Notices
Results 1 to 12 of 12

Thread: Falling object and sound barrier

  1. #1 Falling object and sound barrier 
    Forum Freshman
    Join Date
    Dec 2010
    Posts
    33
    Imagine that some certain object (sphere, hemisphere) is falling down vertically from some altitude, letís say 100 km. If we know its properties (mass, size, density) can we:
    1. Calculate when this object will passes through sound barrier.
    2. What can we say about the shock wave generated during passing through sound barrier? Is this sound too loud? Does its loudness depend on objectís mass?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Comet Dust Collector Moderator
    Join Date
    Mar 2011
    Location
    New Jersey, USA
    Posts
    2,848
    In almost all cases, it won't. The air resistance makes the terminal velocity far lower than the speed of sound.


    Reply With Quote  
     

  4. #3  
    Forum Professor jrmonroe's Avatar
    Join Date
    Mar 2010
    Posts
    1,444
    Also, there's something called transonic drag rise, where the coefficient of drag rises at and around the speed of sound. This is what is meant by the "sound barrier", which planes or pilots must "punch through". Supersonic airplanes require added thrust to succeed at flying through the transonic region. Once through, they can actually throttle back and conserve fuel.

    Here's more on it from NASA.

    Grief is the price we pay for love. (CM Parkes) Our postillion has been struck by lightning. (Unknown) War is always the choice of the chosen who will not have to fight. (Bono) The years tell much what the days never knew. (RW Emerson) Reality is not always probable, or likely. (JL Borges)
    Reply With Quote  
     

  5. #4  
    Forum Freshman
    Join Date
    Dec 2010
    Posts
    33
    MeteorWayne
    In almost all cases, it won't. The air resistance makes the terminal velocity far lower than the speed of sound.
    As far as I guess this refers to my question:
    can we:
    1. Calculate when this object will passes through sound barrier.
    Right?

    jrmonroe
    Your information is interesting but it has got nothing to do with my two questions
    Reply With Quote  
     

  6. #5 Re: Falling object and sound barrier 
    Forum Professor jrmonroe's Avatar
    Join Date
    Mar 2010
    Posts
    1,444
    Quote Originally Posted by Eagle9
    1. Calculate when this object will passes through sound barrier.
    2. What can we say about the shock wave generated during passing through sound barrier? Is this sound too loud? Does its loudness depend on objectís mass?
    1. As MeteorWayne said, most objects probably won't go transonic.
    2.a. Thus, no shock wave will occur.
    2.b. What is meant by "too loud"?
    2.c. If an object does go transonic, the aural characteristics of the boom depend on the object's size and probably its shape. Consider the difference between the crack of a rifle shot versus the sonic boom of fighter aircraft.
    Grief is the price we pay for love. (CM Parkes) Our postillion has been struck by lightning. (Unknown) War is always the choice of the chosen who will not have to fight. (Bono) The years tell much what the days never knew. (RW Emerson) Reality is not always probable, or likely. (JL Borges)
    Reply With Quote  
     

  7. #6  
    Comet Dust Collector Moderator
    Join Date
    Mar 2011
    Location
    New Jersey, USA
    Posts
    2,848
    Quote Originally Posted by Eagle9
    MeteorWayne
    In almost all cases, it won't. The air resistance makes the terminal velocity far lower than the speed of sound.
    As far as I guess this refers to my question:
    can we:
    1. Calculate when this object will passes through sound barrier.
    Right?

    jrmonroe
    Your information is interesting but it has got nothing to do with my two questions
    Actually it does specifically. I told you only a specially designed object would exceed the sound barrier. A round meteorite (which is quite dense) the size of a basketball falling from 60 km reaches a terminal velocity at most a few hundred mph, well below the sound barrier. And a less round or less dense object would be even slower.

    Only a specifically designed projectile could exceed the speed of sound, I doubt it. But don't have the time to spend to do the physics. If you want to know, you do it.



    Wayne
    Reply With Quote  
     

  8. #7  
    Forum Freshman
    Join Date
    Dec 2010
    Posts
    33
    jrmonroe
    1. As MeteorWayne said, most objects probably won't go transonic.
    2.a. Thus, no shock wave will occur.
    Wow! Because of objectís shape? So, the sphere (or hemisphere) will be braked enough during falling down so that its speed will remain subsonic? Did I understand correctly?

    MeteorWayne
    Actually it does specifically. I told you only a specially designed object would exceed the sound barrier. A round meteorite (which is quite dense) the size of a basketball falling from 60 km reaches a terminal velocity at most a few hundred mph, well below the sound barrier. And a less round or less dense object would be even slower.
    I see could you please tell me from which law of Physics derives this? I mean if somebody asks me why the sphere (meteorite or other) will not pass sound barrier I want to know the answer
    Reply With Quote  
     

  9. #8  
    Comet Dust Collector Moderator
    Join Date
    Mar 2011
    Location
    New Jersey, USA
    Posts
    2,848
    Quote Originally Posted by Eagle9
    jrmonroe
    1. As MeteorWayne said, most objects probably won't go transonic.
    2.a. Thus, no shock wave will occur.
    Wow! Because of objectís shape? So, the sphere (or hemisphere) will be braked enough during falling down so that its speed will remain subsonic? Did I understand correctly?
    Yes, that's what I said. I study meteorics and meteoritics.

    MeteorWayne
    Actually it does specifically. I told you only a specially designed object would exceed the sound barrier. A round meteorite (which is quite dense) the size of a basketball falling from 60 km reaches a terminal velocity at most a few hundred mph, well below the sound barrier. And a less round or less dense object would be even slower.
    I see could you please tell me from which law of Physics derives this? I mean if somebody asks me why the sphere (meteorite or other) will not pass sound barrier I want to know the answer
    It's high school level aerodynamics.
    Reply With Quote  
     

  10. #9  
    Forum Professor jrmonroe's Avatar
    Join Date
    Mar 2010
    Posts
    1,444
    Quote Originally Posted by Eagle9
    jrmonroe
    1. As MeteorWayne said, most objects probably won't go transonic.
    2.a. Thus, no shock wave will occur.
    Wow! Because of objectís shape? So, the sphere (or hemisphere) will be braked enough during falling down so that its speed will remain subsonic? Did I understand correctly?

    MeteorWayne
    Actually it does specifically. I told you only a specially designed object would exceed the sound barrier. A round meteorite (which is quite dense) the size of a basketball falling from 60 km reaches a terminal velocity at most a few hundred mph, well below the sound barrier. And a less round or less dense object would be even slower.
    I see could you please tell me from which law of Physics derives this? I mean if somebody asks me why the sphere (meteorite or other) will not pass sound barrier I want to know the answer
    Two forces play tug of war, the object's aerodynamic drag and its weight.

    The aerodynamic drag of an object is D = CD(p/2)V≤S, where CD is the coefficient of drag, which is a function of shape under most conditions; however, as I stated previously, it can increase dramatically in the transonic region of velocity V.

    The weight of an object (W) results from the earth's gravitational field acting upon the object's mass, and it is mostly constant although varying very slightly with altitude.

    The acceleration of the object derives from the famous F = ma equation, or solving for acceleration a = F/m, where F is the total force acting on the object, that is, W Ė D.

    When dropping an object, the intial velocity is zero, and thus, the drag D is also zero and the acceleration equals W/m. As long as W > D, then F will be positive and the object will continue to gain downward speed (and thus drag) until D = W, making F zero, meaning that it has reached its terminal velocity.

    So, speaking in terms of object density and shape (and grossly exaggerating), a sphere of aerogel will never break the sound barrier, but a huge javelin of solid iridium might. The object's ability to "punch" through the sound barrier is mostly a matter of cross-sectional density, so the longer the object, the higher the density (as I hinted at by comparing a sphere to a javelin).
    Grief is the price we pay for love. (CM Parkes) Our postillion has been struck by lightning. (Unknown) War is always the choice of the chosen who will not have to fight. (Bono) The years tell much what the days never knew. (RW Emerson) Reality is not always probable, or likely. (JL Borges)
    Reply With Quote  
     

  11. #10  
    Forum Freshman
    Join Date
    Dec 2010
    Posts
    33
    MeteorWayne
    Yes, that's what I said. I study meteorics and meteoritics.
    Thanks

    jrmonroe
    So, speaking in terms of object density and shape (and grossly exaggerating), a sphere of aerogel will never break the sound barrier, but a huge javelin of solid iridium might. The object's ability to "punch" through the sound barrier is mostly a matter of cross-sectional density, so the longer the object, the higher the density (as I hinted at by comparing a sphere to a javelin).
    I see and do you know the critical cross-sectional density above of which the object can go supersonic?
    Reply With Quote  
     

  12. #11  
    Forum Professor jrmonroe's Avatar
    Join Date
    Mar 2010
    Posts
    1,444
    Quote Originally Posted by Eagle9
    I see and do you know the critical cross-sectional density above of which the object can go supersonic?
    It's not easy to calculate, and it would also depend on the height from which it was dropped.

    At minimum, we can use the standard motion equations, v = gt and s = Ĺat≤, to find that an object dropping in a vacuum from 600 feet will reach 760 mph (Mach 1) as it hits the ground. So, dropping it in air would require a height greater than 600 feet.

    Calculating this supersonic cross-sectional density gets really hairy as the density of the air itself and the speed of sound decreases with altitude.

    Grief is the price we pay for love. (CM Parkes) Our postillion has been struck by lightning. (Unknown) War is always the choice of the chosen who will not have to fight. (Bono) The years tell much what the days never knew. (RW Emerson) Reality is not always probable, or likely. (JL Borges)
    Reply With Quote  
     

  13. #12  
    Forum Freshman
    Join Date
    Dec 2010
    Posts
    33
    Quote Originally Posted by jrmonroe
    Quote Originally Posted by Eagle9
    I see and do you know the critical cross-sectional density above of which the object can go supersonic?
    It's not easy to calculate, and it would also depend on the height from which it was dropped.

    At minimum, we can use the standard motion equations, v = gt and s = Ĺat≤, to find that an object dropping in a vacuum from 600 feet will reach 760 mph (Mach 1) as it hits the ground. So, dropping it in air would require a height greater than 600 feet.

    Calculating this supersonic cross-sectional density gets really hairy as the density of the air itself and the speed of sound decreases with altitude.

    Ok, thanks
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •