Experiment objective

To demonstrate the conservation of angular momentum, a basic principle of physics. (Angular momentum can be thought of as the energy of turning. When a car is turning, you feel angular momentum as a pulling force to the outside. It’s why you lean into a turn.)

Read more background about this activity in Gerhard Herzberg's bio...

You need

  • A revolving chair or stool.

Sit in the revolving chair or stool. Now, holding your arms stretched out at your sides, spin yourself around, or have someone else spin you. As you spin, quickly bring your arms in. You should notice a change in your rate of rotation. Is it faster or slower? This change you feel is caused by the conservation of angular momentum, a basic principle of physics. When you tighten the circle of turning, the energy of turning has to go somewhere and it ends up making you turn faster. Figure skaters use this too, when they bring their arms in to spin faster and faster. Conservation of energy is a law of Nature according to modern science. The energy of turning which is present in all spinning molecules and atoms must go somewhere when things change like they do in a chemical reaction or in burning, heating, or freezing. Among other things, Herzberg showed exactly how the lines in a spectrogram represent the conservation of angular momentum in a molecule as its spinning electrons move between different quantum levels of energy. Certain lines in a spectrogram show the different energy levels that a molecule can have and they also fit into a clever mathematical system called quantum mechanics, a theory that underlies most of modern physics. Quantum mechanics says that many things in nature happen in discrete steps called quantum levels.