Request explanation, Please?

  • Thread starter Grimstone
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  • #1
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I had this elaborate example all set up to lay my question out. and I can hear my x wife in my head. "you over think everything"

So here is my question.
I do not understand free falls 93feet/sec /second.
this sounds to me that Tim would be doing 279 feet per second after only 3 seconds. ??:confused:


If Tim fell 1 mile. how long till he reached ground?

Please. K.I.S.I.S keep it simple I'm stupid.
 

Answers and Replies

  • #2
Doc Al
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So here is my question.
I do not understand free falls 93feet/sec /second.
The acceleration of a free falling body (ignoring air resistance) is about 32 ft/sec/sec.
 
  • #3
Dale
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Here is an online calculator for this stuff, but you have to convert it to SI units, not feet and miles.

http://planetcalc.com/981/
 
  • #4
BruceW
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I agree with Doc Al's approximate value.

To Grimstone (a small explanation, hopefully I can help in case you are confused about free fall stuff): The meaning of 32 ft/sec/sec is essentially the acceleration of the object in free-fall. This value is simply due to Earth's gravity.

If free-fall acceleration was 93 ft/sec/sec as you said, then yes the object would be doing 279 feet per second after only 3 seconds. (But this value for the acceleration is roughly three times more than what it actually is for Earth).

Lastly, about Tim falling a mile, and how much time this would take: To work this out, you need to do a bit of calculus, or use the equations for constant acceleration (the suvat equations, was the name used when they were taught to me).
 
  • #5
HallsofIvy
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Perhaps Grimstone meant that terminal velocity, the speed at which air resistance equals the force of gravity so we have a constant speed, is 93 feet per second (not "per second per second). At 93 feet per second, it would take 279/93= 3 seconds to fall 279 feet. At that terminal velocity, it would take Tim 5280/93= 56.8 seconds to fall a mile- almost a minute. Of course, Tim isn't really going to be concerned about that!
 
  • #6
BruceW
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poor tim. he will always be remembered.
 
  • #7
TheDemx27
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poor tim. he will always be remembered.

as the man who sacrificed himself to create a slightly more interesting math problem.
 
  • #8
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my answer is 18.11 secs??
 
  • #9
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here's my solution
1mi=5280 ft
y=-1/2gt^2
-5280 = -1/2(32.2)t^2

t^2= -5280/(-16.1)

t^2 = 327.95

t=18.11 secs......
 
  • #10
BruceW
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looks good to me.
 
  • #11
Integral
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here's my solution
1mi=5280 ft
y=-1/2gt^2
-5280 = -1/2(32.2)t^2

t^2= -5280/(-16.1)

t^2 = 327.95

t=18.11 secs......

You forget, he is at terminal velocity! It is not an acceleration problem, but one of constant velocity.
 
  • #12
BruceW
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Yes, Grimstone's tim is travelling at terminal velocity, and killme22's tim is falling without air resistance. I forgot the earlier posts. Both answers are right really, but it depends on the assumptions made. That's one of the things I like about physics, the assumptions we use can often be explicitly stated. But in philosophy, literature, e.t.c. the assumptions are often vague or not explicitly stated.
 
  • #13
sophiecentaur
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That'll be why Philosophers couldn't put a Rover on Mars - but they could discuss the significance all night.
 

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