Grade 11 Energy Transformation with External Forces

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Wajeeha
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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?

 
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Wajeeha said:
Ek = W = Efinal - Einitial
*Einitial cancels out because the child was at rest right? So its 0m/s

The child may have been at rest, but are you sure that the child did not have any potential energy?
 
AlephNumbers said:
The child may have been at rest, but are you sure that the child did not have any potential energy?

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 that's how the I got the Force of Friction. Thanks!