The Guinea and Feather is one of the most important experiments in classical physics and it is still performed today as a lecture demonstration for thousands of introductory physics students. It shows that the Earth's gravity accelerates all objects equally, regardless of mass i.e. that a heavy object (a British guinea) and a light object (a feather) will fall at exactly the same rate. This concept of uniform gravitational acceleration was proposed by Galileo Galilei (1564-1642) and directly contradicted the previous claim by Aristotle (384-322 B.C.) that heavier objects fall faster. Of course if you drop a coin and a feather under normal (atmospheric) conditions, the coin will hit the ground first. But Galileo reasoned that there was another force at work slowing down the feather. That force was air resistance and Galileo claimed that under conditions without air resistance, all objects would accelerate equally. Unfortunately the techniques for creating a sufficient vacuum did not exist at the time to prove Galileo correct.

Isaac Newton (1643-1727) proved mathematically that Galileo was correct. Newton's law of universal gravitation states that the attraction between two objects M and m placed a distance r apart (from center to center) is given by F = GMm/r2 , where G is the gravitational constant. But Newton's second law states that the force (F) on an object (m) is related to it's acceleration (a) by F = ma. Combining these two equations we get F = GMm/r2 = ma, or a = GM/r2. Thus, the gravitational acceleration of an object is only dependent on the mass of the Earth (M), it's distance from the Earth's center (r), and the gravitational constant G. We call this acceleration due to gravity g to distiguish it from acceleration due to some other force. The precise value of g varies with location and the local Earth density, but standard gravity is defined as g = 9.80665 m/s2.

Newtons third law states that forces exist in equal and opposite pairs. This means that for a given force F1 acting on a small mass m resulting in an acceleration a1, the Earth also experiences a force F2 on its mass M resulting in an acceleration a2. The total acceleration of the Earth and the small mass m is just the sum a1 + a2 = G(M + m)/r2. However, the mass of the Earth (5.98 x 1024 kg) is many many orders of magnitude greater than the small mass m so M + m is essentially M and a2 is essentially 0. If the mathematics are hard to read here, check out the Wikipedia articles on Universal Gravitation and Earth's gravity.

This experiment has even been performed on the moon by Apollo 15 Commander David Scott using a 1.32 kg geological hammer and a 0.03 kg falcon feather (Falcon was the name of the lunar module). Because there's no atmosphere on the moon, the hammer and the feather landed on the lunar surface simultaneously. This was broadcast live in 1971 and a link to the video is below.

On a historical note, the guinea was the British gold coin first used in this experiment and in the US it's sometimes called the Penny and Feather. Many current versions use a piece of ordinary metal instead of a coin, and a piece of paper instead of a feather.

Please note that the title picture is from the PIRA 200 demonstration list and is copyright 1993 by the University of Minnesota and the Physics Instructional Resource Association. According to this webpage, the title image is released for nonprofit educational use only. All pictures used for other nonprofit purposes, need to retain the signature of their creators...

Step 1: Motivation for a new apparatus

Typically, the Guinea and Feather demonstration consists of a transparent tube containing two different masses and a method of attaching a vacuum pump. The tube is held vertically with both objects at the bottom, and then quickly flipped upside down. This is a fast and reliable way of starting both objects falling from close to the same height at close to the same time.

Several companies sell Guinea and Feather demonstrations, placing both objects together in the same tube. The problem with this configuration is that, at atmosphere, the heavy object can push the light object down underneath it, so it appears that they are falling at the same rate. Another major problem with the commercial models is that the tube diameter does not permit large objects. This means that someone sitting in the back of a large lecture hall may not be able to see the full effect of the demonstration. Finally, some single tube models are designed so that it is impossible to replace the objects within. While it may not be necessary to replace the objects, it's nice to at least have that option.

A far more instructive demonstration places the two objects, each in their own tube, next to each other for a direct comparison. This eliminates the possibility of the heavier object being situated above the light object and interferring during free fall.

There are a few pictures of dual tube variations online at other universities, but as far as I know, they are not commercially available. We used the dual tube apparatus below until it fell off the table and became a one-time-only demonstration of the exact same principle. The two problems with the old dual tube apparatus were: (1) The shortened tube length did not permit a long enough fall time, and (2) It was impossible to replace the light object (dried up and disentigrating masking tape).
That is a beautiful thing. Thank you.
well done!! very nice gadget that shows exactly what its meant to show.
Out<em>standing</em>!<br/><br/>I am so jealous. I teach science to kids aged 9-13 - this is such an important experiment, such a wonderful piece of equipment, and we don't even have a working vacuum pump.<br/><br/>(Serious point - you ought to consider selling this thing, or licensing somebody to make and sell them for you)<br/>
<a rel="nofollow" href="http://www.instructables.com/id/E791HNXF23Z39P6/?ALLSTEPS">Make a vacuum pump</a>Kiteman, This may help in your class.<br/>
It didn't seem all that great, until I saw the video...that changed everything.
I love it.

About This Instructable


4 favorites


Bio: I have B.S. degrees in both Physics and Electrical Engineering. I do Lecture Demonstrations for the University of Washington Department of Physics. I don ... More »
More by sbtroy: The Chaos Machine (Double Pendulum) Building a better Guinea and Feather Build a 15,000 rpm Tesla Turbine using hard drive platters
Add instructable to: