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Significant figures question

  1. Nov 24, 2004 #1

    tony873004

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    11. [Walker2 9.P.010.] A 0.50 kg croquet ball is initially at rest on the grass. When the ball is struck by a mallet, the average force exerted on it is 235 N. If the ball's speed after being struck is 2.6 m/s, how long was the mallet in contact with the ball? (in miliseconds)

    F = ma
    235 = 0.50 * a
    a = 235 / 0.5
    a = 470
    v = at
    2.6 = 470 t
    t = 2.6 / 470
    t = 0.00553191489361702 s
    t = 5.53 ms

    I got this correct according to the online grading system, but if they were picky about significant figures wouldn't I have done?:

    t = 0.00553191489361702 s
    2.6 is the value with the least amount of significant digits with 2, therefore
    t = 0.0 s
    t = 0 ms
     
  2. jcsd
  3. Nov 24, 2004 #2

    jamesrc

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    No, those placeholder zeros in front of the 5 are not significant.

    Also, I think using the impulse-momentum theorem is a better (albeit mathematically equivalent) way to think of the problem:

    [tex] F_{\rm avg}t = \Delta mv [/tex]
    [tex] t = m\frac{v-v_o}{F_{\rm avg}} [/tex]
     
  4. Nov 24, 2004 #3

    tony873004

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    Thanks, that makes sense. We spent the first week of class on significant figures, and now he doesn't enforce it so we all got lazy.

    Thanks for the impulse formulas. I did this problem before I read the impulse chapter by using the force formulas, but your way looks more elegant. :tongue:
     
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