Do Objects in Free Fall Encounter Air Resistance?

  • #1
davenn
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www.physicsclassroom.com/class/1DKin/Lesson-5/The-Big-Misconception

"Free-fall is the motion of objects that move under the sole influence of gravity; free-falling objects do not encounter air resistance.

Huh ?? of course it encounters air resistance, doesnt it ?
An object, regardless of what it is, encounters resistance, it's what keeps the freefall speed constant for that object

os is truer freefall definition , falling in a vacuum ? everything else has a terminal velocity ?

Dave

PS> I think I tripped myself up haha
 
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  • #2
davenn said:
is truer freefall definition , falling in a vacuum ?
Yes
 
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  • #3
davenn said:
Huh ?? of course it encounters air resistance, doesnt it ?
Not on the Moon. The quoted exerpt says nothing about "near the surface of the Earth".
 
  • #4
A.T. said:
Yes

thanks, Initially when i read that statement online, I was forgetting the about terminal velocity,
and that was what was leading me astray

Old age and and senility is my excuse haha

Dave

and to show how bad that is .... I typed this response 2 hrs ago and just found that I
hadnt sent it haha
Long before Kuruman responded LOL
 
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  • #5
Free-fall is defined as moving only under the influence of gravity, so strictly speaking nothing falling in atmosphere is in free fall because it always experiences drag force. So I think "free-falling objects do not encounter air resistance" (as quoted in the OP) is stated the wrong way round - it would be better to say "if it encounters air resistance, it's not in free-fall".

The usual "eh, close enough" rules apply to stuff like dropping a rock a short distance from rest, and we use free-fall calculations for that kind of thing because the error is negligible.
 
  • #6
It's a physics definition vs a colloquial/laymen's definition. Skydivers refer to what they are doing as "freefall".
 
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  • #7
russ_watters said:
Skydivers refer to what they are doing as "freefall".
And the videos they take of each other show that opening a parachute will send you upwards. Just what sort of a world are they living in?
 
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  • #8
sophiecentaur said:
And the videos they take of each other show that opening a parachute will send you upwards. Just what sort of a world are they living in?
Relative?
 
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  • #9
russ_watters said:
Skydivers refer to what they are doing as "freefall".
And so did Tom Petty.
 
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  • #10
davenn said:
freefall definition , falling in a vacuum ?
Yes. Note that ##g## is properly called the free fall acceleration.
 
  • #11
Mister T said:
Yes. Note that ##g## is properly called the free fall acceleration.
A huge change of scale and we think of motion in deep space as being 'free fall' but the g forces are minuscule and I'd imagine that the 'drag' due to momentum exchange in very low density gas and dust must often be of the same order of magnitude - which explains why nebulae evolve to form planetary discs.
 

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