What Was Johnny's Initial Speed Sliding Down the Slope?

[pureouchies4717]

Johnny jumps off a swing, lands sitting down on a grassy 20 degree slope, and slides 3.5 m down the slope before stopping. The coefficient of kinetic friction between grass and the seat of Johnny's pants is 0.5.What was his initial speed on the grass?

now i know the answer is 2.96m/s, but i don't know how to get at it...heres what i tried:

Fn= mgsin20

Ff=.5mgcos20

mgsin20-Ff= ma

mgsin20- .5mgcos20 = ma

(m's cancel out)
a=2.88 m/s^2

Vi^2= 0 + 2(-2.88)(3.5)
Vi= 4.49 m/s

but this is definitely wrong... how do i get the real answer?

 

[Doc Al]

heres what i tried:

Fn= mgsin20

Ff=.5mgcos20

mgsin20-Ff= ma

mgsin20- .5mgcos20 = ma

(m's cancel out)
a=2.88 m/s^2

Your method is perfect, but redo the calculation for "a".

 

[pureouchies4717]

thank you, my friend