Activity 4 Accelerometers

OBJECTIVE:

To measure the acceleration environments created by different motions.

BACKGROUND: As the Space Shuttle orbits Earth, it collides with thinly spaced gas molecules that produce a minuscule braking effect and eventually causes the Shuttle's orbit to decay. Although not noticeable to astronauts, this braking effect produces a force that is felt by objects inside as an acceleration. Acceleration is the rate at which an object's velocity changes with time. Velocity is defined as both a speed and a direction. If speed changes, direction changes, or both speed and direction change, the object is said to be undergoing an acceleration. In the case of objects inside of a Space Shuttle, the acceleration causes them to slowly drift from one position in the cabin to another. To avoid this problem objects are usually tethered or stuck to the wall with velcro. However, even a very slight acceleration is a significant problem to sensitive microgravity experiments.

In many microgravity experiments, knowing the magnitude and direction of the acceleration inside an orbiting Space Shuttle is important. At what acceleration do gravity-dependent fluid phenomena, such as buoyancy and sedimentation, become insignificant and other phenomena, such as surface tension, predominate? These and many other questions are important areas of microgravity research. In this activity, we will experiment with several methods for measuring acceleration.

MATERIALS NEEDED: Cardboard tube Corugated cardboard Glue (hot glue works best) Rubberband 3 lead fishing sinkers (1 oz. "drilled egg") Marker pen Metric ruler Sharp knife or scissors   PROCEDURE: Step 1. Trim one end of the cardboard tube so that the tube is about 25 ters long. Cut a 3 by 15 centimeter window into the side of the tube as shown in the diagram. (The width of the window may have to be modified depending upon the diameter of the tube.) Step 2. Cut six circles out of the corrugated cardboard equal to the inside eter of the tube. Test the circles to see that they will fit snugly into the tube ends. Step 3. Cut the rubberband so that it forms a long elastic cord. Thread one end of the rubberband through the sinker. You may need a straightened paper clip to help you thread the sinker. Slide the sinker to the middle of the cord and stretch the rubberband. Put a drop of glue in both ends of the sinker. Keep the rubberband stretched until the glue hardens. Step 4. Punch a small hole through the center of the cardboard circles. Glue three of the circles together. As you glue them, aligning the corrugations in different directions to increase strength. You will end up with two circles, each being three layers thick. When the glue has dried, thread one end of the elastic cord through the holes and knot the end. Repeat this step with the other three circles of cardboard. Step 5. Insert the cardboard circles into the opposite tube ends and glue them in place. It is not important to have the elastic cord stretched tight at this stage. Step 6. Lay the tube on its side. Stretch the elastic and tie new knots into its ends so that the lead sinker is positioned in the center of the window. Use the marker pen to mark the edges of the tube where the middle of the sinker is located. Label this position "0."

Step 7. Stand the tube upright and mark where the middle of the sinker is located now. Label this position "1." Turn the tube upside down and mark the tube again where the middle of the sinker is located. Label this position "-1." Step 8. Using a small piece of tape, attach the second sinker to the first and follow step 7 again. This time, mark the positions "2" and "-2." Add the third sinker to the first two and repeat step 7 again. Label the new positions "3" and -3." Remove the extra sinkers. The accelerometer is now calibrated from three times the pull of gravity to negative three times the pull of gravity. Step 9. Use the accelerometer in various motion situations to measure the accelerations produced. To operate, the long direction of the accelerometer must be parallel with the direction of the acceleration which, as in the turning automobile example, may or may not be in the direction of motion. Read the acceleration value of the device by comparing the middle of the sinker to the marks on the tube.

QUESTIONS:

1. What unit is the accelerometer calibrated in? Why did you use the additional sinkers to calibrate the device?
2. What does the device read if you toss it into the air?
3. How does the inner ear work as an accelerometer?
4. Can Faraday's Law be employed to measure acceleration? (Refer to a physics textbook for a discussion of Faraday's Law.)
5. What will the accelerometer read when acceleration stops (such as when a car is moving at a constant speed and direction)?

FOR FURTHER RESEARCH:

1. Take the accelerometer to an amusement park and measure the accelerations you experience when you ride a roller coaster and other fast rides.
2. Construct some of the other accelerometers pictured here. How do they work?
3. Design and construct an accelerometer for measuring very slight accelerations such as those that might be encountered on the Space Shuttle.