configured as an ammeter to measure the current running to the motor. Make sure the switch is open! Change the red probe to the 200 milliampere jack and set the range to 200 milliamperes DC. Connect the ammeter to the circuit as show next.

Figure. Block diagram of the circuit setup for current measurement

Step 6

Measure the Current and the Distance

• Unwind the string so that the object is near but not touching the floor.
• Using a meter stick, measure the distance the object is from the floor. Record in the table below as distance 1. (It doesn't matter if you measure to the top or bottom of the object but be sure your eye is level with the object. Try to be accurate!)
• Close the switch and start the stopwatch., As the object rises, read the current and record in the table (as amperes, not milliamperes. For example 150ma = .15a)
• Before it goes all the way up, stop the object by opening the switch and at the same time stop the stopwatch . Measure the distance from the floor (measure to the same point as in the first measurement). Record in the table as distance 2.
• Read the stopwatch and record in the table.

Step 7

Calculate the Efficiency of the Motor

Do the calculations as shown in the  table to determine the Work Input and Work Output.

Finally, Calculate the efficiency of the motor system. Use the following formula. Your answer will be in percent.

If you have time you can do some additional trials and average the efficiencies.

Step 8

Questions

• Can the efficiency of a system be 100%? Why?
• You probably (hopefully) calculated an efficiency of less than 100%. That meant that only some of the energy input to the motor system did work lifting the weight.
• What happened to the rest of the energy?