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Homework Help: MY First Post what force is exerted on a astronaut in space that ?

  1. Jul 21, 2009 #1
    MY First Post....what force is exerted on a astronaut in space that...??

    hi im prepping for a upcoming exam and im trying to understand some concepts of physics and the force of gravity....which my exam will be on and i need to understand these problems so that i can know how to do the rest being that i was left in the dark without anything to guide my self by...and if you could help me out i would really appreciate it alot..... i will be asking alot of questions in the upcoming days about all kinds of topics

    29 ) What force does earth's gravity exerts on a 81-kg astronaut who is orbiting in a space station at a distance from the SURFACE of the earth equal to 2 earth radii? Show your calculations.Your answer must be in newtons........PLEASE TELL ME IF THIS IS RIGHT..these are just appoximate numbers that i will be using

    F=G Mm/r^2 = 81kg x (6 x 10 ^24) / 2 (EARTH RADII) =
    243 x 10^24 (x G = 6.67 x 10 ^-11) = 1620.81 x 10^13 N....this is the answer i got but i feel i am really off......please help me ...i need to know how to do this before i take the exam........

    30) The astronaut from the question above (the one that weighs 81 kg )was launched into space by a Saturn rocket. The upward acceleration of a Saturn rocket shortly after blast off is 80 km/s^2. When the Saturn rocket is accelerating, what is the apparent experience as the weight of his body in other words what does the astronaut experience as the weight of his body? Show your calculations keeping in mind the definition of weight and that more than acceleration due to gravity is acting on the astronaut.Your answer must be in newtons.....this is the answer i got...........

    ..F=MxA......so 81 kg x 80 m/s^2 =6480 N....but also think this is wrong....i dont have a lot of practice with this so please help me out .........
  2. jcsd
  3. Jul 21, 2009 #2
    Re: MY First Post....what force is exerted on a astronaut in space that...??

    First of all, welcome to PF! It is always nice to see new members log on!

    Second, I would like to inform you that this is not where we make questions concerning homework, that has a special section you can access from the PF home page called Homework and Coursework Questions.

    Third I looked over your question and you are doing it almost correct. The only problem is that you are plugging in two earth radii in for your distance. Remember that gravity is always measured from the CENTER of a body to the center of the other body(hint hint)!
  4. Jul 22, 2009 #3
    Re: MY First Post....what force is exerted on a astronaut in space that...??

    thanks jqnsmart....well i did not know that so from now on ill be making my questions there but thanks though for replying back ill check it out and thanks for the tip i think someone helped me figure it out...
  5. Jul 22, 2009 #4
    Re: MY First Post....what force is exerted on a astronaut in space that...??

    Yes, your first answer was almost correct. You needed to use 3 earth radii since you need to measure the distance to the center of the object exerting the gravitational pull.

    As for your second answer, you are very off.
    I'd just like to ask two questions before we go a bit more in depth.
    What is the definition of apparent weight?
    Are you familiar with fictitious forces (D'alembert forces)?

    If you're not familiar with fictitious forces, then we'll approach the problem from an inertial frame (I'll be glad to explain how to approach the problem from an accelerated frame as well, if you're interested).
    Draw a free-body diagram of the astronaut as he is seen by an observer traveling at a constant velocity (Remember that the astronaut is being accelerated!)
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