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
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).
Step 2: Before We Begin
The stock material needed for this includes acrylic, aluminum, and 303 stainless steel.
Before we use any power tools, let's take a moment to talk about shop safety. Be sure to read, understand, and follow all the safety rules that come with your power tools. Knowing how to use your power tools properly will greatly reduce the risk of personal injury. And remember this: there is no more important safety rule than to wear safety glasses.
Step 3: Top O-ring Block
The O-rings are 2 7/8" ID x 3 1/4" OD x 3/16" wide, McMaster-Carr P/N 9452K56. The through hole, counterbore, and O-ring grooves were machined on a lathe using a 4-jaw chuck. To cut O-ring grooves you can either buy parting/grooving tools, or grind your own from high speed steel tool blanks. I ground my own from a 3/16" HSS tool blank.
There are eight untapped through holes in this piece, the top cap block, and the counterweight for 1/4"-20 x 3" long stainless steel socket head cap screws.
Step 4: Top Cap Block
The second picture is a section view showing the top two blocks and the O-rings. The scaled image is dim and hard to see, but it looks better enlarged (click on the i in the upper left corner).
Step 5: Counterweight
Step 6: Top Assembly
Step 7: Bottom O-ring Block
Step 8: Bottom Cap Block and Object Platforms
Step 9: Bottom Two Blocks and Object Platforms
Step 10: Vacuum Hardware
Another piece was used to mount a simple manifold.
See the complete assembly pictures to see how the tubing will connect the two tubes to the vacuum pump.
Step 11: Pivot Block
Before inserting the dowel pins, cut two short lengths of tube and turn two aluminum donuts to provide support. Acrylic can be brittle and could break if too much pressure is applied when pushing in the dowel pins.
Step 12: Bearings, Bearing Blocks, and Aluminum Support Tubes
There are two 2" square aluminum support tubes with 1/8" thick walls. There are four more holes and another slot on the opposite parallel face. The bearing support blocks are screwed to the aluminum support tubes so that the screw heads are completely hidden from the outside.
Step 13: Base Plate and Support Tubes
The short posts are 1.76" square with a 0.025" chamfer to accomodate the inside radius of the 2" square tubes. They're mirror opposites of each other and are screwed to the base plate from the bottom. The support tubes are screwed to the posts from the side and back by 1/4"-20 socket head cap screws.
The base plate is fastened to the table by two 1/2"-13 x 3 1/2" socket head cap screws. There's a short piece of aluminum with the same hole location and thread pattern underneath the top of the cart.
Step 14: Dowel Extensions, Handles, Bolts
Instead of flipping the apparatus upside down by hand, possibly breaking the brittle acrylic, people turn the handles quickly to rotate the apparatus.
The threaded hole on the handle lines up with the through hole in the sides of the support tubes. The locking bolt holds the whole apparatus in the upright and locked position during storage and transport.
Step 15: Vacuum Pump, Gauge, and Cart
Step 16: Objects
I was also going to use real feathers, but there was too much static cling between them and the acrylic tubes. The bright pink and green paper is much more visable anyways.
Both the aluminum and paper disks have a diameter of 1.75" (the tube ID is 2.25"), but the mass of the aluminum disk is over 60 times greater. This combination of the metal and bright paper disks works quite well because the metal makes a loud thud as it hits the bottom of the tube, and the paper is large enough to be seen fluttering down. When the air is pumped out, you can still hear the aluminum disk land, but you can see that the paper lands at the same time. Just watching the paper, it can appear as if the paper is making the loud thud even though it's really the aluminum disk landing at the same time.
The electronic scale was surplus equipment which is why it has a Do Not Use sticker. It still works fine and is accurate even if the calibration isn't official.
Step 17: Complete Apparatus
After some sanding, the aluminum parts look pretty good.
To demonstrate the different fall times at atmosphere, all three toggle valves are opened. The hose is then connected and the pump is turned on. After sufficient vacuum is reached, the valves are closed and the hose is disconnected. Most of the air has now been removed, and the gauge still reads the pressure in the tubes.
Step 18: Operation and Video
Free fall in atmosphere
1) Open all three toggle valves and disconnect the hose from the manifold.
2) Unscrew both of the locking bolts. The apparatus is now free to rotate.
3) Grasp one of the handles and quickly rotate the whole apparatus 180 degrees.
4) Watch and listen for the thud as the aluminum disk lands first while the paper floats down.
5) Repeat a couple times to show that air resistance always causes the paper to land second.
Creating the vacuum
It helps to screw in one of the locking bolts to keep the apparatus in place during this stage.
1) Connect the hose to the vacuum manifold.
2) Turn on the vacuum pump.
3) Watch the vacuum gauge change as the air is removed from both tubes.
5) Wait until the gauge reads -76 cmHg. Remember, this is relative to atmosphere (76 cmHg) so the actual pressure in the tubes is close to (but still slightly above) 0 cmHg.
6) Close all three toggle valves, turn off the vacuum pump, and disconnect the hose from the manifold.
Because the vacuum gauge is on the low pressure side of the cart/base plate toggle valve, it will read the same pressure as the tubes.
Free fall in vacuum
1) Unscrew the locking bolt that kept the apparatus upright. The apparatus is again free to rotate.
2) Grasp one of the handles and quickly rotate the whole apparatus 180 degrees.
3) Watch and listen as both objects always land simultaneously.
4) Repeat a couple times to verify the effect.
Repeat at atmosphere
1) Open the toggle valves to the tubes and listen to the air rush back in. Watch as the rushing air blows the paper disk upwards.
2) Repeat the experiment at atmosphere. The paper again lands second.
When completely finished, screw in both locking bolts to secure the apparatus and open the cart/base plate toggle valve.
Step 19: CAD Files
Step 20: Support Amazon.com
Step 21: The End
Luke Luck isn't my real name of course, it's really Steven. I probably have the coolest job ever as a full time lecture demonstrator for the University of WashingtonDepartment of Physics. We provide demonstrations for undergraduate physics classes on everything from basic mechanics, electromagnetism, thermodynamics, quantum mechanics, and everything in between. Essentially, I get to play with toys (uhh I mean teaching tools) all day, as well as design and build new demonstrations. This new Guinea and Feather apparatus took about a year to design and build (working off and on, in between regular classes and other projects) and was just completed recently.
Thank you reading this instructable. Please post any questions and comments and I'll try to reply in a timely manner.
University of Washington Department of Physics