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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.


DOLLAR BILL GRAB

Figure I. Figure II. Figure III.

Description

Equipment: 2 ordinary 8 oz. glass Coke bottles
Dollar bill

Step 1: Two Coke bottles and a dollar bill are arranged as follows. One Coke bottle, upside down and filled with water, has been placed on top of another Coke bottle, with a dollar bill placed in between the two bottles. (See Figure I. -- Note that blue food coloring has been added to the water to make the water visible in the pictures.) The audience is asked what will happen if the dollar bill is removed.

Step 2: The dollar bill is yanked out (See Figure II.) and the water from the top bottle slowly drains into the bottom bottle. The demonstrator must be careful and ready at any moment to catch the top bottle. Sometimes, a small amount of water is spilled. After all the water has drained, the bottom bottle is now full of water and the top bottle has fallen to the table. (See Figure III.)

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.

Dollar bills are made out of a material which acts as a water sealant.

Step 1: When the equipment is set up, nothing is moving. Dollar bills are made out of a material which acts as a water sealant. This means that no water can pass through the dollar bill. The water in the top bottle is experiencing a downward force due to gravity. The water is being supported, however, by the dollar bill. The water is experiencing an upward force due to the dollar bill. Since the forces acting on the water are balanced, the water does not move.

Step 2: When the demonstrator yanks out the dollar bill, the water in the top bottle is no longer being supported by the dollar bill. The force of gravity is now the only force acting on the water. Since there is an unbalanced force acting on the water, the water falls straight down, slowly draining into the bottom bottle. The demonstrator must be careful when the water is draining. Water drains in such a manner that causes the top bottle to teeter back and forth, and the bottle usually falls. In general a small amount of water is spilled, but not very much.

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.

Dollar bills are made out of a material which acts as a water sealant.

To perform the calculation, we have measured the mass of the water in the top Coke bottle to be 250 grams.

Step 1: When the equipment is set up, nothing is moving. Dollar bills are made out of a material which acts as a water sealant. This means that no water can pass through the dollar bill. The water in the top bottle is experiencing a downward force due to gravity. This force is calculated (see calculation below) to be 2.45 Newtons. The water is being supported, however, by the dollar bill. The water is experiencing an upward force due to the dollar bill. We know that this force must be 2.45 Newtons as well. Since the net, external force acting on the water is equal to zero, the water (which is at rest) does not move.

Fg = 0.25 (kg) * 9.8 (m/s2) = 2.45 (N)

Step 2: When the demonstrator yanks out the dollar bill, the water in the top bottle is no longer being supported by the dollar bill. The force of gravity is now the only force acting on the water. Since there is a net, external force acting on the water, the water falls straight down, slowly draining into the bottom bottle. The demonstrator must be careful when the water is draining. Water drains in such a manner that causes the top bottle to teeter back and forth, and the bottle usually falls. In general a small amount of water is spilled, but not very much.

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