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Grade 11 Energy Transformation with External Forces

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  1. May 6, 2015 #1
    1. Question.

    A 15 kg child slides from rest, down a playground slide that is 4.0m long. The slide makes a 40degree angle with the horizontal. The child's speed at the bottom is 3.2 m/s. What was the force that the slide was exerting on the child?
    Answer: 75 N [up the slide]

    2. My Attempt.

    Ek = W = Efinal - Einitial
    *Einitial cancels out because the child was at rest right? So its 0m/s
    = 1/2(15kg)(3.2m/s)squared
    = 76.8 J

    Then I did:

    W = Ff(costheta)(deltad)
    Ff= W/(costheta)(deltad)
    = 76.8 J/(cos180)(4)
    = -19.2 N

    3. Confusion:

    I'm confused as to what I or like what my next step is? Could you explain what I might've done wrong or what I have to do after this and why? Maybe explain me the concept behind it?

     
  2. jcsd
  3. May 6, 2015 #2
    The child may have been at rest, but are you sure that the child did not have any potential energy?
     
  4. May 6, 2015 #3
    Oh yeah! I did Eg = mgh; I found the haight using sinelaw then plugged that in. Then I found the Kinetic energy and subtracted that from the Eg which was the W then used the equation W = Ff(costheta)(deltad) and thats how the I got the Force of Friction. Thanks!
     
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