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Homework Help: Acceleration question please help!

  1. Nov 8, 2007 #1
    When the space shuttle is flying in space, away from the influence of any gravitational fields, its engines provide a total thrust of 2.4 X 10^7 N. If the shuttle weighs 2.0 X 10^7 N on the surface of the earth, what is its acceleration? (use g=10 m/s^2)

    Fg= (2.0 X 10^7 N)(10 m/s^2)
    = 2.0 X 10^8 N m/s^2

    Fnet= (2.0 X 10^8 N m/s^2)-(2.4 X 10^7 N)
    = 1.76 X 10^8 N m/s^2

    a= 1.76 X 10^8

    I have absolutely no clue how to do this but this is what I have figured out can someone please explain for me?
     
  2. jcsd
  3. Nov 8, 2007 #2
    If the shuttle WEIGHS 2 x 10^7 N on earth, then this is the force.

    What you need to calculate is the mass of the shuttle using F = mg

    The question states that it is away from the influence of any gravitational fields, so the net force is given by the thrust.

    Using F = ma you can then determine the acceleration.
     
  4. Nov 8, 2007 #3
    I'm sorry but I still don't understand.

    so the mass of the shuttle would be 2 x 10^8 N then?
    and Fnet= 2.4 X10^7?
     
  5. Nov 8, 2007 #4
    hey, lol this looks awefully similar to a problem i have in my textbook.. but anyway
    its like how he said, u have to remember that mass doesn't change, weight does. So in earth, you are given a weight. Therefore, you can find the mass of the shuttle. you see

    W=mg (use 10 for your g), then once you have that..use F(thrust)= ma

    simple heh
     
  6. Nov 8, 2007 #5
    mass of the shuttle is in KG, and force is in NEWTONS (N), it is important to get the units right..
     
  7. Nov 8, 2007 #6
    OK.... If something weighs 2 x 10^7 N, then this is the force with which gravity is pulling it towards Earth.

    Its mass can be determined from a rearrangement of F= mg (mass is measured in kg)

    Now, if we have a mass shooting through space, being pushed by an engine with a force of 2.4 x 10^7 N, we can figure out its acceleration using a rearrangement of F = ma.


    Note that these are two different cases, so the F in each is different.
     
  8. Nov 8, 2007 #7
    right, use W=mg, because well its not wrong technically, but weight is a force, and in physics you generally use W for weight instead of Fg. anyway tell me what u get
     
  9. Nov 8, 2007 #8
    and remember the mass doesn't change
     
  10. Nov 8, 2007 #9
    I think my head is all goofed up today I just cant seem to do math correctly haa..

    So I would then take,
    2.4 X 10^7 N = (2.0 X 10^8 N)(a)
    -2.0 X 10^8 N -2.0 X 10^8 N
    -1.76 X 10^8 N = a


    ??
     
  11. Nov 8, 2007 #10
    lol, atleast you realized that is wrong.. but look acceleration is (m/s^2) force is (kg m/s^2) aka: mass x acceleration. anyway lol
    2.0x10^7 N= mg (10=g)
    then take that mass
    2.4x10^7N=ma (a is what you solving for) if i do anything more, i will end up solving the problem
     
  12. Nov 8, 2007 #11
    Not quite.

    Your 2.0 x 10^8 is wrong.

    W = mg

    2.0 x 10^7 = m (10)

    m=?

    then use F(thrust) = ma
     
  13. Nov 8, 2007 #12
    okay so then...
    2.0 X 10^7 = m(10)
    -10 -10
    1.9 X 10^7 = m

    2.4 X 10^7 N = 1.9 X 10^7(a)
    -1.9X10^7N -1.9X10^7N

    5.0 X 10^6 N = a
     
  14. Nov 8, 2007 #13
    You are subtracting where you need to divide. - do the calculations again.

    Other than that it looks ok.

    Also, remember your units! acceleration is measured in m/s^2
     
  15. Nov 8, 2007 #14
    LOL!! sorry, not to be mean, but that was funny
     
  16. Nov 8, 2007 #15
    alright so. For the 8th try haha! 2.4 X 10^7 N = 1.9 X 10^7 N a
    then I divide 2.4 X 10^7 N by 1.9 X 10 ^7 N and get a
    so a = 1.3 m/s^2
     
  17. Nov 8, 2007 #16
    nope.. i dont know what to say lol,
     
  18. Nov 8, 2007 #17
    okay so then...
    2.0 X 10^7 = m(10)
    -10 -10 (ERROR)-- fix that
    1.9 X 10^7 = m (fix that)

    2.4 X 10^7 N = 1.9 X 10^7 (fix that) (a)
    -1.9X10^7N -1.9X10^7N (DIVIDE not multiply) 6=2x3 --> 3=6/2 or 2=6/3 (6-3 doesn't = 2-3)

    5.0 X 10^6 N = a
     
  19. Nov 8, 2007 #18
    Thanks a lot for the help guys. I appreciate it!
     
  20. Nov 8, 2007 #19
    alright I think I figured it out now I hope to the lord.

    2.0 X 10^7 N = m(10)
    ------------ -------
    10 10

    m= 2.0 X 10^6 N

    2.4 X 10^7N = 2.0 X 10^6 N a
    ------------- ---------------
    2.0X10^6N 2.0X10^6N

    a= 12 m/s^2
     
  21. Nov 8, 2007 #20
    good lol :)
     
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