A simple and convincing demonstration of the intermediate axis theorem. Consider an object (a tennis racquet in this case) with three unequal principle moments of inertia. If the racquet is set into rotation about either the axis of greatest moment or least moment and is thereafter subject to no external torques, the resulting motion is stable. However, rotation about the axis of intermediate principle moment of inertia is unstable — the smallest perturbation grows and the rotation axis does not remain close to the initial axis of rotation.
This demonstration allows you to compare chalk’s compressive strength with its tensile strength.
How it works:
We use railroad chalk, which although being softer and harder to work, is nice and big and easy to see. A sample is placed in each of the two types of testing assembly (details in Setting it Up), and loads carefully applied. Railroad chalk has a tensile strength of 195kNm-2 ± 30kNm-2 (a load of 2.5 to 3.5kg) and a compressive strength of 500kNm-2 ± 65kNm-2 ( a load of 7 to 9kg).
A large square metal plate, supported and harmonically driven at its center, is made to vibrate in any one of its numerous normal modes of vibration. As with the regular Chladni Plates, the two-dimensional standing wave patterns are made visible by sand accumulating along the nodal lines. What is different in this demonstration is that a multitude of resonances (across the entire audio range and lower ultrasonic frequencies) can easily be excited. Being a two-dimensional oscillator, the various resonance frequencies are not simply multiples of the...
Balloons filled with helium, CO2, or SF6 act as diverging and converging lenses, respectively.
What it shows:
A balloon, filled with a gas different from air, will refract sound waves. A gas denser than air turns the balloon into a converging lens and a lighter gas makes it a diverging lens. An air-filled balloon has little effect.
How it works:
The refraction phenomenon occurs whenever waves travel from one medium to another in which the velocity of the wave changes. The amount of refraction at...
Stable levitation of one magnet by another is usually prohibited by Earnshaw's Theorem, but the introduction of diamagnetic material at special locations can stabilize such levitation. The demonstration is a replica of an experiment described by M.D. Simon and A.K. Geim1 and is pictured in the photograph. The illustration is from their paper.