Simple Accelerometer

1 Simple Accelerometer

It is possible to make an accelerometer in about 20 minutes, using a lead weight, two rubber bands, a dowel rod, and some bailing wire.

Figure 1: Simple Home-Made Accelerometer

The dowel rod is 24 inches long and 0.5 inches in diameter.

First, make a “carrier” as shown in the middle of the diagram. It is made of a single piece of bailing wire. The carrier wire has a straight portion about 4 inches long in this example; adjust this to fit the length of your weight. The straight portion segues into loop at each end. Each loop slides freely on the rod. During construction, twist the loop a couple of times to provide structural strength, in accordance with time-honored bailing-wire construction principles. Use the very last bit of length at each end, as it emerges from the twist, to make pegs to which the lead weight attaches.

For the weight, I used a 3 ounce piece of lead that had fallen off somebody’s tire. I picked it up from the roadside, and straightened it using an anvil and a small hammer. I drilled small holes to receive the pegs.

In the figure, I pulled the weight off one of the pegs, so you can see the straight section of the carrier wire. Normally the weight would be flush against the straight wire.

Near each end of the stick, install a loop of bailing wire, twisted tight so it doesn’t slide, to serve as a retainer for the rubber band.

Cut open each rubber band to make a single filament, rather than a loop. Attach one end to the carrier using two half hitches. Attached the other end to the retainer near the end of the stick; you can use half hitches, or just pinch it between the wire and the wood.

Using flimsier rubber bands and/or a heavier weight will maximize the sensitivity of the instrument.

The instrument measures the projection of g along its length. When horizontal, as shown in the figure, it reads zero. If you hold it with the “top” end up, it reads +1. If you hold it the other way up, it reads -1.

Select one of the loops on the carrier to use as a pointer, and mark the stick with appropriate readings. (My markings are not visible in figure 1.)

For starters, just play with the instrument. Hold it horizontal, and wiggle it back and forth along its length, to convince yourself that it really does measure acceleration. Then hold it vertically right-side-up, then turn it upside-down. Dandle it up and down.

You can also attach a string and swing the instrument around and around to create a centrifugal field. Observe the angle of the string, and the corresponding accelerometer reading: a 45 degree angle should correspond to 1.41 Gees, and a 60 degree angle should correspond to 2 Gees.

To use the instrument to measure the weightlessness of free fall, hold it high with one hand, drop it, and catch it with the other hand. Catch it so that you clutch the weight and the stick together, so as to preserve the free-fall reading, which you can then examine at leisure.

2 References

1.: John Denker “Can You Feel Gravity?” ./gravity-perception.htm
2.: Joachim Köppen, “The Flimsy AcceleroMeter” http://astro.u-strasbg.fr/~koppen/weightless/fam/index.html

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