| Author |
Message
|
| JaneBennet |
 Posted: Wed Apr 16, 2008 2:55 am Post subject: Acceleration of free fall / Acceleration due to gravity |
 |
|
Forum Ph.D.
Joined: 06 Apr 2008
Posts: 878
|
I’ve always thought the the two terms mean the same thing. However, in an edition of the previous series of “University Challenge” (a British university quiz show hosted by Jeremy Paxman), when one of the two contesting teams gave “acceleration due to gravity” as an answer to one question, the answer was disallowed by Jeremy Paxman because the answer on his card was “acceleration of free fall”. 
I was totally stunned. 
Fortunately the team that had that answer disallowed won by a comfortable margin in the end and so there wasn’t a fuss over that particular question. Well then, is there really a technical difference in sense between the terms “acceleration of free fall” and “acceleration due to gravity”? Have I been right all along to consider both expressions as synonymous with each other? 
_________________
Did You Know?
Random Fact: Electromagnetic waves |
|
| Back to top |
|
 |
| bit4bit |
| Posted: Wed Apr 16, 2008 4:42 am Post subject: |
|
|
Forum Ph.D.
Joined: 14 Jul 2007
Posts: 625
|
You're right, they mean exactly the same thing....g = 9.81ms-2 near the Earths surface. I think Jeremey Paxman was either completely ignorant of it, or was just being too harsh.
_________________
Chance favours the prepared mind. |
|
| Back to top |
|
|
| Ophiolite |
| Posted: Wed Apr 16, 2008 5:09 am Post subject: |
|
|
Forum Radioactive Isotope
Joined: 25 Jul 2005
Posts: 4804
Location: Scotland
|
The very phrase "acceleration of free fall" seems clumsy and ambiguous to me. It implies that the strange entity free fall is the thing doing the accelration.
Something like "acceleration in free fall" makes more sense.
It is difficult, without knowing the original question, whether the disitinction between the two is valid or not. It seems to me that I have typically (perhaps only) heard free fall referred to falling within the atmosphere. In that case the acceleration is going to be strongly modified and eventually limited by air resistance. In this interpretation acceleration due to gravity and acceleration during free fall would be different.
Just a thought.
_________________
The Universe is not only weirder than we imagine it is weirder than we can imagine. J.B.S.Haldane. |
|
| Back to top |
|
|
| SteveF |
| Posted: Wed Apr 16, 2008 5:16 am Post subject: |
|
|
Forum Bachelors Degree
Joined: 28 May 2007
Posts: 483
Location: NC USA
|
Â
Certainly there is a difference!
An object sliding down a ramp is accelerating due to gravity. It is definitely not in free fall.
You didn't state the quiz question. It is possible that either answer may have been acceptable, as you say. Or maybe not. The program ought to have had a seated panel of experts who can rule on an answer if the moderator is uncertain. That's how it's done on many US quiz programs. |
|
| Back to top |
|
|
| bit4bit |
| Posted: Wed Apr 16, 2008 5:34 am Post subject: |
|
|
Forum Ph.D.
Joined: 14 Jul 2007
Posts: 625
|
Numerically, "acceleration due to gravity" and "acceleration of free fall" are exactly the same (9.81ms-2), since neither takes into account any effects of wind resistance or drag.
If you are in free fall, negleting the effects of drag, your acceleration (of free fall) is due to gravity, and is 9.81ms-2.
For the case of an object going down a ramp, the acceleration is only acting perpendicularly to the Earth's surface, and so it only gets factored into the vertical component of the objects motion. It is still numerically equal to "acceleration of free fall", though I agree there is some ambiguity to the context of the question, since for that example, "acceleration of free fall" is not particularly clear.
_________________
Chance favours the prepared mind. |
|
| Back to top |
|
|
| William McCormick |
| Posted: Wed Apr 16, 2008 4:25 pm Post subject: |
|
|
Forum Professor
Joined: 03 Apr 2008
Posts: 1301
|
| bit4bit wrote: |
Numerically, "acceleration due to gravity" and "acceleration of free fall" are exactly the same (9.81ms-2), since neither takes into account any effects of wind resistance or drag.
If you are in free fall, negleting the effects of drag, your acceleration (of free fall) is due to gravity, and is 9.81ms-2.
For the case of an object going down a ramp, the acceleration is only acting perpendicularly to the Earth's surface, and so it only gets factored into the vertical component of the objects motion. It is still numerically equal to "acceleration of free fall", though I agree there is some ambiguity to the context of the question, since for that example, "acceleration of free fall" is not particularly clear. |
You would think "Acceleration during free fall" would be a better way to clarify it.
However I have noticed that "Great Scientists, highly decorated". Often put a more complex even bewildering term on things that we have used for years and years. And have communicated it already, without problem or incident for many years.
In comes the new term and we have to joke around with it for months sometimes, until it is part of our vocabulary. And sometimes it causes a problem.
Simple wins as far as I am concerned. Complex is for the error prone.
Sincerely,
William McCormick
_________________
Warning: Any information contained in this post could be part of a conspiracy to make you stupid. So only use it if you understand it. Use at your own risk.
http://www.Rockwelder.com |
|
| Back to top |
|
|
| 425 Chaotic Requisition |
| Posted: Wed Apr 16, 2008 4:28 pm Post subject: |
|
|
The Doctor
Joined: 18 Jun 2007
Posts: 3320
Location: England, UK.
|
Distance from location of beginning of freefall to terminal velocity, I'm sure theres an equation in there somewhere  .
_________________
"Victory is in trying. Defeat is in not". - SVRDW. |
|
| Back to top |
|
|
| JaneBennet |
| Posted: Wed Apr 16, 2008 4:36 pm Post subject: |
|
|
Forum Ph.D.
Joined: 06 Apr 2008
Posts: 878
|
| SteveF wrote: |
Certainly there is a difference!
An object sliding down a ramp is accelerating due to gravity. It is definitely not in free fall. |
Hmm, I’ve never thought about it this way before. But it makes sense. 
_________________
Did You Know?
Random Fact: Electromagnetic waves |
|
| Back to top |
|
|
| i_feel_tiredsleepy |
| Posted: Wed Apr 16, 2008 4:49 pm Post subject: |
|
|
Forum Ph.D.
Joined: 21 Mar 2008
Posts: 659
Location: Montreal
|
| JaneBennet wrote: |
| SteveF wrote: |
Certainly there is a difference!
An object sliding down a ramp is accelerating due to gravity. It is definitely not in free fall. |
Hmm, I’ve never thought about it this way before. But it makes sense. |
Look at Bit4Bit's post, the acceleration is still due to gravity. |
|
| Back to top |
|
|
| JaneBennet |
| Posted: Wed Apr 16, 2008 5:01 pm Post subject: |
|
|
Forum Ph.D.
Joined: 06 Apr 2008
Posts: 878
|
| i_feel_tiredsleepy wrote: |
| JaneBennet wrote: |
| SteveF wrote: |
Certainly there is a difference!
An object sliding down a ramp is accelerating due to gravity. It is definitely not in free fall. |
Hmm, I’ve never thought about it this way before. But it makes sense. |
Look at Bit4Bit's post, the acceleration is still due to gravity. |
Exactly. That’s what SteveF said.
_________________
Did You Know?
Random Fact: Electromagnetic waves |
|
| Back to top |
|
|
| Harold14370 |
| Posted: Wed Apr 16, 2008 5:51 pm Post subject: |
|
|
Forum Professor
Joined: 13 Apr 2007
Posts: 1675
Location: Pennsylvania
|
| Technically, Steve is right, but it would really be nitpicky to disallow the answer on that basis. As I remember from physics classes, g is called "the acceleration due to gravity" even if the object is sitting on the ground. Then its weight is "mass multiplied by the acceleration due to gravity." |
|
| Back to top |
|
|
| William McCormick |
| Posted: Wed Apr 16, 2008 6:23 pm Post subject: |
|
|
Forum Professor
Joined: 03 Apr 2008
Posts: 1301
|
| 425 Chaotic Requisition wrote: |
| Distance from location of beginning of freefall to terminal velocity, I'm sure theres an equation in there somewhere . |
Terminal velocity is only a valid thing passing through a substance that can slow it down. In space it could probably reach almost any speed.
The terms we are discussing are way to vague to describe something like that.
And even then the objects terminal velocity is determined by its temperature, structure, and or its ability to emit radio, laser or light beams ahead of it. To cut a path, much like lightning does through air.
Some objects can actually use the air passing through the object, to detonate air behind it. They are often dangerous but realistic.
Sincerely,
William McCormick
_________________
Warning: Any information contained in this post could be part of a conspiracy to make you stupid. So only use it if you understand it. Use at your own risk.
http://www.Rockwelder.com |
|
| Back to top |
|
|
| 425 Chaotic Requisition |
| Posted: Thu Apr 17, 2008 12:35 am Post subject: |
|
|
The Doctor
Joined: 18 Jun 2007
Posts: 3320
Location: England, UK.
|
Yeah but something or someone in freefall usually is pulled into the object that is causing the gravity. I was merely applying naturally this question to Earth. Logically that would to most people be easier to comprehend.
_________________
"Victory is in trying. Defeat is in not". - SVRDW. |
|
| Back to top |
|
|
| KALSTER |
| Posted: Thu Apr 17, 2008 7:14 am Post subject: |
|
|
Forum Cosmic Wizard
Joined: 08 Sep 2007
Posts: 2222
Location: South Africa
|
Maybe there was an orbital element to the question? While the numerical value is still the same, some ambiguity might have arisen between linear and vector acceleration? An object in orbit for instance has almost no linear acceleration, but it does vectorily. Such an object is in free fall, but undergoes no linear acceleration. An object falling linearly toward the gravitational centre of the earth does undergo linear acceleration. Both are due to gravity obviously, so I guess it comes down to the quizzer’s lack of understanding. IMHO
_________________
"Gullibility kills" - Carl Sagan
As jy dom is, moet jy kak. |
|
| Back to top |
|
|
| William McCormick |
| Posted: Thu Apr 17, 2008 8:37 pm Post subject: |
|
|
Forum Professor
Joined: 03 Apr 2008
Posts: 1301
|
| 425 Chaotic Requisition wrote: |
| Yeah but something or someone in freefall usually is pulled into the object that is causing the gravity. I was merely applying naturally this question to Earth. Logically that would to most people be easier to comprehend. |
I have yet to have anyone ever, explain attraction or pulling. And don't get me wrong, I say the vacuum sucks, even though it is the pressure outside the vacuum hose in the room, pushing air into the vacuum hose.
But I am saying that it appears that something from above the planet is pinning us to the planet, rather then the planet sucking us to it. That is also how I was taught.
Sincerely,
William McCormick
_________________
Warning: Any information contained in this post could be part of a conspiracy to make you stupid. So only use it if you understand it. Use at your own risk.
http://www.Rockwelder.com |
|
| Back to top |
|
|
|
|
