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Physics question on motion

  1. Jul 11, 2012 #1
    1. The problem statement, all variables and given/known data
    An object that is acclerating uniformly travels 22.5 m in the eighth second of its motion.
    a) what is the acceleration of the object?

    b) how far does the object travel in 6 seconds?


    2. Relevant equations
    I'm not exactly sure what equation would be necessary-
    perhaps -- a=v-u/t
    v=u+at
    x=ut+1/2at^2


    3. The attempt at a solution

    i attempted to use the a=v-u/t equation, however, it wasn't successful. I don't even think you can use that equation since we dont have the values for velocity;initial nor final.
     
  2. jcsd
  3. Jul 11, 2012 #2

    PhanthomJay

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    The object has no motion at t = 0. How far does it travel in 8 seconds? How far does it travel in 7 seconds? Its displacement between the. 7th and 8th seconds is given as 22.5 m. Use your last equation.
     
  4. Jul 11, 2012 #3

    NascentOxygen

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    Hi einstein101. The eighth second is that time difference between seven seconds and eight seconds.

    travel during first 7 seconds= u.7 + (1/2)a.72

    travel during first 8 seconds= ????
     
  5. Jul 12, 2012 #4
    But how do I find the initial velocity for the equation?
    And how would I exactly go about finding the acceleration?
     
  6. Jul 12, 2012 #5
    It says nowhere that the object started from rest.
     
  7. Jul 12, 2012 #6

    Doc Al

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    It says "in the eighth second of its motion". I would interpret that to mean that it started from rest.
     
  8. Jul 12, 2012 #7

    NascentOxygen

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    Can you finish the equation I left uncompleted?
     
  9. Jul 12, 2012 #8
    Hmm, I suppose you're right.
     
  10. Jul 12, 2012 #9
    so travel during first 8 seconds=u x 8 +1/2(a)(8)^2

    then what do i do?
     
  11. Jul 13, 2012 #10

    NascentOxygen

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    Good. Now subtract that from the companion equation I gave you, viz., of distance travelled during first 7 seconds.
     
  12. Jul 13, 2012 #11

    PhanthomJay

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    It has been established that the object starts from rest and thus u = 0 and the distance traveled in 8 seconds is 32(a). In a similar manner, find the distance traveled in 7 seconds. Subtract this 2nd equation from the first equation and set the result equal to 22.5 to solve for a.
     
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