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Homework Help: What horizontal force is required to accelerate the block

  1. Jul 17, 2010 #1
    A block of mass 4.0kg rests on a horizontal surface. What horizontal force is required to accelerate the block at 5.0m/s2 if the coefficient of kinetic friction is 0.25?
     
  2. jcsd
  3. Jul 17, 2010 #2
    Re: help :(

    start with writing a free body diagram.
    & any equations you may know that you think would be useful here.
     
  4. Jul 17, 2010 #3
    Re: help :(

    ΣF=ma

    Fk=mu * N
     
  5. Jul 17, 2010 #4
    Re: help :(

    ma=4.0kg*5.0m/s^2
    =20 N
    Fk= 0.25 * 20
    = 5 N


    Is it correct???
     
  6. Jul 18, 2010 #5
    Re: help :(

    so, how much force do you need to apply in order to overcome the frictional force?
     
  7. Jul 28, 2010 #6
    Re: help :(

    ΣF=F-fk=mg
    F=mg-fk
    =(4.0)*(9.8)-(0.25)(39.2)
    =29.4 N
     
  8. Jul 28, 2010 #7
    Re: help :(

    In your last post, you've got a vertical force (mg) and a horizontal force (fk) in the same equation. Not allowed.

    In your previous post, you found the frictional force's maximum value. You also found that in order to accelerate at 5m/s/s, you need 20 Newtons of NET force.

    So how much TOTAL force must be applied (horizontally) to overcome friction and give the block enough force to accelerate?
     
    Last edited: Jul 28, 2010
  9. Jul 28, 2010 #8
    Re: help :(

    ohh 0_o

    it's
    ΣF=F-fk=ma
    F=ma+fk
    =(4.0)*(5)+(0.25)(39.2)
    =20+9.8
    =29.8 N
     
  10. Jul 28, 2010 #9
    Re: help :(

    looks correct to me
     
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