Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Physics of falling objects

Tech Log The very best in practical technical discussion on the web

Physics of falling objects

Old 10th Mar 2014, 01:37
  #1 (permalink)  
Thread Starter
 
Join Date: Feb 2000
Location: 500 miles from Chaikhosi, Yogistan
Posts: 3,513
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.
compressor stall is online now  
Old 10th Mar 2014, 01:47
  #2 (permalink)  
 
Join Date: May 2013
Location: have I forgotten or am I lost?
Age: 66
Posts: 1,129
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.
dubbleyew eight is offline  
Old 10th Mar 2014, 02:04
  #3 (permalink)  
 
Join Date: Nov 2012
Location: In the room next to the lift
Posts: 41
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

Last edited by CaptainEmad; 10th Mar 2014 at 02:06. Reason: Removed the Eiffel tower reference.
CaptainEmad is offline  
Old 10th Mar 2014, 02:13
  #4 (permalink)  
 
Join Date: Sep 2010
Location: England
Posts: 1,955
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.
Lord Spandex Masher is offline  
Old 10th Mar 2014, 02:15
  #5 (permalink)  
 
Join Date: Jan 2011
Location: Seattle
Posts: 628
objects were dropped off the Eiffel tower
Is it possible to drop anything off the Eiffel tower without having it bounce off the side a couple of times?



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.
EEngr is offline  
Old 10th Mar 2014, 02:44
  #6 (permalink)  
Thread Starter
 
Join Date: Feb 2000
Location: 500 miles from Chaikhosi, Yogistan
Posts: 3,513
Originally Posted by dubbleyew eight View Post

...
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?
compressor stall is online now  
Old 10th Mar 2014, 02:45
  #7 (permalink)  
 
Join Date: Dec 2007
Location: Pasadena
Posts: 633
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.)
awblain is offline  
Old 10th Mar 2014, 02:56
  #8 (permalink)  
 
Join Date: Dec 1998
Location: Escapee from Ultima Thule
Posts: 4,230
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.

Last edited by Tinstaafl; 12th Mar 2014 at 11:30.
Tinstaafl is offline  
Old 10th Mar 2014, 02:59
  #9 (permalink)  
 
Join Date: Dec 2007
Location: Pasadena
Posts: 633
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.

Last edited by awblain; 10th Mar 2014 at 08:37. Reason: Being clearer on "Drag is affected by mass"
awblain is offline  
Old 10th Mar 2014, 03:44
  #10 (permalink)  
 
Join Date: Jan 2008
Location: Denver
Posts: 952
Is it possible to drop anything off the Eiffel tower without having it bounce off the side a couple of times?
Yes, actually. The first elevated floor is a square "doughnut" with a hole in the center overlooking the plaza underneath. Easy to drop something 190 feet from there without it hitting anything before the ground (there is a glass wall, previously a low railing, to prevent amateur gravity experiments. )

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.
Uh, no.

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).
pattern_is_full is offline  
Old 10th Mar 2014, 05:22
  #11 (permalink)  
 
Join Date: Oct 2011
Location: United Kingdom
Posts: 277
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.
Induced drag depends on lift, not mass. If you put two wings in a wind-tunnel at a fixed AoA and filled one with concrete the induced drag would be the same in each. For aircraft though more mass means more lift is required and induced drag increases.
ManUtd1999 is offline  
Old 10th Mar 2014, 05:29
  #12 (permalink)  
 
Join Date: Aug 2003
Location: Sale, Australia
Age: 75
Posts: 3,829
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
Exactly...
Brian Abraham is offline  
Old 10th Mar 2014, 06:14
  #13 (permalink)  
 
Join Date: Feb 2000
Location: Alaska, PNG, etc.
Age: 56
Posts: 1,518
Originally Posted by compressor stall View Post
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.
Unless you're misrepresenting the discussion (and I have n reason to believe that you are, just covering that possibility) Skippiness 1 E hasn't a clue what he's talking about.
A Squared is offline  
Old 10th Mar 2014, 06:58
  #14 (permalink)  
 
Join Date: Feb 2000
Location: Alaska, PNG, etc.
Age: 56
Posts: 1,518
Originally Posted by dubbleyew eight View Post
there was an experiment many years ago where objects were dropped off the Eiffel tower to explore the effect. you are wrong.
Well, no, he's not wrong. I don't know about the Eiffel tower experiment, but Galileo is famously (and perhaps apocryphally) credited with doing the same thing at the Tower of Pisa. Some accounts have the balls falling at the same speed, other accounts have the heavier ball falling perceptibly faster.

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.



Originally Posted by dubbleyew eight View Post
you don't get any more influence exerted on the concrete ball.
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.
A Squared is offline  
Old 10th Mar 2014, 07:00
  #15 (permalink)  
 
Join Date: Feb 2000
Location: Alaska, PNG, etc.
Age: 56
Posts: 1,518
Originally Posted by compressor stall View Post
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?
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.
A Squared is offline  
Old 10th Mar 2014, 07:11
  #16 (permalink)  
 
Join Date: Nov 2003
Location: UK/OZ
Posts: 1,672
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.
mickjoebill is offline  
Old 10th Mar 2014, 07:12
  #17 (permalink)  
 
Join Date: Feb 2000
Location: Alaska, PNG, etc.
Age: 56
Posts: 1,518
Originally Posted by Tinstaafl View Post
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.
It's true as far as it goes. Drag is not affected by mass, but the *force* of gravity certainly is; in fact it's proportional to mass.

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.
A Squared is offline  
Old 10th Mar 2014, 07:41
  #18 (permalink)  
 
Join Date: Dec 2001
Location: England
Posts: 1,384
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.
You are correct. When they reach terminal velocity Drag = weight. If the shape is the same the only way for drag to be different is if the velocity is different.

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 :-)
cwatters is offline  
Old 10th Mar 2014, 07:45
  #19 (permalink)  
 
Join Date: Feb 2000
Location: Alaska, PNG, etc.
Age: 56
Posts: 1,518
Originally Posted by cwatters View Post
They would be correct on the moon..
https://www.youtube.com/watch?v=5C5_dOEyAfk
When you're faking the footage on a soundstage in Burbank, you can make anything fall at any speed you want.
A Squared is offline  
Old 10th Mar 2014, 08:10
  #20 (permalink)  
 
Join Date: Dec 2007
Location: Pasadena
Posts: 633
pattern…

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).
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.
awblain is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.