Acoustic Horn

What it shows:

As a passive amplification device, the exponential horn is amazing. Using a "talking" greeting card as a feeble source of music, the intensity of the sound gets amplified by about 18 dB when the greeting card is coupled to the horn ... a dramatic effect.

How it works:

The multicellular horn is a cluster of eight smaller exponential horns, each with a small mouth to avoid beaming in a large frequency range, but together they form a sector of a sphere large enough to control directivity at low frequencies — the...

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Hear the Wall Bend

What it shows:  A room-size laser interferometer with audio signal output. A standing wave is produced whenever a wave is reflected back on itself. A resonant cavity requires a second reflection so that the twice reflected wave has the opportunity to be in phase with the original wave. Here, laser light is reflected from a half-silvered mirror (mounted on a wall) so as to return to the laser and be reflected again by the laser. Movement of the wall by half a wavelength is sufficient to change the cavity formed between laser mirror and wall mirror from one resonant...

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Kepler's Machine

Working model to show Kepler's 1st and 2nd laws and the equivalence of the area law to the law of equants.

What it shows:

A demonstration illustrating the equivalence of Kepler's second law, the Law of Areas, with the Law of Angles.

How it works:

In order to determine the orbit of Mars using circular orbits, Kepler had to offset the focus of Mars' orbit from the Sun to a point C (figure 1). Kepler's 2nd Law of planetary motion states that a planet's orbit around the Sun will sweep out equal areas in equal times....

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Inertia of Rest

Concrete block smashed on lecturer's chest with sledge hammer.

What it shows:

The lecturer (or someone else) lies on a bed-of-nails without discomfort, thus demonstrating the concept of pressure, which is the force per unit area. For added drama the person is sandwiched between two beds of nails with the added weight of a cinder block on top. The cinder block can be broken with a sledge hammer.

How it works:

The forces (weight of the body, cinder block, etc.) are distributed over the total area of all the nail...

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Parallel-Axis Theorem

What it shows:

One can show that the period of oscillation of an object doesn't change for different suspension points, as long as they're the same distance from the COM. This is consistent with what the parallel-axis theorem tells us about the moment of inertia of the object.

How it works:

The parallel-axis theorm states that if \(I_{cm}\) is the moment-of-inertia of an object about an axis through its center-of-mass, then \(I\), the moment of inertia about any axis parallel to that...

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Organ Pipes

Selection of single organ pipes, open and close-ended, to blow through.

organ pipes

Atwood's Machine

Combinations of weights suspended over pulley to show that asymmetry causes acceleration.

atwood's machine

Image on the left, of a lightweight plastic pulley with balanced 50 g brass weights, and on the right, the pulley in motion as the unbalanced weights accelerate. 

Hula Hoop Rotational Inertia

What it shows:

A suspended hula hoop has the same period of oscillation as a pendulum whose length is equal to the diameter of the hoop.

How it works:

The parallel-axis theorem allows us to readily deduce the rotational inertia of a hoop about an axis that passes through its circumference and is given by

\(I = I_{cm} + MR^2 = 2MR^2\)

where M is the mass of the hoop and R is its radius. The period of oscillation thus becomes


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