Physical Clocks
from It's About Time Colloquy funded by the Burroughs Wellcome Fund

The following stations allow you to build several types of physical clocks and evaluate their precision, accuracy and ease of use. The stations may be completed in any order and you may not get to do every station. Try to spend no more than 30 minutes at a station.

Station 1: Falling Objects

CBL2 Set-Up

• Turn on the calculator. Press (APPS) and choose DATAMATE. The program will start and check for sensors. It should find a MOTION detector in DIG1.
• Press (1) for Setup. Press the up arrow and press (ENTER) to select MODE.
• Press (2) for time graph. We want to use a TIME INTERVAL of .05 seconds with a NUMBER OF SAMPLES of 60. If this appears on the screen press (1) and skip the next step. If not press (2) and do the next step.
• Enter .05 and press (ENTER). Enter 60 and press (ENTER). Press (1) for OK.
• Press (1) for OK and return to the main menu. You are now ready to collect data.

You will drop 3 objects - coffee filter, beach ball and book - as potential clocks. The time you are using as a standard is the time it takes the object to fall.

For each object:

• Hold it as high as you can (stand on a chair if you like).
• Press (2) for Start. Wait for the CBL2 to beep and drop the object onto the motion detector.
• After a few seconds you will be able to choose a graph to look at. Press (ENTER) for DISTANCE.
• Identify the area of the graph where the object was falling. Use the arrow keys to trace along the graph. The X and Y values appear at the bottom.
• You may adjust the sampling rate if you wish and repeat the measurement.

? How are the graphs different for the 3 objects? How are they the same?

? Do all the objects fall at the same rate? Why do you think that is?

? Choose the object you think would make the best clock. Explain your choice.

 

Station 2: Springs

CBL2 Set-Up

• Use the same steps as Station 1 EXCEPT use a 100 as the NUMBER OF SAMPLES.

You can use the motion of a spring to keep time fairly reliably and easily.

• Hook the spring onto the ring stand and hang the weight holder on the end. Pull the holder straight down gently to get a steady bouncing with no side to side motion.
• Place the motion detector directly below the holder at least 50 cm away.
• Press (2) for START and wait for the detector to collect the data. After a few seconds you will be able to choose a graph to look at. Press (ENTER) for DISTANCE.
• Observe the graph. Determine the average time for one complete oscillation (also called the period).

? What factors might affect the period of your clock? Test out your ideas and describe your results below.

? Choose the set up you think would make the best clock. Explain your choice.

 

Station 3: Pendulums

CBL2 Set-Up

• Use the same steps as Station 1 EXCEPT use a 100 as the NUMBER OF SAMPLES.

You can use the motion of a pendulum to keep time fairly reliably and easily.

• Tie the string to the tennis ball and the ring stand to create a pendulum between 30 and 50 cm long. Pull the ball back slightly to create a steady motion. You may need to hold the ring stand.
• Place the motion detector in front of the swinging ball so that it comes no closer than 50 cm.
• Press (2) for START and wait for the detector to collect the data. After a few seconds you will be able to choose a graph to look at. Press (ENTER) for DISTANCE.
• Observe the graph. Determine the average time for one complete oscillation (also called the period).

? What factors might affect the period of your clock? Test out your ideas and describe your results.

? Choose the pendulum you think would make the best clock. Explain your choice here.

 

Station 4: Vibrating Saw BE CAREFUL - THE SAW IS SHARP!

CBL2 Set-Up

• Use the same steps as Station 1 EXCEPT use a 100 as the NUMBER OF SAMPLES.

You can use the motion of a spring to keep time fairly reliably and easily.

• Clamp the saw to the table. Put a ball of play dough on the end of the saw. Pull the end of the saw straight down gently to get a steady bouncing.
• Place the motion detector directly below the ball of dough at least 50 cm away.
• Press (2) for START and wait for the detector to collect the data. After a few seconds you will be able to choose a graph to look at. Press (ENTER) for DISTANCE.
• Observe the graph. Determine the average time for one complete oscillation (also called the period).

? What factors might affect the period of your clock? Test out your ideas and describe your results.

? Choose the arrangement you think would make the best clock. Explain your choice here.

 

Station 5: Tuning Forks

CBL2 Set-Up

• Turn on the calculator. Press (APPS) and choose DATAMATE. The program will start and check for sensors. It should find a MICROPHONE in CH1.
• Press (1) for Setup. Press the up arrow and press (ENTER) to select MODE.
• Press (2) for time graph. We want to use a TIME INTERVAL of .0001 seconds with a NUMBER OF SAMPLES of 200. If this appears on the screen press (1) and skip the next step. If not press (2) and do the next step.
• Enter .0001 and press (ENTER). Enter 200 and press (ENTER). Press (1) for OK.
• Press (1) for OK and return to the main menu. You are now ready to collect data.

Sound vibrations are used quite often in clocks.

• Strike the forked end of the tuning fork against the block of wood and hold it near the microphone.
• Press (2) to START collecting data. After a few seconds a graph will appear.
• Observe the graph. Determine the average time for one complete oscillation (also called the period).
• You may want to experiment with different tuning forks to see how this affects the period.

? How is the size of the tuning fork related to the period of the wave? Why do you think that is?

? What other factors might affect the period of your clock? Test out your ideas and describe your results below.

? Choose the tuning fork you think would make the best clock. Explain your choice here.

 

Station 6: Echo Tube

CBL2 Set-Up

• Turn on the calculator. Press (APPS) and choose DATAMATE. The program will start and check for sensors. It should find a MICROPHONE in CH1.
• This calculator is already set up for the TRIGGER function. That means when you press (2) to START the microphone will wait for a sound to start collecting data.
• It is important that you are very quiet after you select START as other noises may activate the microphone.

The speed of sound can be a tool for measuring time. Here you will measure the time for a sound to traverse down a tube and back (echo).

• Place the tube so the closed end is against a wall.
• Place the microphone in the opening of the tube with the sensor facing down the tube.
• Press (2) for START. When the calculator displays WAITING FOR TRIGGER, make a sharp sound using the wood block or clapping your hands.
• After a few seconds the graph will display. Identify the initial sound and the echo. Use the arrow keys to trace the graph and determine the time it took the echo to return to the microscope.

? If the speed of sound is 330 m/s + (.6 * temp.), how accurate is your average measurement?

? What factors might affect your results?