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Accelerometers
CONTENTS
Activity Management: This activity provides students with the plans for making a one-axis accelerometer that can be used to measure acceleration in different environments ranging from +3 g to -3 g. The device consists of a triangular shaped poster board box they construct with a lead fishing sinker suspended in its middle with a single strand of a rubber band. Before using the device, students must calibrate it for the range of accelerations it can measure. The pattern for making the accelerometer box is included in this guide. It must be doubled in size. It is recommended that several patterns be available for the students to share. To save on materials, students can work in teams to make a single accelerometer. Old file folders can be substituted for the poster board. The student reader can be used at any time during the activity. The instructions call for three egg (shaped) sinkers. Actually, only one is needed for the accelerometer. The other two are used for caiibrating the accelerometer and can be shared between teams. When the boxes are being assembled, the three sides are brought together to form a prism shape and held securely with masking tape. The ends should not be folded down yet. A rubber band is cut and one end is inserted into a hole punched into one of the box ends. Tie the rubber band to a small paper clip. This will prevent the end of the rubber band from sliding through the hole. The other end of the rubber band is slipped through the sinker first and then tied off at the other end of the box with another paper clip. As each rubber band end is tied, the box ends are closed and held with more tape. The two flaps on each end overlap the prism part of the box on the outside. It is likely that the rubber band will need some adjustment so it is at the right tension. This can be easily done by rolling one paper clip over so the rubber band winds up on it. When the rubber band is lightly stretched, tape the clip down. After gluing the sinker in place on the rubber band, the accelerometer must be calibrated. The position of the sinker when the box is standing on one end indicates the acceleration of 1 gravity (1 g). By making a paper clip hook, a second sinker is hung from the first and the new position of the first sinker indicates an acceleration of 2g9. A third sinker indicates 3 g. Inverting the box and repeating the procedure yields positions for negative 1, 2, and 3 g. Be sure the students understand that a negative g acceleration is an acceleration in a direction opposite gravity's pull. Finally, the half way position of the sinker when the box is laid on its side is 0 g. Students are then challenged to use their accelerometers to measure various accelerations. They will discover that tossing the device or letting it fall will cause the sinker to move, but it will be difficult to read the scale. It is easier to read if the students jump with the meter. In this case, they must keep the meter in front of their faces through the entire jump. Better still would be to take the accelerometer on a fast elevator, on a trampoline, or a roller coaster at an amusement park. Assessment:
Acceleration is the rate at which an object's velocity is changing. The change can be in how fast the object is moving, a direction change, or both. If you are driving an automobile and press down on the gas pedal (called the accelerator), your velocity changes. Let's say you go from 0 kilometers to 50 kilometers per hour in 10 seconds. Your acceleration is said to be 5 kilometers per hour per second. In other words, each second you are going 5 kilometers per hour faster than the second before. In 10 seconds, you reach 50 kilometers per hour. You feel this acceleration by being pressed into the back of your car seat. Actually, it is the car seat pressing against you. Because of the property of inertia, your body resists acceleration. You also experience acceleration when there is a change in direction. Let's say you are driving again but this time at a constant speed in a straight line. Then, the road curves sharply to the right. Without changing speed, you make the turn and feel your body pushed into the left wall of the car. Again, it is actually the car pushing on you. This time, your acceleration was a change in direction. Can you think of situations in which acceleration is both a change in speed and direction? The reason for this discussion on acceleration is that it is important to understand that the force of gravity produces an acceleration on objects. Imagine you are standing at the edge of a cliff and you drop a baseball over the edge. Gravity accelerates the ball as it falls. The acceleration is 9.8 meters per second per second. After 5 seconds, the ball is traveling at a rate of nearly 50 meters per second. To create a microgravity environment where the effects of gravity on an experiment are reduced to zero, NASA would have to accelerate that experiment (make it fall) at exactly the same rate gravity does. In practice, this is hard to do. When you jump into the air, the microgravity environment you experience is about 1/100th the acceleration of Earth's gravity. The best microgravity environment that NASA's parabolic aircraft can create is about 1/1000th g. On the Space Shuttle in Earth orbit, microgravity is about one-millionth g. In practical terms, if you dropped a ball there, the ball would take about 17 minutes just to fall 5 meters! Accelerometer Construction and Calibration The instructions below are for making a measuring device called an accelerometer. Accelerometers are used to measure how fast an object changes its speed in one or more directions. This accelerometer uses a lead weight suspended by a rubber band to sense changes in an object's motion.
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