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Work done on a block

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  1. Feb 26, 2016 #1
    1. The problem statement, all variables and given/known data

    A 100 N net force acts on a 100 kg block (initially at rest) for 4s
    a. How much work was done on the block? (1)
    b. What is the block’s velocity after the 4 s? (1)
    c. How much energy does the block have after this time?(1)
    d. The 100 N net force is no longer acting on the block. If the block slides an additional 3 m before coming to rest, what average force must have acted on the block over those 3 m? (1)

    2. Relevant equations
    P=W/t
    work done = change in energy
    Hooke's Law: Fs=kΔx
    Ws=(1/2)kΔx2
    KE=(1/2)mv2
    PEg=mgh
    SE=(1/2)kΔx2
    Total energy = KE+PE+SE
    W=FΔd

    3. The attempt at a solution
    The only information I have is:
    F=100N
    m=100kg
    Δt=4s

    All of the equations I have require information I don't have so I am very confused as to how to attempt this question.

    W=FΔd was what I thought I should be using, but I don't have Δd. Then I thought maybe this one: Ws=(1/2)kΔx2, but I don't have k OR Δx.... :frown:
     
  2. jcsd
  3. Feb 26, 2016 #2

    cnh1995

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    Homework Helper

    You can calculate acceleration from this. Use kinematical equations to find the displacement.
     
  4. Feb 26, 2016 #3
    The kinematic equation I have for acceleration is
    a=(vf-vi)/Δt
    a=(vf-0)/4

    I don't have a final velocity though, just F, m and Δt.
     
  5. Feb 26, 2016 #4

    cnh1995

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    Remember Mr. Newton..
     
  6. Feb 26, 2016 #5
    Like the three laws?

    f=ma
    for every action there's an equal and opposite reaction.
    an object in uniform motion will stay in moth in until a force act on it.

    EDIT: Okay, I found this page on here https://www.physicsforums.com/threads/find-velocity-given-force-and-mass.300413/ and now understand

    vf=Ft/m
    vf=(100)(4)/100
    vf=4m/s
     
  7. Feb 26, 2016 #6

    cnh1995

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    That is true because initial velocity=0.
    F=m(vf-vi/t.
    In general, F=ma
    You have m=100kg, F=100N.
    This gives a=1m/s2...
    Since
    v=u+at, and u=0,
    v=at=1*4=4m/s...
     
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