A 15 kg child slides, from rest, down a playground slide tht is 4.0 m long, as shown in the figure. The slide makes a 40 degrees angle with the horizontal. The child's speed at the bottom is 3.2 m/s. What was teh force of friction that the sldie was exerting on the child?

Ok. The answer in the texbook says 75N.

Here is my WROng attempt

work=Kinetic Energy 2 - Kinetic Energy 1

work = 1/2 m (V2squared - v1squared)

w=1/2 (15 kg) (3.2 m/s squared - 0)

w=7.5 (10.24 m/s)

w=76.8 J

then I..

Work=force (distance) (cos40)

f= w/d (cos 40)

f=76.8 J / 4.0 (cos 40)

f=14.7 N

Where is my error.. HELP PLEASE