Do objects of different size or mass fall at different rates? In the everyday world, the answer is “yes.” However, as first proposed by Galileo, in a vacuum all objects fall at the same rate. The Guinea and Feather Tube recreates the historical device that demonstrated the validity of Galileo’s theory!
- Free-falling objects
- Air resistance
- Air pressure
End plugs, 2 (one with with stopcock valve)*
Guinea and Feather Tube, acrylic, 3 ft*
Vacuum pump and vacuum tubing
*Materials included in kit.
Care should be used when operating a vacuum pump. Please follow the specific directions to your vacuum pump very carefully. Do not evacuate the guinea and feather tube if it is cracked. Do not pressurize the Guinea and Feather Tube; this may cause the end plugs to fly off the ends of the tube. Wear protective eyewear when performing this demonstration.
All materials may be saved for future use.
Coin and Feather Free Fall Demonstration
- Place the feather and coin inside the guinea and feather tube. Snuggly cap both ends of the tube with the appropriate end plugs.
- Hold the guinea and feather tube assembly with one hand on each end plug to prevent the plugs from coming out during the demonstration.
- Demonstrate the free-fall motion of the coin and the feather under normal conditions. Hold the tube in the vertical position, and then quickly flip the tube 180° to make the coin and feather start to fall at the same time. Notice that the feather falls at a much slower rate compared to the coin. Repeat several times.
- Next, connect one end of vacuum tubing to the serrated hose connection on the stopcock valve and the other end to the vacuum valve on a vacuum pump.
- Open the stopcock valve and evacuate the air from the tube with the vacuum pump. Once a vacuum has been established inside the tube, close the stopcock valve and disconnect the guinea and feather tube assembly from the vacuum pump setup.
- Repeat procedure steps 2 and 3. Notice that the coin and the feather fall at the same rate and strike the bottom of the tube at the same time! The difference between the vacuum and normal atmospheric pressure environment should be dramatic.
- After the demonstration, open the stopcock valve to reestablish atmospheric pressure inside the tube.
Fountain in a Vacuum Demonstration
- Remove the end plugs and take out the coin and feather from the tube. Replace the end plugs on the tube.
- Connect one end of vacuum tubing to the serrated hose connection on the stopcock valve of the tube and the other end to the vacuum valve on a vacuum pump.
- Open the stopcock valve and evacuate the air from the tube with the vacuum pump. Once a vacuum has been established inside the tube, close the stopcock valve and remove the tube from the vacuum pump setup leaving the vacuum tubing connected to the serrated hose connection on the stopcock valve of the tube.
- Immerse the free end of the vacuum tubing in a reservoir of water. (A sink or large bucket filled with water will work.)
- Open the stopcock valve on the guinea and feather tube assembly (with the vacuum tubing underwater). The water from the reservoir will quickly rush into the tube creating a “fountain-like” effect.
- Empty the water from the tube and completely dry the tube, end plugs and stopcock valve before storage.
- Establishing a good vacuum is essential to show the dramatic difference between the free fall of the coin and feather in air compared to in a vacuum. A two-stage vacuum pump provides a very effective vacuum for this demonstration.
- Students can practice this activity themselves. Have each student obtain a textbook and a sheet of notebook paper. Hold the book in one hand and paper in the other with the cover of the book and the sheet of paper parallel to the floor. Raise them to the same height and release them at the same time. The book will reach the ground first and the sheet of paper will “float” down. Then position the sheet of paper on top of the book so that the ends do not extend beyond the edge of the book. Raise the book and paper to the same height as before and release the book. The book and paper reach the ground at the same time. Air resistance did not affect the sheet of paper. If the acceleration of falling objects was dependent on mass, then the paper would have fallen slower than the book.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Planning and carrying out investigations
Developing and using models
Disciplinary Core Ideas
MS-PS2.A: Forces and Motion
HS-PS2.A: Forces and Motion
Cause and effect
Systems and system models
MS-PS2-2: Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object
MS-PS2-4: Construct and present arguments using evidence to support the claim that gravitational interactions are attractive and depend on the masses of interacting objects
HS-PS2-2: Use mathematical representations to support the claim that the total momentum of a system of objects is conserved when there is no net force on the system.
HS-PS2-4: Use mathematical representations of Newton’s Law of Gravitation and Coulomb’s Law to describe and predict the gravitational and electrostatic forces between objects.
Galileo Galilei (1564–1642) was the first person to claim that all objects fall at the same rate, regardless of their size, shape or mass. He had difficulty convincing the people of his generation of this because everyday experience seemed to suggest otherwise. Everyone at the time knew that heavy objects, such as cannon balls, fell faster than light objects, such as feathers. Legend has it that Galileo attempted to convince his skeptics by dropping two cannon balls, one heavier than the other, from the Leaning Tower of Pisa. It was not until the end of the 17th century (after Galileo’s death) that the ability to obatin a vacuum had greatly improved. A device consisting of an enclosed glass tube attached to a vacuum pump, a feather and an English guinea (coin) was built and used by Robert Boyle (1627−1691) in 1660 to prove that finally demonstrated that Galileo was correct. The guinea and feather were released from the same height at the same time and when there was air inside the tube, the guinea and feather fell as they normally would, with the guinea striking the bottom of the glass tube first. However, once a vacuum was established, the coin and the feather fell through the tube at the same rate and reached the bottom at the same time. This proved Galileo’s theory that all objects fall at the same rate due to gravity. The reason why the feather falls slower than a coin is because of air resistance. Air creates friction and drag for all objects that travel through it. This drag has a tendency to slow down lighter objects (or objects with greater surface area) more than heavier objects (or objects with less surface area), which is why a feather “floats” to the ground while a coin “falls” quickly.
The vacuum fountain is a simple demonstration of air pressure. The atmospheric pressure on the surface of the water in the reservoir is 14.7 lb/in2. The pressure inside the evacuated tube is significantly less(approaching 0 lb/in2 for an absolute vacuum). Therefore, when the stopcock valve is opened, atmospheric pressure exerts a large force on the surface of the water which is not balanced by pressure inside the tube. Thus, water is forced into the tube by air pressure above the surface until the pressure equalizes. The small opening and large, unbalanced force causes the equalizing process to create a dramatic fountain-like geyser.
Institute and Museum of History of Science, Florence, Italy, http://galileo.imss.firenze.it/vuoto/eesper7.html (accessed April 2017)
Physics Demonstrations, J. C. Sprott, http://sprott.physics.wisc.edu/demobook/chapter1.htm (accessed April 2017)