Physics of falling objects
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Physics of falling objects
An argument developed off topic in the Malaysian thread about the physics of falling objects. It was off topic so was mostly deleted.
I and one other were taken to task by Skipness One Echo and others for stating that objects of the same size but different densities (eg a basketball and an identical one filled with concrete) would not hit the ground at the same time, dropped from say 35000 feet in an atmosphere.
Discuss.
I and one other were taken to task by Skipness One Echo and others for stating that objects of the same size but different densities (eg a basketball and an identical one filled with concrete) would not hit the ground at the same time, dropped from say 35000 feet in an atmosphere.
Discuss.
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there was an experiment many years ago where objects were dropped off the Eiffel tower to explore the effect. you are wrong.
the fall is due to gravity (whatever that is)
the gravitational attraction acts on every particle of the two balls at the same rate of influence.
so it doesn't matter whether you have a ball full of nothing or a ball full of concrete, every particle of both balls is under the same influence.
you don't get any more influence exerted on the concrete ball.
the fall is due to gravity (whatever that is)
the gravitational attraction acts on every particle of the two balls at the same rate of influence.
so it doesn't matter whether you have a ball full of nothing or a ball full of concrete, every particle of both balls is under the same influence.
you don't get any more influence exerted on the concrete ball.
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It has got to be a wind-up.
In a vacuum, all objects free fall due to gravity at the same rate of acceleration, regardless of their mass or density.
In a vacuum.
Objects dropped on earth, in the atmosphere, behave differently.
Ever see the feather and hammer dropped on the moon?
Feather & Hammer Drop on Moon - YouTube
In a vacuum, all objects free fall due to gravity at the same rate of acceleration, regardless of their mass or density.
In a vacuum.
Objects dropped on earth, in the atmosphere, behave differently.
Ever see the feather and hammer dropped on the moon?
Feather & Hammer Drop on Moon - YouTube
Last edited by CaptainEmad; 10th Mar 2014 at 02:06. Reason: Removed the Eiffel tower reference.
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Actually differing masses will accelerate under the sole force of gravity at exactly the same rate. The more massive object will only accelerate faster if there is reasonable air resistance.
Every kilogram of mass is acted upon by gravity (9.8N/kg on our planet). This is 'f'.
F/m=a
10kg(m) x 10 (9.8 simplified) = 100N/10kg(m) = 10m/s/s
100kg x 10 = 1000N/100kg = 10m/s/s
Factor in enough air resistance and time and the more massive object will hit the ground first. Would it in the OP's case? Dunno. Not enough info.
Every kilogram of mass is acted upon by gravity (9.8N/kg on our planet). This is 'f'.
F/m=a
10kg(m) x 10 (9.8 simplified) = 100N/10kg(m) = 10m/s/s
100kg x 10 = 1000N/100kg = 10m/s/s
Factor in enough air resistance and time and the more massive object will hit the ground first. Would it in the OP's case? Dunno. Not enough info.
objects were dropped off the Eiffel tower
Now the Tower of Pisa I could see working. But that could be too short to measure the effect of drag.
The formula for the terminal velocity of a falling object depends on its density (mass for identically sized objects).
Terminal velocity - Wikipedia, the free encyclopedia
So I would expect that, given sufficient altitude to allow terminal velocity to be approached and enough time for that difference in velocity to manifest itself as a noticeable difference in descent time, that difference would become apparant.
Thread Starter
...
you are wrong
....
the gravitational attraction acts on every particle of the two balls at the same rate of influence.
so it doesn't matter whether you have a ball full of nothing or a ball full of concrete, every particle of both balls is under the same influence.
you don't get any more influence exerted on the concrete ball.
So a balloon filled with air dropped from the Eiffel Tower or even my garage bench would fall at the same rate as a balloon filled with concrete?
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The denser ball will always hit the ground faster, in air, as it has a lower ratio of drag to weight.
Galileo is reported to have used different-sized cannonballs dropped from the tower of Pisa to show that overall mass, popularly thought to determine rate of descent, did not affect the outcome. (Although the bigger cannonball will fall a little faster owing to: a lower ratio of drag to weight.)
Galileo is reported to have used different-sized cannonballs dropped from the tower of Pisa to show that overall mass, popularly thought to determine rate of descent, did not affect the outcome. (Although the bigger cannonball will fall a little faster owing to: a lower ratio of drag to weight.)
Provided the both objects are identical except for mass, they will fall at the same rate. Ignoring drag from the air for a moment, eg in a vacuum, gravity acts to accelerate each particle at the same time.
Consider this thought experiment: Three identical masses, dropped at exactly the same moment. Each will accelerate the same as the other two. Now connect two of the masses by a loose piece of string that is so small that its mass & surface area are negligible (or even non-existent if you posit a thread small enough). Do you think the two connected masses will suddenly behave differently to the independent mass? Now imagine the thread replaced by a rod so now the two masses are locked together. Do you think the connected masses will suddenly change behaviour? If it helps, imagine the connector suddenly appears mid-fall. Now imagine, instead of three identical masses with two of them connected together, two identical masses differing only in density. Gravity's effect is the same, regardless of size or shape.
Now add a fluid such as air. The only effect the fluid has is to provide a retarding force - 'drag' - reducing the acceleration of gravity. Drag is not affected by mass. C=Cd 1/2 rho v^2 s remember. So, as long as the objects *only differ in density* they will fall at the same rate.
Consider this thought experiment: Three identical masses, dropped at exactly the same moment. Each will accelerate the same as the other two. Now connect two of the masses by a loose piece of string that is so small that its mass & surface area are negligible (or even non-existent if you posit a thread small enough). Do you think the two connected masses will suddenly behave differently to the independent mass? Now imagine the thread replaced by a rod so now the two masses are locked together. Do you think the connected masses will suddenly change behaviour? If it helps, imagine the connector suddenly appears mid-fall. Now imagine, instead of three identical masses with two of them connected together, two identical masses differing only in density. Gravity's effect is the same, regardless of size or shape.
Now add a fluid such as air. The only effect the fluid has is to provide a retarding force - 'drag' - reducing the acceleration of gravity. Drag is not affected by mass. C=Cd 1/2 rho v^2 s remember. So, as long as the objects *only differ in density* they will fall at the same rate.
Last edited by Tinstaafl; 12th Mar 2014 at 11:30.
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Drag is affected by mass.
For the same density drag rises as size squared, while mass rises as size cubed.
Added - if shape is the same.
Mass is a good choice of a dependent variable to link that.
Don't forget Tinstaafl that while Cd does have that dependence, the drag force derived from it has an area term too.
If objects only differ in density, and have the same size and shape, they will definitely not fall at the same rate.
For the same density drag rises as size squared, while mass rises as size cubed.
Added - if shape is the same.
Mass is a good choice of a dependent variable to link that.
Don't forget Tinstaafl that while Cd does have that dependence, the drag force derived from it has an area term too.
If objects only differ in density, and have the same size and shape, they will definitely not fall at the same rate.
Last edited by awblain; 10th Mar 2014 at 08:37. Reason: Being clearer on "Drag is affected by mass"
Is it possible to drop anything off the Eiffel tower without having it bounce off the side a couple of times?
)File:Sous la Tour Eiffel 1.jpg - Wikipedia, the free encyclopedia
Drag is affected by mass.
For the same density, drag rises as size squared, while mass rises as size cubed.
For the same density, drag rises as size squared, while mass rises as size cubed.
Drag is affected by size (among other things) and Mass is affected by size (among other things).
But there is no direct correllation between drag and mass (or vice versa) (with the exception where one introduces a wing into the equation, in which case one can say that induced drag (from the wing's lifting effort) is affected by mass.
That has nothing to do with form drag and air resistance in a free fall, however.
You can have high-mass objects with low drag (Saturn/Apollo moon rocket, for example) and low-mass objects with high drag (the average mattress, for example). And objects of equal mass and even equal density, with very different drag (a flat plate of aluminum vs. a needle-shaped piece of aluminum, both massing 5 kilos).
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But there is no direct correllation between drag and mass (or vice versa) (with the exception where one introduces a wing into the equation, in which case one can say that induced drag (from the wing's lifting effort) is affected by mass.
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objects of the same size but different densities (eg a basketball and an identical one filled with concrete) would not hit the ground at the same time
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An argument developed off topic in the Malaysian thread about the physics of falling objects. It was off topic so was mostly deleted.
I and one other were taken to task by Skipness One Echo and others for stating that objects of the same size but different densities (eg a basketball and an identical one filled with concrete) would not hit the ground at the same time, dropped from say 35000 feet in an atmosphere.
Discuss.
I and one other were taken to task by Skipness One Echo and others for stating that objects of the same size but different densities (eg a basketball and an identical one filled with concrete) would not hit the ground at the same time, dropped from say 35000 feet in an atmosphere.
Discuss.
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In either case, it doesn't matter, the experiment was to test the (then) current theory of "Natural Motion" concocted by Aristotle which claimed that the speed of a falling object is proportional to it's mass. And even if the denser ball fell slightly faster, it was still obvious that the difference between the two was not proportional to the respective mass differences.
well, yeah, actually you do. The force of gravity on an object is proportional to it's mass. The basketball filled with air is about .6kg while the one filled with concrete will have a mass of 17 kg. So the force of gravity (in round numbers) near the earth's surface will be 6 newtons on the air ball, and 170 newtons on the concrete ball.
In the absence of any other force, the acceleration will be identical (because the force is proportional to the mass) but the balls are falling thru the atmosphere, not a vacuum. There is also drag to consider.
OK, forget weight for a moment. you have two objects moving through the same air. they have identical drag coefficients and identical frontal areas, and identical wetted area. Aerodynamically, they are identical. One has 6 newtons of force propelling it thru the air, and the other has 170 newtons of force propelling it through the air.
Which will travel faster thru the air?
It should be obvious that the object with 170 newtons propulsive force will travel faster than the one with 6 newtons.
It's the same with falling objects. As the balls fall faster, the drag begins to predominate over inertia, and more and more of the force of gravity is counteracted by the drag and less of it accelerates the balls. And the ball that has the least force will accelerate less than the ball with more force.
Ultimately both balls will reach their terminal velocity*, and the terminal velocity of the concrete ball will be much higher than the terminal velocity of the air filled ball.
*Dangerous territory when dealing with a poor understanding of physics. There is a popular misconception that there is some single "terminal velocity" for any and all falling objects This ain't true. Not even close. Every object has it's own terminal velocity determined by it's density and aerodynamic properties. A feather has a very low terminal velocity. Doesn't matter how high you let it fall, it ain't falling any faster. A high penetration aerial bomb on the other hand has a very high terminal velocity. There are bombs which will exceed the speed of sound while in a free fall. This is because they have a lot of mass and have a very low drag coefficient. (lots of gravitational force vs. not much drag force) The terminal velocity of a skydiver in free fall is somewhere between a feather and a bunker buster bomb. In fact, different individual skydivers will have different terminal velocities depending on their technique.
Last edited by A Squared; 10th Mar 2014 at 08:23.
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That's a good illustration of the complete absurdity of the claim. Frankly I'm astonished that we're even having this discussion on a board of professional pilots.
This discussion started on the ML thread when there was disagreement about the duration of free fall wreckage from 35000ft, with one maths man saying it would only take 40 seconds!
Real world examples differ from pure maths..
The Space Shuttle Challenger cockpit detached at 45,000 feet continued to 65,000 feet then free fell, slowly spinning for 2 min 25 seconds before it hit the water.
Challenger 1
Halo skydivers jump from 35000ft, it takes around 2 minutes to deploy their chute at between 3000 to 5000 feet.
Gopros weight around 100 grams and when not clad in a waterproof housing, are the size of a matchbox.
Unverified go pro footage, falling 2 minutes from 12500 feet, it flutters and rotates at around 7 revs per second.
Unverified go pro footage falling for 1 minute 40 seconds from around 10,500 feet,
Unverified go pro footage, falling for 22 seconds from 2200 feet.
Unverified go pro footage, 27 seconds to fall 3000 feet.
Real world examples differ from pure maths..
The Space Shuttle Challenger cockpit detached at 45,000 feet continued to 65,000 feet then free fell, slowly spinning for 2 min 25 seconds before it hit the water.
Challenger 1
Halo skydivers jump from 35000ft, it takes around 2 minutes to deploy their chute at between 3000 to 5000 feet.
Gopros weight around 100 grams and when not clad in a waterproof housing, are the size of a matchbox.
Unverified go pro footage, falling 2 minutes from 12500 feet, it flutters and rotates at around 7 revs per second.
Unverified go pro footage falling for 1 minute 40 seconds from around 10,500 feet,
Unverified go pro footage, falling for 22 seconds from 2200 feet.
Unverified go pro footage, 27 seconds to fall 3000 feet.
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Now add a fluid such as air. The only effect the fluid has is to provide a retarding force - 'drag' - reducing the acceleration of gravity. Drag is not affected by mass. Cd=1/2 rho v^2 s remember. So, as long as the objects *only differ in density* they will fall at the same rate.
And for objects of different masses, which are otherwise externally identical, the *force of gravity is higher for the object with greater mass, so the *force* of gravity will reach equilibrium with the *force* of drag, at a higher airspeed. And when the *force* of gravity reaches equilibrium with the *force* of drag, the object will no longer accelerate. And like I said that, occurs at a higher airspeed for the more dense object (assuming same volume) .
Last edited by A Squared; 10th Mar 2014 at 07:34.
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I and one other were taken to task by Skipness One Echo and others for stating that objects of the same size but different densities (eg a basketball and an identical one filled with concrete) would not hit the ground at the same time, dropped from say 35000 feet in an atmosphere.
They would be correct on the moon..
https://www.youtube.com/watch?v=5C5_dOEyAfk
EDIT: should be easy to prove by dropping a table tennis ball and something the same size like an egg...or perhaps not an egg as that's streamlined :-)
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They would be correct on the moon..
https://www.youtube.com/watch?v=5C5_dOEyAfk
https://www.youtube.com/watch?v=5C5_dOEyAfk
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pattern…
OK, but we were talking about objects of the same shape with the concrete and air basketballs.
To be more correct:
For the same density, speed and shape, drag rises as size squared, while mass rises as size cubed.
Quote:
Drag is affected by mass.
For the same density, drag rises as size squared, while mass rises as size cubed.
Uh, no.
Drag is affected by size (among other things) and Mass is affected by size (among other things).
Drag is affected by mass.
For the same density, drag rises as size squared, while mass rises as size cubed.
Uh, no.
Drag is affected by size (among other things) and Mass is affected by size (among other things).
To be more correct:
For the same density, speed and shape, drag rises as size squared, while mass rises as size cubed.