Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    I keep getting the wrong answer on this question and can't see why

    A satellite is required to orbit the Earth once every 12 hours. Calculate the necessary radius of its orbit.
    if you use T^2 = [(4xπxπ )/(GM)] x r^3
    you get (43200)^2 = [(4xπxπ )/(4x10^14)] x r^3 //M = 6.0x10^24(kg)
    then r^3 = 1.89x10^22
    so r = 26.6x10^6(m)

    However the answer should be 90.9x10^6(m) but I can't see where I am going wrong.
    Offline

    2
    ReputationRep:
    Seems right to me, but i could be making the same mistake that you are. Is the question from a textbook? Its not unusual for a textbook to publish the answers wrong.
    Offline

    2
    ReputationRep:
    (Original post by Chris2505)
    I keep getting the wrong answer on this question and can't see why



    if you use T^2 = [(4xπxπ )/(GM)] x r^3
    you get (43200)^2 = [(4xπxπ )/(4x10^14)] x r^3 //M = 6.0x10^24(kg)
    then r^3 = 1.89x10^22
    so r = 26.6x10^6(m)

    However the answer should be 90.9x10^6(m) but I can't see where I am going wrong.
    Got the same answer as you, dont think the book will be wrong though as its quite big difference
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by Im_a_cyborg)
    Seems right to me, but i could be making the same mistake that you are. Is the question from a textbook? Its not unusual for a textbook to publish the answers wrong.
    Yeah from a textbook but htere has been the odd mistake in it but usually just a typo, not a calculation error.
    Offline

    20
    Maybe they only had pi once in there? It looks like it's out by a factor of ~3...
    Offline

    2
    ReputationRep:
    (Original post by Chris2505)
    Yeah from a textbook but htere has been the odd mistake in it but usually just a typo, not a calculation error.
    I have come across a few errors in the answers in mine. Especially that OCR Physics 2 one.
    Offline

    2
    ReputationRep:
    well i also got the same answer maybe the mark scheme is wrong
    • Thread Starter
    Offline

    0
    ReputationRep:
    Thanks anyway, must be a book error, again...

    90.9Mm does seem a very big distance though!!
    Offline

    9
    ReputationRep:
    (Original post by Chris2505)
    I keep getting the wrong answer on this question and can't see why



    if you use T^2 = [(4x?x? )/(GM)] x r^3
    you get (43200)^2 = [(4x?x? )/(4x10^14)] x r^3 //M = 6.0x10^24(kg)
    then r^3 = 1.89x10^22
    so r = 26.6x10^6(m)

    However the answer should be 90.9x10^6(m) but I can't see where I am going wrong.
    they didnt use 4pi^2 to get their answer beats me why they didnt include it
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 22, 2010
Poll
Do you like carrot cake?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.