Using a laser along with computer microphone probeware and the appropriate software, students determine the speed of rotating objects.An audio amplifier is connected to a solar cell to change the laser light signal into a sound signal. A simple calculation is required to find the rate of rotation.

Science - Grade 8
*112.24  The student is expected to:
b1A demonstrate safe practices during laboratory investigations.
b2A plan and implement investigative procedures including asking questions and selecting and using equipment and technology.
b2B collect data by observing and measuring.
b2C organize, analyze, evaluate, make inferences, and predict trends from evidence.
b2D communicate valid conclusions.
b2E construct graphs and tables using tools including computers to organize, examine, and evaluate data.
b3C represent the natural world using models and identify their limitations.
b4A collect, analyze, and record information using computers and compuuter probes.
b4B exrapolte from collected information to make predictions.
b5A identify a design problem and propose a solution.
b5B design and test a model to solve the problem.
b5C evaluate the model and make recommendations for improving the model.
b7B recognize that waves are generated and can travel through different media.
b13A describe characteristics of the universe such as stars and galaxies.

Technology Applications (Computer Literacy) - Grades 6-8
*126.12  The stident is expected to:
c1A demonstrate knowledge and appropriate use of operating systems, software applications...
c2A demonstrate proficiency in the use of a variety of input devices.

• The student will set up laboratory equipment to determine the speed of a motor.
• The student will calculate the speed of a motor from collected data.
• The student will evaluate amd make recommendations for applying the experimental procedure to measure other rotating objects.
• This concept will be compared to such natural phenomena as quasars.

• Students will be familiar with pulsars and quasars.
• Students will have experience with the safe use of lasers.
• Students should understand the term frequency.

__1.5 V D.C. motor and battery
__Solar cell with mini-plug attached (Can use a radio earphone and plug with the earphone removed.)
__Radio Shack Audio Amplifier-Speaker (part#277-1008c)
__Laser
__Disk with concentric holes such as a sink strainer
__Microphone computer probeware such as LOGAL, Vernier, or PASCO

Preparation:

1. Attach the mini-plug to the solar cell so that it will plug directly into the input jack of the audio amplifier.
2. Attach the disk (sink strainer) to the motor shaft so that it is centered.
3. Familiarize the students with the microphone computer software and hardware.

Classroom management:
The laboratory group size depends upon the number of setups that are available.

Questions to investigate:

1. How can the rotational speed of an object be measured?
2. How can sound frequency be used to measure light moddulation?
3. Can this method for measuring speed be used in other "real life" situations?
4. Is the rate of rotation the same for the inside edge and the outside edge of a disk?

Data Collection:

1. Arrange the equipment so that as the disk spins, the laser beam shines through the inner most set of concentric holes. The laser sholud be intercepted on the other side of the motor by the solar cell attached to the audio amplifier. The audio amplifier should produce a constant tone.
2. Set the microphone so that it will pick up the sound produced by the audio amplifier. The computer program should be set to measure sound frequency.
3. Move the motor-disk combination a little until the laser beam shines through the next set of holes. Measure the frequency with the microphone probe.
4. Continue moving the disk until all of the frequencies have been measured.

Calculations:

1. Count the number of holes in each coccentric ring, starting with the set closest to the center.
2. Divide the frequency of the inner most ring (pulses/second) by the number of holes (pulse sources/revolution) in the inner most ring. This will give revolutions/second or speed of rotation.
3. Do this measurement for each ring of holes.
4. Compare the rotational speed at each position. The speed of rotation should yield the same result for each calculation.

Communicate results:

1. Draw a graph of frequency versus number of holes. Explain the meaning of the graph.
2. Draw a graph of speed versus distance from the center of the disk. Explain the meaning of the graph.
3. Design a procedure that could be done to measure the speed of another rotating object.
4. Explain how this method could be used to measure quasars.

• Use a checklist to measure laboratory procedure.
• Evaluate each student/group on the explanations of the graphs.
• Give credit for creating an application design using this procedure.

• This procedure could be done by placing small mirrors at various positions on a fan blade and reflecting the laser light.
• The solar cell/amplifier/microphone setup could be replaced by a light probe which is set to measure pulse frequency.