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Homework Help: Question about significant digits

  1. Sep 8, 2007 #1
    1. The problem statement, all variables and given/known data
    A 2.6 kg mass hangs at one end of a rope that is attached to a support on a railroad car. When the car accelerates to the right, the rope makes an angle of 8.5° with the vertical, as shown in Figure P4.68. Find the acceleration of the car.


    2. Relevant equations
    (Fnet)x=max
    (Fnet)y=may

    3. The attempt at a solution

    (Fnet)y=may
    Tcos8.5-mg=0
    T=(2.6 x 9.8)/cos 8.5
    T=25.76


    (Fnet)x=max
    Tsin8.5=max
    Tsin8.5/2.6=ax
    ax=25.76 sin 8.5/2.6
    ax=1.5


    NOW HERE'S MY PROBLEM:

    I am doing homework on webassign.net and it is very picky about the significant digits in your answer. So far I've put in: 1.5, 1.4, 1.46, and 1.47.

    I get 6 tries, which is frusturating because I know I solved it correctly, I just don't know how many significant digits. I assumed I would have 2 significant digits and the answer would be 1.5 because all the given values have 2 significant digits. If someone could help me figure it out, I would appreciate it.

    Also (reason for edit): When I put in 1.5 and 1.4 I got this response:
    Your answer is within 10% of the correct value. This may be due to roundoff error, or you could have a mistake in your calculation. Carry out all intermediate results to at least four-digit accuracy to minimize roundoff error. m/s2

    However, when I bring it out to 1.46 or 1.47 I don't get that message.
     
    Last edited: Sep 8, 2007
  2. jcsd
  3. Sep 8, 2007 #2
    Nevermind! I put in 1.465 and it worked.

    How frusturating though.
     
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