A Simple Accelerometer Made of a Glass Jar and a Cork

Acceleration a (a for acceleration) is an important physical quantity for describing a movement. But how is it defined? Obviously, acceleration a is linked to a change in speed. If, for example, a car increases its speed, it accelerates.

But when it brakes, it also changes its speed and accelerates. In this case, however, the acceleration is negative. The definition of acceleration a is: a = dv/dt. a is therefore the change in speed per second. If a car changes its speed within 2 seconds from v1 = 50 m/s to v2 = 66 m/s, the acceleration is a = 66 - 50 / 2 = 8 m/s².

But does the speed v always have to change in magnitude for acceleration? The speed v and the acceleration a are vectors. This means that they not only have an amount, but also a direction. For an acceleration a not equal to 0, the speed must change in magnitude and/or direction. Let's assume that a child is moving in a circle on a carousel at a constant speed (in terms of magnitude).

However, the direction of the speed is constantly changing. Therefore, the child also accelerates during its circular movement. In this specific case, the acceleration (= change in the speed vector) points inwards and this acceleration is called centripetal acceleration. It is calculated as a_zp = v²/r with the circular orbit radius r.

For the accelerometer you only need

• a pickle jar with a lid
• a cork
• some string
• hot glue and
• water

Use hot glue to attach a piece of string to the bottom of the cork. Then glue the other end of the string to the inside of the lid. The string should be long enough so that when the string is taut, the cork is about 2/3 of the way up the pickle jar. Then fill the pickle jar completely with water and close the lid. Your accelerometer is ready....

Place the glass filled with water on a table with the lid facing down. Now ask your students where the cork will move if you move the glass quickly to the left, for example. The vast majority of students will answer that the cork will move to the right due to its inertia. Now perform the experiment.

To everyone's surprise, the cork will not move to the right as predicted, but instead it will move to the left. Exactly in the direction of the acceleration. But why is this? Well, not only the cork is inert, but also the water inside the pickle jar. If the glass accelerates to the left, the water in the glass remains behind due to its inertia and is pushed to the right. This increases the water pressure in the water from left to right. On the right side of the cork, the water pressure is therefore slightly higher than on the left side. This causes the cork to feel a force to the left and it moves exactly in the direction of the acceleration.

To prove this, you can also move your accelerometer quickly in a circular path. As we learned in the introduction, a circular motion results in an inward acceleration. And that is exactly where the cork moves, voila. With your accelerometer you can not only mislead others, but also have the direction of the acceleration acting during a movement shown to you very clearly. This makes it ideal for science lessons. Have fun experimenting...

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