Small merry-go-round

Exhibit on display in Stanislaw Lem Garden of Experiences, a branch of the Museum of Municipal Engineering in Krakow.

Description written by Olga Odziemczyk

There are two single-person carousels here, both serving the same experiment. The difference is that on one of them you spin while sitting, on the other – standing. The sitting version is safer.

The sitting model is the one closer to the garden path. The merry-go-round is mounted on a metal base plate 90 by 90 centimetres. The plate is slightly above the ground. The merry-go-round is about 70 cm high. Its construction is simple and it consists of a low-mounted, octagonal rotating platform, in the centre of which is a short, vertical post with a U-shaped seat at the top. You should straddle the seat and place your feet on the rotating platform.

The standing model is located further back on the gravel-covered part. This carousel is a round, low-mounted rotating platform surrounded by a metal railing. The railing is made of vertical posts connected at the top and bottom with flat rings. The entire apparatus is about 90 centimetres high. Part of the railing is hinged and serves as a door, which must be opened to step onto the platform. The doors have two horizontal, semi-circular handles on the outside, at the top and bottom. To open the door, you must locate the catch near the handle on the upper ring, and release it by moving the end of the catch toward the handle. After stepping onto the rotating platform, you must close the door. The rotating platform is round and measures 68 cm in diameter.

To perform the experiment, you’ll also need weights. You’ll find the half-kilo dumbbells on the platform or on the ground near it.

To start with, choose which model you want to use, then stand or sit on it accordingly. Take the weights, hold them close to your body and ask someone to spin you. As you’re spinning, start moving the weights away from your body by stretching your arms. Hold your arms in the outstretched position for a while, then start bringing the weights closer to your body again and hold the last phase of that motion for a while again.

Notice how the speed of your spinning changes. Do you spin faster with your arms outstretched or when you hold them close to your body? It should be easy to detect that you spin much faster with your hands close to your body. This is explained by the principle of conservation of angular momentum. Angular momentum is connected with the mass of the system, its dimensions and rotational speed, called angular velocity. During the experiment, the mass of the system does not change, but by moving your arms, you change its dimensions, which results in an inversely proportional change of angular velocity. Moving your hands apart, you increase the system’s dimensions, which causes a decrease in angular velocity. Bringing your hands closer, you decrease the dimensions, so the speed of rotation increases. This effect is used, among others, by dancers performing pirouettes.

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