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Work, energy and power

  1. Nov 29, 2009 #1
    1. The problem statement, all variables and given/known data
    the question and answers are in the attachment, I don't understand question 8b) (ii), why is the work done against friction =f x d and not 10N x d?


    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Nov 29, 2009 #2

    Doc Al

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    For some reason, I cannot view those attachments. Can you describe the problem?
    The obvious question is: What does the friction force equal?
     
  4. Nov 29, 2009 #3
    okay, the question goes like this:
    on a rough horizontal Ground, a 3 kg block at rest is moved be a Force of 10N. The acceleration of the block is 2ms-2.
    if the force ceases to act after 4 seconds,
    what 's the further distance traveled by the block before it comes to rest?
     
  5. Nov 29, 2009 #4

    Doc Al

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    That means the 10 N force is no longer applied.
    What's the only force acting to slow down the block?
     
  6. Nov 29, 2009 #5
    that's got to be the friction, right?
     
  7. Nov 29, 2009 #6

    Doc Al

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    Yes. Now use the given information to figure out the friction force. (Hint: Newton's 2nd law.)
     
  8. Nov 29, 2009 #7
    by Newton's 2nd law
    F=ma
    Let the friction be f
    10-f=3(2)
    f=4N
     
  9. Nov 29, 2009 #8

    Doc Al

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    Good!
     
  10. Nov 29, 2009 #9
    yay! thx a million. but there's another thing, how do I get the further distance traveled by the block before it comes to rest?
     
  11. Nov 29, 2009 #10

    Doc Al

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    Several ways:
    (1) You can find the acceleration and use kinematics. (First figure out how fast the block is moving at the end of 4 seconds.)
    (2) You can set the work done by friction equal to the change in kinetic energy.
     
  12. Nov 29, 2009 #11
    ah, I see. The book must have been using the second way you describe, I get it now, thx a lot.
     
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