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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.


PENCIL DROP

Figure I. Figure II.

Description

Equipment: Ordinary 8 oz. glass Coke bottle
Wooden crochet hoop 10 inches in diameter
Small pencil (2.5 grams)

Step 1: The Coke bottle is placed on a table. The wooden hoop is balanced on top of the Coke bottle and the pencil is balanced on top of the hoop. (See Figure I.) The demonstrators ask how it might be possible to get the pencil in the Coke bottle without touching either the pencil or the Coke bottle.

Step 2: The demonstrator very swiftly grabs the wooden hoop in an angle downwards, bringing the hoop away from the Coke bottle. Considering the top of the hoop 12 o'clock and the bottom of the hoop 6 o'clock, the demonstrator generally grabs the hoop at around 4 o'clock. With a little luck, the pencil drops directly into the Coke bottle. (See Figure II.) (After a little practice, a success rate of about 70% can be acheived.)

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.

A stationary object will not move unless it is acted on by an unbalanced force.

Step 1: When the equipment is set up, nothing is moving. The pencil is experiencing a downward force due to gravity. The pencil is being supported, however, by the hoop. The pencil is experiencing an upward force due to the hoop. Since the forces acting on the pencil are balanced, the pencil does not move.

Step 2: When the demonstrator swifly grabs the hoop, the pencil is no longer being supported by the hoop. The force of gravity is now the only force acting on the pencil. Since there is an unbalanced force acting on the pencil, the pencil falls straight down, landing in the Coke bottle. The demonstrator must be careful to grab the hoop in a downward angle to avoid giving the pencil a little push in another direction. Even a little push will knock the pencil off course.

More Specifically

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

An object at rest will remain at rest unless it is acted on by a net, external force.

Step 1: When the equipment is set up, nothing is moving. The pencil is experiencing a downward force due to gravity. This force is calculated (See calculation below) to be 0.25 Newtons. The pencil is being supported, however, by the hoop. The pencil is experiencing an upward force due to the hoop. We know that this force must be 0.25 Newtons as well. Since the net, external force acting on the pencil is equal to zero, the pencil (which is at rest) does not move.

Fg = 0.025 (kg) * 9.8 (m/s2) = 0.25 (N)

Step 2: When the demonstrator swifly grabs the hoop, the pencil is no longer being supported by the hoop. The force of gravity is now the only force acting on the pencil. Since there is a net, external force acting on the pencil, the pencil falls straight down, landing in the Coke bottle. The demonstrator must be careful to grab the hoop in a downward angle to avoid giving the pencil a little push in another direction. Even a little push will knock the pencil off course.

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