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Homework Help: Finding Final Speed Using

  1. Jan 13, 2008 #1
    Finding Final Speed Using Work

    A person pushes with a horizonal 25N force over a distance of 1.2m on a 4.5kg crate, intially at rest on a frictionless surface. What is the final speed of the crate?

    heres my data list..

    F=25N
    delta d=1.2m
    m=4.5kg
    v1=0
    v2=?
    theta=0 degrees

    i used the W=fxdeltadxcos(theta) formula to find out the work done was 30J.. but once i have work i dont no what to find next to find the final speed..
     
    Last edited: Jan 13, 2008
  2. jcsd
  3. Jan 13, 2008 #2
    There are two ways of approach. One is using the constant accleration equation

    (v2)^2=(v1)^2+2as

    where a is the acceleration of the crate and s is the displacement caused by the accleration, i.e.

    v1 = 0, a = F/m (Newton's Second Law) = 25/4.5 = 50/9, s = 1.2, so

    v2 = sqrt (2.4*50/9) = sqrt (120/9 ) = sqrt (120)/3

    The other is to find the work done by the force = F.d = 25*1.2 = 30J
    and then say this is the additional kinetic energy given to the crate. Since the crate was at rest, we can say

    0.5*m*(v2)^2 = 30
    v2 = sqrt ( 60/4.5 ) = sqrt (120/9) = sqrt (120)/3
     
  4. Jan 13, 2008 #3

    rock.freak667

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    Homework Helper

    What is the work done by the force converted into? (If it is moving what kind of energy does it have?)
     
  5. Jan 13, 2008 #4
    Energy by virtue of its motion is called kinetic energy, and kinetic energy is given by the formula K.E. = 0.5mv^2 where m is the mass of the body and v is the velocity.
     
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