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Work-energy theorem problem

  1. Oct 25, 2008 #1
    I am just confuse in this question may be i forgot about work and energy .I just need little help not the answer ,here is the question

    1.A block of mass 2.2 kg, which has an initial
    velocity of 3.9 m/s at time t = 0, slides on a
    horizontal surface.
    Calculate the work that must be done on
    the block to bring it to rest. Answer in units j.

    solution .

    Is it work = KE WHICH IS 1/2MV^2

    I try to use W = F.d
    but i don.t know the acceleration
     
  2. jcsd
  3. Oct 25, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Re: work

    Good. Use the work-energy theorem: W = ΔKE.

    (What's the change in KE?)
     
  4. Oct 25, 2008 #3
    Re: work

    oh thank you , now i got it, i didn't put negative sign
     
  5. Oct 25, 2008 #4
    Re: work

    how about If a constant friction force of 11 Newtons is
    exerted on the block by the surface, what is
    the acceleration? Answer in units of m/s.

    solution
    F - friction force = mass * acceleration

    is that right
     
  6. Oct 26, 2008 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Re: work

    Is friction the only horizontal force acting on the block? If so, then all you need is F = m*a, where F is the friction force.

    Also: acceleration has units of m/s^2, not m/s (m/s is the units for speed)
     
  7. Oct 26, 2008 #6
    Re: work

    thank you doc i just forgot to write the square
     
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