Home Show Overview Demonstrations Topics Covered Scheduling Info Feedback


The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


Standing on Light Bulbs
Figure I. Figure II.

Description

Equipment: Triangular piece of wood designed with wires and inlets to allow 3 light bulbs to be screwed into the wood. An electrical cord allows the light bulbs to be plugged in. In addition, a remote switch has been added to the circuit. The triangle is an equilateral triangle with sides of length 20 inches.

3 (40 Watt) light bulbs

Step 1: The light bulbs are screwed in and the piece of wood is placed so that it is entirely supported by the light bulbs. The cord is plugged into the wall. A demonstrator carefully places all his weight onto the piece of wood. Remarkably, the light bulbs support him! (See Figure I.)

Step 2: The demonstrator steps off of the wood and asks for a volunteer from the audience to give it a try. The volunteer slowly steps onto the piece of wood. As he/she does so, the light bulbs slowly light up! (See Figure II.)

Explanation

Basic Ideas: At the Earth's surface, all objects experience a downward force due to gravity. This force depends on the object's mass: the greater the mass, the greater the force.

Although light bulbs are made of very thin glass, they are designed in a special shape called a catenary. A catenary shape is the shape which is formed if a string is held at both ends and the string is allowed to sag. As it turns out, this shape is very strong and can support much more weight than other round shapes.

Step 1: When the demonstrator steps onto the piece of wood, his mass is exerting a force downward on the light bulbs. Due to the catenary design of the light bulbs, they are able to support this force.

Step 2: When a volunteer from the audience steps onto the piece of wood, he/she is exerting a force downward on the light bulbs, just as the demonstrator was in Step 1. Again, the light bulbs are able to support this force. This time, however, the light bulbs light up! The reason for this has nothing to do with physics. The demonstrator cleverly controls the light bulbs with the remote lighting switch. As the volunteer steps onto the piece of wood, the demonstrator slowly turns the lights on.

More Specifically

Basic Ideas: At the Earth's surface, Fg = mg, where Fg = force due to gravity, m = mass, and g = gravitational acceleration.

Although light bulbs are made of very thin glass, they are designed in a special shape called a catenary. A catenary shape is the shape which is formed if a string is held at both ends and the string is allowed to sag. Because of the reinforcing nature of this shape, it is very strong and can support much more weight than other round shapes. The exact physics behind the catenary are very complex, but the general idea is that each small portion of the shape is reinforcing the other portions.

To perform the calculation, we have measured the mass of the demonstrator to be approximately 150 pounds, or 70 kilograms.

Step 1: When the demonstrator steps onto the piece of wood, his mass is exerting a force downward on the light bulbs. This force is calculated to be 686 Newtons (See calculation below). This means that each light bulb is supporting approximately 230 Newtons of force. Due to the catenary design of the light bulbs, they are able to support this force.

Fg = 70 (kg) * 9.8 (m/s2) = 686 (N)

Step 2: When a volunteer from the audience steps onto the piece of wood, he/she is exerting a force downward on the light bulbs, just as the demonstrator was in Step 1. Again, the light bulbs are able to support this force. This time, however, the light bulbs light up! The reason for this has nothing to do with physics. The demonstrator cleverly controls the light bulbs with the remote lighting switch. As the volunteer steps onto the piece of wood, the demonstrator slowly turns the lights on.

Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia