Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Request explanation, Please?

  1. Aug 14, 2012 #1
    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.
     
  2. jcsd
  3. Aug 14, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The acceleration of a free falling body (ignoring air resistance) is about 32 ft/sec/sec.
     
  4. Aug 14, 2012 #3

    Dale

    Staff: Mentor

    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/
     
  5. Aug 14, 2012 #4

    BruceW

    User Avatar
    Homework Helper

    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).
     
  6. Aug 14, 2012 #5

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    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!
     
  7. Aug 15, 2012 #6

    BruceW

    User Avatar
    Homework Helper

    poor tim. he will always be remembered.
     
  8. Aug 15, 2012 #7

    TheDemx27

    User Avatar
    Gold Member

    as the man who sacrificed himself to create a slightly more interesting math problem.
     
  9. Aug 19, 2012 #8
    my answer is 18.11 secs??
     
  10. Aug 19, 2012 #9
    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......
     
  11. Aug 19, 2012 #10

    BruceW

    User Avatar
    Homework Helper

    looks good to me.
     
  12. Aug 20, 2012 #11

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You forget, he is at terminal velocity! It is not an acceleration problem, but one of constant velocity.
     
  13. Aug 21, 2012 #12

    BruceW

    User Avatar
    Homework Helper

    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.
     
  14. Aug 21, 2012 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That'll be why Philosophers couldn't put a Rover on Mars - but they could discuss the significance all night.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Request explanation, Please?
  1. Vacuum explanation (Replies: 4)

Loading...