What happens when a feather and bowling ball are dropped at the same time in a void container?

Table of Contents Show

Tools and Materials
To Do and Notice
What’s Going On?
Going Further
The Bowling Ball and Feather in Real Speed
Gravity - Air = Video Gold

Tools and Materials

Three feet (three meters) or more of clear, plastic, rigid-walled tube (available at your local plastics store) with an inner diameter of at least 1 inch (2.5 cm)
Two solid rubber stoppers—one with a hole through it, one without—to fit in the ends of the plastic tube
Copper tubing about 4 in (10 cm) long that fits tightly in the hole in the rubber stopper (glass tubing can be used—just be especially careful so it doesn't break)
About six feet (180 cm) of thick-walled, flexible plastic or rubber vacuum tubing
A coin
A feather or a small piece of paper
Vacuum pump (use a regular lab vacuum pump if available; if not, use a small hand pump such as Mityvac®)
2 hose clamps
A partner (preferably an adult)

Assembly

Insert the solid rubber stopper (the one without the hole) firmly into one end of the plastic tube. Turn the tube so the stopper is on the bottom.
Put the coin and the feather (or piece of paper) in the tube.
Push the copper tube through the one-hole stopper and firmly insert the stopper into the open end of the plastic tube.
Push the vacuum tubing over the copper tube and secure it with a hose clamp, if needed. Attach the other end of the vacuum tubing to the pump; again, use a hose clamp if needed.
The final assembly should look like the diagram below (click to enlarge).

To Do and Notice

Invert the tube and let the objects fall. Notice that the feather falls much more slowly than the coin.

Now pump the air out of the tube and invert it again (the pump can remain attached while you invert the tube). Notice that the feather falls much more rapidly than before—in fact, it falls almost as fast as the coin.

Let the air back into the tube and repeat the experiment. (Try to avoid rubbing the wall of the tube; otherwise, static electricity may make the feather stick to it.)

What’s Going On?

Galileo predicted that heavy objects and light objects would fall at the same rate. The reason for this is simple. Suppose the coin has 50 times as much mass as the feather. This means that the earth pulls 50 times as hard on the coin as it does on the feather. You might think this would cause the coin to fall faster. But because of the coin's greater mass, it's also much harder to accelerate the coin than the feather—50 times harder, in fact! The two effects exactly cancel out, and the two objects therefore fall with the same acceleration.

This rule holds true only if gravity is the only force acting on the two objects. But if the objects fall through air, then air resistance must also be taken into account.

Larger objects experience more air resistance than smaller objects. Also, the faster an object falls, the more air resistance it encounters. When the retarding force of the air just balances the downward pull of gravity, the object will no longer gain speed; it will have reached what is called its terminal velocity.

Since the feather is so much lighter than the coin, the air resistance on it very quickly builds up to equal the pull of gravity. After that, the feather gains no more speed, but just drifts slowly downward. The heavier coin, meanwhile, must fall much longer before it gathers enough speed so that air resistance will balance the gravitational force on it. The coin quickly pulls away from the feather.

Going Further

The terminal velocity of a human being falling through the air with arms and legs outstretched is about 120 miles per hour (192 kilometers per hour)—slower than a lead balloon, but a good deal faster than a feather!

Components of force make a toy walk and stop at just the right time.

“g,” that’s interesting! Investigating gravity.

See. The masses cancel. Mass doesn't matter even though matter is made of mass (physics pun). Also, I wrote these equations as scalar instead of vectors just to make it look simpler.

The Bowling Ball and Feather in Real Speed

The bowling ball and feather drop in the BBC Human Universe video looks awesome. However, they ran the shot in slow motion to make it look more dramatic. Wouldn't in be cool to see it in real time? I think I can make that happen.

Normally, I would take a video like this and find the real frame rate. I've done this before with some of the MythBusters videos. The basic idea is to look at a falling object. Since you know the acceleration should be -9.8 m/s2, you can just find the correct frame rate to give you that acceleration. It's pretty simple. However, that doesn't work in this case. The problem here is that there are two things I don't know. I don't know the distance scale and I don't know the frame rate. This means I need another strategy.

Luckily, the video shows the same bowling ball and feather dropping with air and in real time. I can use that to find the scale of the video. In this case, I will use the close up shot that shows the bowling ball and I will find the diameter.

If I use a bowling ball diameter of 21.59 cm, the falling ball seems to have the correct acceleration. Here is a plot of the vertical motion of that first fall.

Of course this is using the free video analysis program Tracker. Also, remember that the kinematic equation for an object with a constant acceleration (in the y-direction) is:

The term in front of t2 in the fitting equation would be 1/2 of the acceleration. So, a coefficient of -4.73 would give an acceleration of 9.46 m/s2. This isn't 9.8 m/s2 like I would expect, but it's close enough.

I can also get the total falling time from the video with a value of 2.04 seconds. This means that I can solve for the drop height of the ball.

However, I ignored the air resistance on the bowling ball during this drop. Is that ok? Let's say the ball has a mass of 6 kg. If you then create a numerical calculation for a falling ball both with and without air resistance, you get a time difference of just 0.048 seconds. Yes, you can try this calculation for yourself (as a homework exercise).

Moving on to the slow motion video (without air), I get the following plot for the vertical position of the bowling ball.

This gives an acceleration of 0.018 m/s2 - but that's not a real second, that's a fake second (since the video isn't in real time). If I call this time unit s', I can set this acceleration equal to 9.8 m/s2 (real seconds here) and solve for the relationship between real and fake time.

(Credit: sumroeng chinnapan/Shutterstock)

Over 400 years ago, the story goes, Galileo stood atop the Leaning Tower of Pisa and dropped two balls of different masses over the edge. As we all know, both balls smacked the ground at the same time, proving that gravity affects objects’ acceleration regardless of mass. (Though whether that was a real experiment or merely a thought experiment is still debated.)

Regardless, it’s a great, memorable visual. But be prepared to replace it with an even better one.

Gravity - Air = Video Gold

To demonstrate the effects of air — not gravity — on falling objects, physicist Brian Cox of the BBC Two program Human Universe visited the largest vacuum chamber in the world: NASA’s Space Power Facility in Ohio.

In this video, you see Galileo’s centuries-old concept illustrated quite dramatically. A bowling ball and a feather both fall at the same speed when all the air has been removed from the massive chamber.

Read more: 20 Things You Didn’t Know About Gravity

Yeah, it makes sense, but it’s still surreal to see a massive bowling ball and a delicate feather fall at an identical speed.

Although the demonstration is certainly impressive here on Earth, let’s not forget Apollo 15 astronaut David Scott’s rendition of the famous experiment — from the moon. Both a falcon feather and a hammer fall at the same speed, but without the encumbrance of a massive vacuum chamber.

Gravity: it has a certain pull on the human curiosity.