# Objects will travel at a constant speed towards the earth no matter

## Main Question or Discussion Point

When releasing two different objects at the same time, the objects will travel at a constant speed towards the earth no matter the mass. I have a hard time believing this. If I were to release a brick and a feather at the same time, the brick will reach the ground before the feather. I know the wind will affect the feather. If this experiment would be conducted in a vacuum would both objects hit the ground at the same time?

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jtbell
Mentor
Yes, in a vacuum the two objects fall at the same rate. This is sometimes done as a class demonstration using evacuated tubes. There's also a video of it being done on the Moon (no air there, of course) by one of the Apollo astronauts back in the 1970s.

http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html" [Broken]

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Danger
Gold Member
I first saw this done at the Ontario Science Centre back in the 60's and it blew me away. The reasoning behind it is that the heavier object has more inertia for gravity to fight against.

When releasing 2 objects at the same time, they will both have the same constant acceleration (no matter the masses). They will not travel at a constant speed unless they hit terminal velocity - in which case they probably will travel at different speeds.

The reason the feather takes longer in experiment, is because of air resistance (wind); as said above, in a vacuum they would hit the ground at the same time.

If i'm interpreting Danger's explanation correctly - then he's totally right.
The force of gravity depends on mass, but so does acceleration - they cancel out.

Danger
Gold Member
The force of gravity depends on mass, but so does acceleration - they cancel out.
Thanks for the clarification. That is indeed what I meant, but I'm at work and so was a little rushed on that post.

I understand now. Thank you guys.

Danger
Gold Member
You're more than welcome. We aim to please.
And, as the sign on the bar bathroom door says: 'If you aim to please, please aim.'

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You can try putting a feather on a brick (maybe a book is more convenient) and drop them. The brick then shield the feather from air resistance and they will fall at the same rate.

Danger
Gold Member
And there's always the one about gluing a piece of buttered toast onto a cat's back, and see which one ends upon the bottom.

And there's always the one about gluing a piece of buttered toast onto a cat's back, and see which one ends upon the bottom.
Um actually danger I believe you haven't been keeping up with the latest mythbuster episodes:http://mythbustersresults.com/episode28" [Broken]
In episode 28 the butter toast myth was definitively busted! :rofl:

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russ_watters
Mentor
Have they done the cat experiment, though? My sister did when she was a little kid and discovered a cat needs to be dropped from higher than about a foot and a half to land on its feet.

Danger
Gold Member
I doubt that Mythbusters would do the cat experiment... too many freaks like PETA would complain.
It's actually very cool how a cat does it; a good demonstration of angular momentum conservation which comes naturally to them. They tuck in their back legs and splay the front ones, then torque their spines. Less resistance to angular displacement means that their ass ends spin and the head ends don't. Then they immediately splay their rear legs, tuck in the front ones, and repeat the torque. End result: right-side-up cat.
When I was about 15 I experimented with my cat, dropping him on the bed. (He didn't mind a bit, and seemed to rather enjoy it.) He managed to right himself from a height of slightly less than a foot, and it took about 1/10th of a second. Remarkable.

Isn't it also because it is gravity which gives an object weight. In a place where there is no gravity wouldn't two objects which have the same weight fall at the same speed?

Hootenanny
Staff Emeritus
Gold Member
Isn't it also because it is gravity which gives an object weight.
Yes.
In a place where there is no gravity wouldn't two objects which have the same weight fall at the same speed?
In a place where there's no gravity, two objects wouldn't fall at all! They would be weightless, like Astronauts in space. And secondly in the absence of fluid friction (e.g. air resistance) all objects fall (under gravity) at the same rate.

Danger
Gold Member
Incidentally, if anyone out there is contemplating the cat experiment, either be very fast or wear leather gloves. The cat's first instinct is to grab onto your hands with every available claw and hang on for dear life.

Yes.

In a place where there's no gravity, two objects wouldn't fall at all! They would be weightless, like Astronauts in space. And secondly in the absence of fluid friction (e.g. air resistance) all objects fall (under gravity) at the same rate.
I really don't want to be a pain; but i feel its important in a learning environment like this to clarify (although what hootenanny is trying to say is completely correct) -> astronauts most certainly are not weightless, and they are falling (rapidly) - although indeed at the same speed as their significantly heavier spacecraft.

Cheers.

The OP says that when two objects are travelling towards the Earth. Isn't the Earth's gravity restricted to the limits of its atmosphere? Then how do those two objects fall?

Hootenanny
Staff Emeritus
Gold Member
I really don't want to be a pain; but i feel its important in a learning environment like this to clarify (although what hootenanny is trying to say is completely correct) -> astronauts most certainly are not weightless, and they are falling (rapidly) - although indeed at the same speed as their significantly heavier spacecraft.

Cheers.
Quite true, Astronauts where perhaps a bad analogy, but to be honest I couldn't think of another one.
The OP says that when two objects are travelling towards the Earth. Isn't the Earth's gravity restricted to the limits of its atmosphere?
No, gravity is not restricted to the limits of the upper atmosphere, it extends infinitely beyond it. Yes, it does becoming very weak very quickly, but that doesn't mean the Earth's gravity just stops at the edge of space.
Then how do those two objects fall?
Which two objects are we talking about, the two in free-fall, or the two in zero gravity?

No, gravity is not restricted to the limits of the upper atmosphere, it extends infinitely beyond it. Yes, it does becoming very weak very quickly, but that doesn't mean the Earth's gravity just stops at the edge of space.
For example, the Solar and Heliospheric Observatory (SOHO) satellite:
"SOHO moves around the Sun in step with the Earth, by slowly orbiting around the First Lagrangian Point (L1), where the combined gravity of the Earth and Sun keep SOHO in an orbit locked to the Earth-Sun line. The L1 point is approximately 1.5 million kilometres away from Earth (about four times the distance of the Moon), in the direction of the Sun."​