Inside the large plexiglas tube are a steel cube and a pair of feathers, each of which has a steel wire embedded in its spine. At each end of the tube is an electromagnet, which when energized can suspend the cube and feathers when you rotate the cylinder to place that end on top. When you flip the switch (at right in the middle of the tube, on the box with the orange cord hanging from it), this cuts the current to the top electromagnet, releasing the cube and feathers, which fall to the bottom of the tube. The electromagnet at the bottom is now energized, and if you rotate the tube to place this end on top, you can repeat the process. The valve on the left (connected by a hose to the mechanical pump) allows you to pump the air out of the tube and seal it under vacuum. A small vent valve just behind the pivot where the main valve is, allows you to refill the tube with air. (You can also do this by slowly opening the large valve, if the hose is not connected.) With the tube filled with air, when you drop the cube and feathers as described above, the cube plummets to the bottom, while the feathers float down after it. When the tube is evacuated and you drop them, the cube and feathers all fall together. You may choose to have the apparatus evacuated before class time, or to pump it out during class. Before you use the apparatus, you can drop the green wood block and brown feather, shown in front of the power supply, to show how they fall.

The large power supply on the table provides current for the magnets, and is plugged into the wall via the timer switch, shown above just to the right of the power supply. The magnet coils draw 15 amperes, and if they are left on too long, they get hot enough to be damaged, and to damage the endcaps of the apparatus. For this reason, always use the timer switch, and set it for a maximum of 10 minutes.

Questions regarding the motion of falling objects date at least to the ancient Greeks. As quoted in Physics, Part 1, Third Edition, by Robert Resnick and David Halliday, Aristotle held that “the downward movement . . . of any body endowed with weight is quicker in proportion to its size.” Much later, Galileo Galilei performed experiments, both with objects falling vertically and with balls rolling down an inclined plane. From these, he was able to determine that the way balls accelerated as they rolled down an incline was similar to the way they accelerated when dropped vertically. Since the acceleration down the incline is a component of the (vertical) acceleration of gravity, g, its magnitude is smaller than g, so the motion of the balls as they roll down the incline is slower than that when they fall vertically. Rolling the balls down an incline thus made it easier for Galileo to time their motion. From these experiments, Galileo found that the acceleration was constant, and that it was independent of the mass of the balls.

It might perhaps still be tempting to think that heavier objects fall faster than lighter ones, especially while watching such things as leaves, feathers and even beach balls float to the ground. All objects that are released from some height and fall back to earth are subject to drag from the earth’s atmosphere. For most objects, this drag is small enough relative to the downward force of gravity, that we do not notice its effect. For objects such as leaves and feathers, however, this drag is significant compared to the force of gravity, so that when they are released from some height, we can see that they fall with an acceleration that is less than g. This demonstration shows in a rather impressive way, that absent the drag due to the atmosphere, a steel cube and a pair of feathers fall at exactly the same rate.

Before you perform the demonstration with the cube and feathers in the apparatus, you can take the green wood block in one hand and the feather in the other, and holding them at the same height, release them. While the block accelerates quickly to the floor, the feather floats down gently and lands a bit later.

To operate the apparatus, turn the timer switch past, and then back to, 10 minutes (or some shorter time). Do not set the timer for more than 10 minutes! Now, by means of the long aluminum rods at the center of the apparatus, slowly rotate the tube until the cube and feathers are at the top (and the tube is vertical). Now flip the toggle switch on the box that is between the two handles. (It will be pointing up; when you flip it, it will point down.) If the apparatus has been evacuated, the cube and feathers fall together, and all strike the bottom plate at exactly the same time. If the apparatus has not been evacuated, the cube plummets to the bottom, while the feathers float down after it. Either way, if you wish to repeat the demonstration, rotate the tube back to its original position, so that the cube and feathers are again on top, and flip the switch. Please note that if the vacuum pump is connected to the apparatus, it is best not to keep rotating the tube in one direction, but rather to rotate it one way and then back in the opposite direction.

If you started with the tube evacuated, you can open the vent to allow air in. Once the tube is filled with air, you can repeat the demonstration to show that the behavior of the cube and feathers is the same as what you and the class observed for the green block and brown feather.

If you started with the tube filled with air, you can now evacuate it by turning on the pump. (Make sure that the main valve is open.) After several minutes, perform the demonstration as described above. The cube and feathers now fall together and reach the bottom at exactly the same time.

At the end of the last moon walk of the Apollo 15 mission, Commander David Scott performed what is probably the ultimate version of this demonstration, by dropping simultaneously a hammer and a feather. The video below is from NASA, and you can find it, with some explanatory text, at this link: https://moon.nasa.gov/resources/331/the-apollo-15-hammer-feather-drop/

References:

1) Resnick, Robert and Halliday, David. Physics, Part One, Third Edition (New York: John Wiley and Sons, 1977), pp. 43-44.