How to explain to a child what a Foucault pendulum is

Recall that the Foucault pendulum is an experimental device with which you can visually observe the daily rotation of the Earth. It is a fairly long (in the original design of Jean Foucault length was 67 m) steel wire, on which the load is suspended. Over time, the plane of oscillation of the pendulum changes, slowly turning in the direction opposite to the direction of rotation of the Earth, and the geographical position (latitude) of the device affects the rate of change.

It is very difficult to imagine a plane motionless relative to stars and, correspondingly, rotating relative to the Earth. The Earth is too large, its apparent “plane” is too familiar to us, and it is completely impossible to feel the rotation on ourselves. The Foucault pendulum clearly shows us the effect of daily rotation, but even looking at it, it is not always easy to understand and accept its “testimonies”.

Tool kit Plate; three forks; wine cork; lime or any other object of similar parameters, easily pierced by a needle; two sewing needles; spool of thread; salt.

But imagine a situation: a son comes up to you and asks: dad, I read about some Foucault pendulum, which proves that the Earth is spinning, and did not understand anything. Can you explain something simpler to me? Of course, you answer and build a model of the pendulum right in the kitchen.

Limes and needles

You can build a model from almost anything, you can make it more beautiful, bigger, more photogenic. We preferred to use simple objects that can be found in just about any kitchen in a matter of minutes. You don’t even need to go to the store.

If you rotate the plate evenly (for example, by placing it on a spinning disk), the tip of our pendulum will describe in salt a figure similar to that described by a real Foucault pendulum.

So, a plate, three forks, two needles, a cork, some kind of cargo (lime, potato, a small apple), a spool of thread, salt. A plate plays the role of the Earth, and the table on which it stands is a fixed coordinate system in which the Earth rotates (in other words, stars). From all this, it is easy to build the design shown in the first photo. The most difficult thing is to choose the length of the thread so that the tip of the needle almost touches the surface of the plate. It is very important to maintain alignment, that is, make sure that the tip of the needle comes out of the very middle of the fetus, which is used as a load.

Then we start the system - it is best to pull the load aside and let go. The pendulum begins to hesitate. If we rotate the plate around the axis, it turns out that the pendulum does not rotate with it, but continues to oscillate in a constant plane! Salt in this case is used for clarity - when you turn the plate, the tip of the needle draws a new trajectory.

The longer the thread, the longer the pendulum will oscillate with sufficient amplitude so that it would be interesting to observe the experiment from the side.

Now it’s enough to imagine that the plate is very large - with a diameter of the Earth. And she spins around, as Galileo used to say, according to legend, on her own, just as we rotate the plate with our hand. And the Foucault pendulum, going down from the dome of the Moscow planetarium or the Paris Pantheon, writes out an intricate figure, constantly changing the plane of oscillations relative to the Earth. More precisely, this Earth is changing its position relative to the pendulum. Like a plate.

Also on the topic is worth reading about our experiments with kinetic sand and flame propagation through paraffin vapor.

The article “Foucault's Kitchen Pendulum” was published in the journal Popular Mechanics (No. 3, March 2015).