DO objects fall at the same speed? NO!

[Janus]

Actually, you are wrong. MattC had the correct answer with:

 

 

 

Please note that the differences are miniscule, but they do exist.

 

 

 

It really depends on how you define the word "fall". If you define it as the acceleration of an object due to gravity, then a light object and heavy object "fall" at the same speed. If you define it as the closing speed between the Earth and the objects as measured from the Earth, then this will be slightly greater for a heavier object and it will "fall" faster. The first use is the more technical usage while the second is the common language usage.

[baric]

It really depends on how you define the word "fall". If you define it as the acceleration of an object due to gravity, then a light object and heavy object "fall" at the same speed. If you define it as the closing speed between the Earth and the objects as measured from the Earth, then this will be slightly greater for a heavier object and it will "fall" faster. The first use is the more technical usage while the second is the common language usage.

 

I would think that questions about gravity on a science forum should probably stick with the technically correct answers, even if that means providing a disclaimer that it's a miniscule difference.

[J.C.MacSwell]

It really depends on how you define the word "fall". If you define it as the acceleration of an object due to gravity, then a light object and heavy object "fall" at the same speed. If you define it as the closing speed between the Earth and the objects as measured from the Earth, then this will be slightly greater for a heavier object and it will "fall" faster. The first use is the more technical usage while the second is the common language usage.

 

In common language usage, does anyone make the distinction?

[mooeypoo]

A cursory look at the parachute would tend to reinforce the idea, though — the light material falls slowly and the heavy one falls faster, and when tying them together the light one slows down the heavy one. You have to show show something else — like that there is no tension in the string when you tie two objects together.

True, until you look at parachuting accidents where the chute does not open right, twirls around above the object and they both fall uncontrollably with greater speed. If it was strictly mass, the shape of the parachute wouldn't matter.

[PSM5]

I would think that questions about gravity on a science forum should probably stick with the technically correct answers, even if that means providing a disclaimer that it's a miniscule difference.

Both of Janus's scenarios are technically correct.

 

In common language usage, does anyone make the distinction?

I will try.

 

The first is called the universality of free fall. It is the acceleration of either body (the earth or the falling object) relative to their common center of mass.

 

[math]A_{m1}=\frac{Gm2}{r^2}[/math]

 

[math]A_{m2}=-\frac{Gm1}{r^2}[/math]

 

The second is called the relative acceleration. It is the acceleration of either body relative to the other.

 

[math]A_{rel}=\frac{G(m1+m2)}{r^2}[/math]

 

The difference between the two scenarios is only in the frame of reference. The time to impact will be the same either way. I think the reason Janus declaired the first scenario to be a more technical usage is because the frame of reference for the first scenario is inertial, the second is not.

Edited July 25, 2011 by PSM5

[johnreed]

know that objects with the same surface area but different densities fall at the same speed in a vacuum.

 

 

However, there is actually a big argument between me and my girlfriend, at the point of I feel like strangling her to death.

 

 

I say heavier objects do fall faster (I provided calculations). She (appears) to think that this is nonsense and am full-of-shit and she knows better because she knows more about the topic and am just a uni student (She was my lecturer, before we started dating. Now shes just my bitch)

 

 

I dropped a piece of paper, and a piece of cardboard with similar SA. The cardboard fell much faster.

 

 

I could explain this by stating that heavier objects have more force to 'push' air molecules out of the way.

 

 

In other words, heavier objects are not as affected by air resistance.

 

If [g] in the expession [mg] for weight is a consequence of location then all atoms must fall at the rate of [g] at that location. Forget pressing air. Think vacuum.

[mooeypoo]

If [g] in the expession [mg] for weight is a consequence of location then all atoms must fall at the rate of [g] at that location. Forget pressing air. Think vacuum.

 

Sorry,what do you mean? g is acceleration, and in this context of newtonian mechanics it's constant acceleration regardless of location... Also, are you suggesting the atoms are falling = the air is falling?

 

I'm not quite clear as to what you're saying.... can you explain?

[matty]

know that objects with the same surface area but different densities fall at the same speed in a vacuum.

 

However, there is actually a big argument between me and my girlfriend, at the point of I feel like strangling her to death.

 

I say heavier objects do fall faster (I provided calculations). She (appears) to think that this is nonsense and am full-of-shit and she knows better because she knows more about the topic and am just a uni student (She was my lecturer, before we started dating. Now shes just my bitch.)

 

...In other words, heavier objects are not as affected by air resistance.

 

 

Heavier objects fall faster, doubly so in a vacuum, the rate of acceleration being acted upon by another force.

 

If [g] in the expession [mg] for weight is a consequence of location then all atoms must fall at the rate of [g] at that location. Forget pressing air. Think vacuum.

 

He means mass multiplied by gravity and introduces err talking about location, presumably the objects are being dropped from the same locale. A vacuum adds variable g-force, no constant there and would act to bring the denser object down even quicker than it had in the experiment with the lighter one without the added element of gravity.

 

variable force, that is, from vacuum to vacuum

Edited October 17, 2011 by matty

[swansont]

Heavier objects fall faster, doubly so in a vacuum, the rate of acceleration being acted upon by another force.

 

 

 

He means mass multiplied by gravity and introduces err talking about location, presumably the objects are being dropped from the same locale. A vacuum adds variable g-force, no constant there and would act to bring the denser object down even quicker than it had in the experiment with the lighter one without the added element of gravity.

 

variable force, that is, from vacuum to vacuum

 

Um, no.

 

Acceleration from gravity is independent of the mass, and it does not vary from vacuum to vacuum.

[matty]

Uhm, not all vacuums are created equal. Yes ~ Try a little housekeeping, a vacuum is only as powerful as it's engineered to be.

 

And are you actually saying that two objects with differing mass would fall at an equivalent rate because they've been placed in a vacuum?

 

~Acceleration and gravity are totally in cahootz in the event of objects with differing mass having been dropped down a vacuum, side by side and all.

 

That'd be to say a vacuum, any vacuum levels the playing field of mass, lol. Quitit

[swansont]

The presence or absence of a vacuum does not affect the gravitational acceleration. Two objects in a vacuum will fall with the same acceleration. It's been tested.

 

Here's a crude experiment

http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html

 

The list of tests of the weak equivalence principle is fairly long

http://en.wikipedia.org/wiki/Equivalence_principle#Tests_of_the_weak_equivalence_principle

[mooeypoo]

Uhm, not all vacuums are created equal. Yes ~ Try a little housekeeping, a vacuum is only as powerful as it's engineered to be.

 

 

That's not the vacuum we're talking about, matty.

[questionposter]

You're just as bad as her. They don't try it!

 

This is basic stuff you should have learned in high school. The only reason things wouldn't fall at the same speed from the same height is because of air resistance, such as with a feather and a bowling ball. Your sort of semi-right with the force thing, but it's density, not just force. I could launch a feather out of a rocket launcher and it still wouldn't necessarily fall at the same speed as a bowling ball on Earth. What's happening is the same force of gravity is being applied to both objects. Since that's true, in a vaccume, both the objects fall at the same speed. Why would the Earth pull on a bowling ball with more force than a feather? There's more atoms to be pulled on, but those atoms in the bowling ball are being pulled with the same force as the atoms in the feather.

Edited October 18, 2011 by questionposter

[matty]

The presence or absence of a vacuum does not affect the gravitational acceleration. Two objects in a vacuum will fall with the same acceleration. It's been tested.

 

Here's a crude experiment

http://nssdc.gsfc.na...ather_drop.html

 

The list of tests of the weak equivalence principle is fairly long

http://en.wikipedia....lence_principle

 

 

That's not the vacuum we're talking about, matty.

 

 

Thanks, I'll play catchup, lessee...

[L42yB]

While the earths gravitational effect on all objects is constant, all objects are also creating their own (admittedly tiny) gravitational fields because they also have mass. Therefore, the mass of the object does have an effect. It may be considered insignificant, but *technically* it is still there.

 

An object with sufficient mass (say, a piece of a neutron star) would fall at a much greater speed than a bowling ball, even in a vacuum.

 

So doesn't that mean that mass does matter, it's just that nothing has enough mass for it to be significant?

[swansont]

While the earths gravitational effect on all objects is constant, all objects are also creating their own (admittedly tiny) gravitational fields because they also have mass. Therefore, the mass of the object does have an effect. It may be considered insignificant, but *technically* it is still there.

 

An object with sufficient mass (say, a piece of a neutron star) would fall at a much greater speed than a bowling ball, even in a vacuum.

 

So doesn't that mean that mass does matter, it's just that nothing has enough mass for it to be significant?

 

It depends on how you phrase the question. If the two objects are side-by-side, they will hit at the same time. If the experiment is done serially, the more massive object hits first, even if that time difference is exceedingly small. But that wasn't the question — did it have a higher speed? No, it hit first because your target moved. The speed was the same in the rest frame of the experiment.

[johnreed]

IS GRAVITY THE UNIFORM ATTRACTION OF NON-UNIFORM ATOMS ALLOWING THE COMPARATIVE CUMULATIVE RESISTANCE OF NON-UNIFORM ATOMS WE MEASURE ON THE BALANCE SCALE AND CALL MASS?

 

by

 

John Lawrence Reed Jr

 

In response to a question asked by Robert Allan

 

(mod note) Deleted; this is a duplication of another post

http://www.scienceforums.net/topic/60574-johnreed-studies/page__p__631631__fromsearch__1#entry631631

 

It's also not clear it's on-topic; please review the rules, specifically rules 5 and 10(end mod note)

 

That's not the vacuum we're talking about, matty.

 

 

Well I am new at posting in ongoing dialogues here so bear with me.

If we place a balance scale on the moon the [g] factor is different than on earth. In addition the [g] factor can change in magnitude at different locations on earth altho' at a much smaller magnitude. Except for theoretical locations like black hole horizons where we place a balance scale the factor of [g] will be the same on both pans to start with. This being the practical case the only quantity the balance scale compares is mass [m] which remains the same on earth, moon and any location we can use it. So the mass [m] remains the same but [g] can vary depending on location. If [g] varied with mass then we could jump from a plane and hold hands to either increase or decrease our rate of fall Further if [g] varied with mass [m] once on the planet surface we would be a part of the earths mass and could not even walk. We would ooze and thinly try to cover the planet surface. But as someone said this is all high school stuff. Your girl friend is right and you are wrong. This is all high school stuff that you should learn before you state a proposition in a public forum. And the earth is not flat, in case that comes next.

Edited November 8, 2011 by swansont
deletions

[swansont]

This is all high school stuff that you should learn before you state a proposition in a public forum.

 

!

Moderator Note

Don't take the discussion in that direction. Asking questions and learning is what this forum is for.

[johnreed]

I guess I don't get the hang of this posting ongoing dialogues. I read one person saying [g] is measured on the balance scale. One saying different masses fall at different rates... then the guy starts out with insults to his girlfriend... and I get reprimanded for advising him of proper action in public forums. He is not asking he is telling and he is going to believe he is right regardless what anyone says for a long time. An undereducated crack pot that insults his girl friend in public? I don't know.

[swansont]

!

Moderator Note

The first thing to get the hang of is the rules.

[The Crow]

there's some nonsense about g of a stationary object being different from that of a falling one. Obviously g is only defined for a body in free fall, not for one eating hot dogs.Sisyphus goes so far as to balance it by the normal reaction. g IS a constant, but does not appear to be one due to the fact that it is always in free fall.

About the A+B conundrum, one can say that a is pulling b up, but the reverse is also true. and in fact both forces are internal to the system of falling masses and so dont affect the motion.

[mooeypoo]

there's some nonsense about g of a stationary object being different from that of a falling one. Obviously g is only defined for a body in free fall, not for one eating hot dogs.Sisyphus goes so far as to balance it by the normal reaction. g IS a constant, but does not appear to be one due to the fact that it is always in free fall.

About the A+B conundrum, one can say that a is pulling b up, but the reverse is also true. and in fact both forces are internal to the system of falling masses and so dont affect the motion.

 

What? Wait, I am losing you. Are you claiming the gravitational constant doesn't matter when I'm eating a hotdog?

 

The only reason you don't use the symbol "g" in equations that talk about stationary objects is because the normal force and the gravitational force are cancelling each other out, and we built "simplified" equations that stem out of that.

 

That doesn't mean g doesn't exist in that case...

[core433]

Why do replies to this kind of questions always either miss the point entirely or go off on some tangent about stuff we already know?

 

Here's my theory. The OP and his girlfriend are both wrong. The former brings up the empirically correct observation but justifies it poorly. The latter blindly stuck to an elementary fact that was spoon-fed to her and didn't consider that there can be multiple forces acting on an object in this complex universe.

 

Let's state what we know.

 

1) All objects accelerate equally in an ideal vacuum.

2) A paper, cardboard, and steel sheet of the exact same shape have the same surface area.

3) A steel square falls faster than a cardboard square falls faster than a paper square in air.

 

Saying all 3 will fall at the same rate in air is stupid because empirically we can prove this is not true. Saying air resistance as a function of surface area is the only opposing force to gravity is wrong because all 3 have the same surface area. What I feel can only be the proper answer is therefore aerodynamics, which dictates how an object deforms or changes orientation as it is falling, which dictates turbulence of air which affects its rate of fall.

 

A paper sheet bends like crazy in air due to being structurally incapable of maintaining a stable form in the face of pressure from air, thus moving in a sinusoidal pattern and creating some amount of lift in its travel. Its speed is a function of both surface air resistance and lift.

 

A cardboard sheet exhibits similar behavior but to a much lesser extent.

 

A steel sheet exhibits no such behavior, thus it creates no lift due to aerodynamics and its slowdown is purely due to air resistance.

 

Structural integrity is often correlated with mass, so the OP attributed this phenomenon to mass, which makes him wrong. That is to say, if I take a material that is both light and structurally sound - such as a sheet of carbon nanotubes - and dropped it, assuming it did not deform in air, it would fall at the rate more similar to a heavy object of the same shape than a light, malleable object. This is difficult to envision intuitively because such material rarely occurs naturally.

Edited October 28, 2011 by core433

[mooeypoo]

What you say is true, but the reason we stuck to "elemetary physics" is because the thread went there. People did point out that there are other factors that apply, but in general, the same acceleration acts at all objects, which is true.

 

Sometimes, however, we tend to go to the 'simplistic' in these type of questions to make sure we avoid going to issues the originator of the question isn't getting confused. "In principle", all objects fall at the same rate, and "in principle" the fact they don't on earth is due to air resistance.

 

Still, you summarize the issue well.

[Axioms]

Ali Algebra know that objects with the same surface area but different densities fall at the same speed in a vacuum.

 

However, there is actually a big argument between me and my girlfriend, at the point of I feel like strangling her to death.

 

I say heavier objects do fall faster (I provided calculations). She (appears) to think that this is nonsense and am full-of-shit and she knows better because she knows more about the topic and am just a uni student (She was my lecturer, before we started dating. Now shes just my bitch)

 

I dropped a piece of paper, and a piece of cardboard with similar SA. The cardboard fell much faster.

 

I could explain this by stating that heavier objects have more force to 'push' air molecules out of the way.

 

In other words, heavier objects are not as affected by air resistance.

 

I'm not sure if anyone spoke about drag. I just skimmed through the posts. The question did include air resistance though because he stated "know that objects with the same surface area but different densities fall at the same speed in a vacuume" and he did include air resistance in his answer. The paper and cardboard (same shape and different mass) will have different terminal velocities. It is wrong in saying that heavier objects are less affected by air resistance but he has the right idea, kind of.

 

Terminal velocity is where drag is equal to the force of gravity. Fg_y=ma. It requires less drag force for the lighter object to reach terminal velocity.

 

proof: v_paper = sqrt(2fg/CpA) (C is the drag coefficient p is the air density and A is the cross-sectional area of the object) (Drag equation manipulated from D=0.5*CpAv^2 and D-Fg=ma)

= sqrt(2(9.8*1*10^-3)/0.5*1*(20*45)) (not sure what A or what p is and cannot be bothered to measure but is meaningless if keep constant)

=6.6*10^-3 m/s

the only thing in the equation that changes is fg so lets say the cardboard is 10* the mass.

v_cardboard = 0.021 m/s

 

Please note the values I used are not accurate but does not make the calculations wrong. In both equations the only thing that changes is fg so the values are constant in these circumstances.

 

The objects do accelerate at the same speed initially but the velocity at which they hit the ground will be different. It is dependent on the surface area of the object though. If you take the paper and cardboard and drop it where the maximum SA is expossed and then compare it to where the smallest SA is expossed you will see the difference in results.

 

Why do replies to this kind of questions always either miss the point entirely or go off on some tangent about stuff we already know?

 

Here's my theory. The OP and his girlfriend are both wrong. The former brings up the empirically correct observation but justifies it poorly. The latter blindly stuck to an elementary fact that was spoon-fed to her and didn't consider that there can be multiple forces acting on an object in this complex universe.

 

Let's state what we know.

 

1) All objects accelerate equally in an ideal vacuum.

2) A paper, cardboard, and steel sheet of the exact same shape have the same surface area.

3) A steel square falls faster than a cardboard square falls faster than a paper square in air.

 

Saying all 3 will fall at the same rate in air is stupid because empirically we can prove this is not true. Saying air resistance as a function of surface area is the only opposing force to gravity is wrong because all 3 have the same surface area. What I feel can only be the proper answer is therefore aerodynamics, which dictates how an object deforms or changes orientation as it is falling, which dictates turbulence of air which affects its rate of fall.

 

A paper sheet bends like crazy in air due to being structurally incapable of maintaining a stable form in the face of pressure from air, thus moving in a sinusoidal pattern and creating some amount of lift in its travel. Its speed is a function of both surface air resistance and lift.

 

A cardboard sheet exhibits similar behavior but to a much lesser extent.

 

A steel sheet exhibits no such behavior, thus it creates no lift due to aerodynamics and its slowdown is purely due to air resistance.

 

Structural integrity is often correlated with mass, so the OP attributed this phenomenon to mass, which makes him wrong. That is to say, if I take a material that is both light and structurally sound - such as a sheet of carbon nanotubes - and dropped it, assuming it did not deform in air, it would fall at the rate more similar to a heavy object of the same shape than a light, malleable object. This is difficult to envision intuitively because such material rarely occurs naturally.

 

I disagree to a extent. It is related to the mass in the form of force. Lets say the paper, cardboard and steel were all 100% structually sound. We now drop it off a building where all the dimentions of the maximum SA acts towards the ground. The terminal velocity can be determined of each. Drag - Fg =ma. This is the amount of force needed for the air resistance to counteract the force of gravity. Lighter objects will require much less force by air resistance to reach terminal velocity.

Read my last post.