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Kinematics again!

  1. Sep 4, 2006 #1
    The initial speed of a body is 4.37 m/s. What is the speed after 2.06s if it accelerates uniformly at -3.93 m/s^2. Answer in units of m/s.

    would I not use the kinamatic equ. v=v.+a(t) so v=4.37+-3.93(2.06)? cause when I do that I get the wrong solution?
  2. jcsd
  3. Sep 4, 2006 #2


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    What is the answer?
    What did you get?
  4. Sep 4, 2006 #3
    -3.7258 is what i got.
  5. Sep 4, 2006 #4
    You are undoubtfully correct. Just be careful with significant figures: the solution is -3.73 m/s.

    Why do you claim you are wrong?
  6. Sep 4, 2006 #5
    its an assignment through a college online, and when i submit the answer -3.7258 or -3.73 it says im wrong.
  7. Sep 4, 2006 #6
    Try -3.72 m/s, then.
  8. Sep 4, 2006 #7
    nope ....... that didnt work either. I dont know what im doing wrong? Im sure im right.
  9. Sep 5, 2006 #8
    I think it's -3.72 m/s, even numbers don't get rounded up when the next digit is 5.

    In any case, could you give us the exact statement of the problem? Also check that you are not making a mistake in reading the problem. Finally, there's the possibility that the answer known by the computer is wrong.
  10. Sep 5, 2006 #9


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    It gets a little trickier because it asks "What is the speed", not "What is the velocity". Then it gives initial speed of 4.37 m/s, but since it is a speed, it implies no direction. Then it gives acceleration with a minus sign, implying direction, but not giving you a reference axis. So you don't know what direction relative to the initial speed it is accelerating.

    Speed is a scalar, not a vector, and should be a positive number.

    I'd be tempted to try 3.73 m/s. (no negative sign).
  11. Sep 5, 2006 #10


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    Are you sure about that? The digit after the 5 is an 8. I imagine 3.7258 rounds to 3.73.
  12. Sep 6, 2006 #11
    I don't know about professional conventions, but I was taught that when rounding to the nth digit one should only consider upto the n+1)th digit, ignoring later digits. By this convention, you wouldn't round 5 to 6, you would treat it as a 5.
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