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Circular Motion Question

  1. Nov 17, 2004 #1
    Hi guys.what do you think of the answers i have got for the questions below.If you think i have made any mistakes plz do say so.I think my answers are weird,

    a) A space station orbits the earth in every 91 mins .calculate the angular speed of the space station
    ans) 91/60 times 2pie=9.52 radians

    b)The space station orbit is 210 km above the surface of the earth which has the radius of 6370 km.Fine the acceleration of the space station.
    ans) a=rw(squared)
    r=(6370000 + 210000)m
    w=9.52(from above)

    Therefore a= 6.2 times 10^7

    I thin the value of a is toooooooo large...I am confused here

    c)A box of mass 4.1 kg is located inside the space station.What is the size and the direction of the resultant force acting on the box in the space station?
    Direction of Resultant force=towards the earth

    size=F=ma=4.1 times 6.2 times 10^7=2.5 times 10^8.....I think this is incredbly a very large force...I have got a feeling that all my answers are wrong..Plz help me out!!!
    Last edited by a moderator: May 28, 2015
  2. jcsd
  3. Nov 17, 2004 #2
    (a) angular speed = 2pi/period
    = 2pi/(91 x 60)
    = 1.15 x 10^-3 rad/s
    (b) acceleration = w^2 r
    = (1.15 x 10^-3)^2 (210000 + 6370000)
    = 8.71 m/s^2
    (c) magnitude = 4.1 (8.71)
    = 35.7 N
    direction : towards centre of the earth
  4. Nov 17, 2004 #3


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    First of all, this is not a criticism, but my curiosity has gotten the best of me. So against my better judgement, I'm going to ask.

    Is there a reason why a lot of your questions on here seems to be of the nature of "ASAP" or "Urgent", or of those immediate in nature? Considering that I skim through this section of PF rather quickly, it has happened often enough for me to notice it.

    Based on my experience, whenever I come across things like this online, it is either (i) from a student doing an online quiz, homework, test, or something of that sort, and wanting an answer immediately to enter into that "box"; or (ii) from a student doing last minute cram study and suddenly discovering he/she doesn't know or understand something.

    Either way, this isn't good or the most effective means of leanring. It should not happen, at least not this frequently, that you need "urgent" help in your studies. You will not understand things well when you're in a hurry.

  5. Nov 17, 2004 #4
    I am revising loads of things,all at the same time for my exams in jan.thats the main reason i wanted immediate response.I am sorry if that pissed you off.
  6. Nov 17, 2004 #5


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    Well now, reread what I said carefully or this will turn into something else. I said it wasn't a criticism, so I wasn't "pissed". I was just curious since this appears to be a very common theme in almost all the questions you asked. I wanted to advice (even if it wasn't asked for) that if it happens that you're doing all these in a hurry, there's a good chance you will not learn much from them.

    So then, the reason why you want all of these ASAP, Urgently, etc. is because you are .......... er ........... impatient?

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