APOD: Hammer Versus Feather on the Moon (2011 Nov 01)

[Chris Peterson]

Has that ever been tried in a really big vacuum chamber on earth?

Certainly- the equivalence principle has been tested in hundreds of experiments, typically involving accelerated motion in high quality vacuums. Heck, I remember doing this in high school physics in the 1970s with a long, evacuated glass tube containing a feather and a rock.

As suggested elsewhere, this stunt on the Moon was not an experiment at all, but merely a demonstration. And it's a good demonstration: I've used this NASA video in the classroom for years, and every kid loves it.

[Boomer12k]

Supposedly on the Moon , Scott drops the feather and the hammer while Irwin is seen moving around . The question is : who is holding and operating the movie camera ???

Earth telemetry. The guys at the space center. Remote control camera. Just like today with robot space craft on mars, etc...


:----===== *

[DCStone]

What I find interesting is his comment just before the drop: "And hopefully, they'll hit the ground at the same time ... How about that!" Hopefully? You mean they hadn't rehearsed this at least 50 times in the giant vacuum chamber back at NASA on earth? Or did he not fully believe the physics he'd learnt in high school? Why did he sound surprised when the experiment actually worked? Then again, "If it smells, it's chemistry. If it moves, it's biology. And if it doesn't work, it's physics!"

[Stickler]

With the difference in mass between each object, the difference between the feather and the ball is grossly insignificant. If you were to measure the difference between a five-pound ball and a 50,000-pound ball in the gravitational attraction to the moon, I suspect you would see a difference. The same is true on earth - if one selected objects of a significant difference, relative to the mass of the earth, the gravitational force would effect an increase in attractive force.

[Chris Peterson]

The same is true on earth - if one selected objects of a significant difference, relative to the mass of the earth, the gravitational force would effect an increase in attractive force.

Every experiment ever conducted, using test masses ranging from atomic to galactic, has failed to show any dependence between mass and the acceleration of gravity on a body.

[Yoshi]

I clocked the time between the release and the hit and came up with 0.7-0.8 seconds. Assuming it dropped from ~0.8 m, the acceleration of gravity calculates to 2-3 m/s2, which is much larger than 1.63 by wikipedia. Was the test done at the low gravity anomally? Or is the screen on APOD running faster than the actual?

[rstevenson]

I clocked the time between the release and the hit and came up with 0.7-0.8 seconds. Assuming it dropped from ~0.8 m, the acceleration of gravity calculates to 2-3 m/s2, which is much larger than 1.63 by wikipedia. Was the test done at the low gravity anomally? Or is the screen on APOD running faster than the actual?

You also need to calculate the uncertainties. Can you tell exactly when he let go of the objects? When they landed? Is the time seen on the video an accurate reproduction of the time that would have been recorded in reality? Then you need to propagate these uncertainties to a final tally. It seems likely that the standard figure of 1.63 m/s^2 would be well within your calculated 2-3 m/s^2 ± the total uncertainty.

Rob, showing off his freshman Physics

[islader2]

@ DCStone The drop is pure theater for the masses who pay for the moon exploration. It was rehearsed very well==even the falcon feather was symbolic. As Chris said, many of us have done the same experiment here on Earth. It was a lovely piece of theater. I loved it, and still do.

[iamlucky13]

What I find interesting is his comment just before the drop: "And hopefully, they'll hit the ground at the same time ... How about that!" Hopefully? You mean they hadn't rehearsed this at least 50 times in the giant vacuum chamber back at NASA on earth? Or did he not fully believe the physics he'd learnt in high school? Why did he sound surprised when the experiment actually worked? Then again, "If it smells, it's chemistry. If it moves, it's biology. And if it doesn't work, it's physics!"

"Hopefully" possibly because he wanted to create just a hint of suspense for viewers, or possibly because he wasn't quite confident in his ability to release a feather of almost no weight at exactly the same time as a hammer as a result of the inability to really feel either through the thick, pressurized gloves.

They don't rehearse every thing 50 times over, much less in a vacuum chamber. They do little to no practice in a vacuum chamber, because the vacuum adds almost nothing to the experience versus wearing the suit in a normal environment, but makes it much more difficult and expensive to practice.

The extensive training astronauts go through is for learning how to use the many and varied pieces of specialized equipment they are called upon to use so they get the most utility out of them in the limited time they have available, and to make sure they able to respond correctly to a wide range of emergency situations that may arise. There is no need to practice simple tasks like dropping objects.

But yeah, it always seems like when you try to do a simple experiment, something comes up to throw the results off.

[neptunium]

For some reason, this image isn't loading on my computer. There's a message where the image should be that says "Internet explorer cannot display the webpage." Why is the image not coming up?

[bystander]

It's a YouTube video. You need flash player.

[neptunium]

Ooooohhhhh, I somehow had my flash player disabled. Thanks, bystander.

[mactavish]

Even if this wasn't the case the solar photon pressure pushing the feather downward produces feather accelerations that are some 15 orders of magnitude greater than the acceleration of the moon being pulled upward (by it's attraction to the hammer).

Well said. But, what did you say?

[deepstar1]

The equivalency principle is stated as the "acceleration an object feels due to gravity does not depend on its mass..." This is confusing. Planets with different masses produce different gravitational forces, and if I remember correctly, the force due to gravity equals the product of the two masses in question times a constant and divided by the distance between them squared. For two small objects on the same planet, the difference between the product of the masses would be negligible, but I don't see how one can say that the acceleration is entirely independent of mass.

[Chris Peterson]

The equivalency principle is stated as the "acceleration an object feels due to gravity does not depend on its mass..." This is confusing. Planets with different masses produce different gravitational forces, and if I remember correctly, the force due to gravity equals the product of the two masses in question times a constant and divided by the distance between them squared. For two small objects on the same planet, the difference between the product of the masses would be negligible, but I don't see how one can say that the acceleration is entirely independent of mass.

Where is the confusion? The equivalency principle simply states that given some particular gravitational field, two objects will experience the same acceleration regardless of their individual masses. It doesn't say that objects of different masses won't produce different gravitational fields.

[NoelC]

I am shaking my head at the folks assuming the "experiment" had to be practiced in order to make sure it came out "right", while simultaneously ignoring the fact that we actually got the astronauts all the way to the moon. Do you think that would have been possible if the scientists, engineers, and astronauts didn't understand the physics and physicalities well enough to know ahead of time whether the feather and the hammer would fall together?

Has anyone calculated the cost of delivering that feather to the moon (e.g., determine the weight of the feather vs. the overall weight of what was delivered to the lunar surface, then multiply that fraction times the mission cost)?

-Noel

[neptunium]

With the difference in mass between each object, the difference between the feather and the ball is grossly insignificant. If you were to measure the difference between a five-pound ball and a 50,000-pound ball in the gravitational attraction to the moon, I suspect you would see a difference. The same is true on earth - if one selected objects of a significant difference, relative to the mass of the earth, the gravitational force would effect an increase in attractive force.

Actually, this isn't true. It does not matter if a five-pound ball and a ten-ton ball are dropped at the same time, they are still going to hit the ground at the same time, because objects fall at a rate of 9.4 meters faster every second. However, having a lot more mass than the five-pound ball, the ten-ton ball will hit the ground with a lot more force.

I clocked the time between the release and the hit and came up with 0.7-0.8 seconds. assuming it dropped from ~0.8 m, the acceleration of gravity calculates to 2-3 m/s2, which is much larger than 1.63 by wikipedia.

First of all, you have to figure out the exact distance from his dropping point to the ground. It is rather hard to just estimate instead of using proportions. Next, you have to figure out how much less gravity is on the moon than on earth. Now, use the rate of 9.8 meters to figure out how long it takes for these objects to touch the ground. However, you have to divide 9.8 by the amount of gravity on the moon to have an answer of how fast the objects fall using that rate. Like I said before, the objects fall at the same speed, but the hammer has a greater force upon hitting the ground.

[geckzilla]

I clocked the time between the release and the hit and came up with 0.7-0.8 seconds. assuming it dropped from ~0.8 m, the acceleration of gravity calculates to 2-3 m/s2, which is much larger than 1.63 by wikipedia.

First of all, you have to figure out the exact distance from his dropping point to the ground. It is rather hard to just estimate instead of using proportions. Next, you have to figure out how much less gravity is on the moon than on earth. Now, use the rate of 9.8 meters to figure out how long it takes for these objects to touch the ground. However, you have to divide 9.8 by the amount of gravity on the moon to have an answer of how fast the objects fall using that rate. Like I said before, the objects fall at the same speed, but the hammer has a greater force upon hitting the ground.

Just adding my two tiny cents to this calculation. Don't forget the frame rate of the video would also affect the timing.

[neptunium]

So this means that the only way to have an accurate answer is to actually be on the moon.

[Chris Peterson]

So this means that the only way to have an accurate answer is to actually be on the moon.

No. You could research the video system used to determine the correct frame rate, examine any intermediate codecs that might have dropped or inserted frames, and come up with an accurate time stamp for each frame. You could then scale some objects to get an accurate height for the drop. It's a lot of effort, but there's nothing preventing somebody from doing it.

Speaking for myself, as a physicist, I'd start with the known gravitational acceleration of the Moon and the known laws of physics, and work the other way- use physics to figure out the unknowns, like frame rate.

[zloq]

People were talking about measuring the times really carefully. If you did - sure - you would find that a heavy object dropped by itself will hit the ground sooner than a lighter object of the same size - with or without air resistance. No relativity involved or anything.

zloq

[neufer]

People were talking about measuring the times really carefully. If you did - sure - you would find that a heavy object dropped by itself will hit the ground sooner than a lighter object of the same size - with or without air resistance. No relativity involved or anything.

Photon pressure from the sun causes the feather to fall in a shorter period of time.

Even at night the photon pressure from planets & stars causes the feather to fall in a shorter period of time.

[Beyond]

More surface area to weight

[Chris Peterson]

Photon pressure from the sun causes the feather to fall in a shorter period of time.

Even at night the photon pressure from planets & stars causes the feather to fall in a shorter period of time.

Well... if you are going to look at the situation that closely, you also have to consider that the lunar surface itself is radiating photons as well as charged particles. To determine the precise fall time of the feather compared with the hammer, you would have to calculate how the photon flux from the sky balances the photon flux from the ground. Very complicated, especially as you'd need to consider the time of day or night. Also, the surface of the Moon carries a substantial electrical charge, and the feather certainly won't be perfectly neutral. No doubt there are other effects as well, when considering things orders of magnitude less significant than gravity.

[neufer]

Photon pressure from the sun causes the feather to fall in a shorter period of time.

Even at night the photon pressure from planets & stars causes the feather to fall in a shorter period of time.

Well... if you are going to look at the situation that closely, you also have to consider that the lunar surface itself is radiating photons as well as charged particles. To determine the precise fall time of the feather compared with the hammer, you would have to calculate how the photon flux from the sky balances the photon flux from the ground. Very complicated, especially as you'd need to consider the time of day or night. Also, the surface of the Moon carries a substantial electrical charge, and the feather certainly won't be perfectly neutral. No doubt there are other effects as well, when considering things orders of magnitude less significant than gravity.

There is certainly something to be said for lunar photons (Lambertian scattered + IR) dominating the situation at low sun angles as well as just after sunset.

My gut feeling is that dielectric polarization would probably dominate the electrical situation and cause the feather to fall faster in any event. (It is like the dust settling on your computer screen; please clean it off now...it is getting disgusting!)