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a) Draw and label a free body diagram.

b) Calculate the acceleration of the wagon.

c) If the acceleration remains constant, what will the final velocity of the wagon be when t = 7.0s?

d) If the work done by air resistance is 15J, determine the final velocity of the wagon when t=7.0s.

e) What is the wagon's displacement when t=7.0s?

So I drew a FBD. And found that a=1.12 m/s^2 (by finding Fnormal using Fnety = Fnormal + Fapplied - Fgravity = 0 and then using Fnormal in Fnetx = Fapplied*cos35 - Ffriction. Then Fnetx = ma)

c) V=Vo +at

t = 7.0s

Vo = 0

a = 1.12

Therefore V = 7.9 m/s

d) ?? Here's the answer the teacher gave me, but I don't understand it:

1/2mv^2 -15 = 1/2mv2^2

1/2(25)(7.9)^2 -15 = 1/2(25)(V2^2)

765.125 = 1/2(25)(V2^2)

V2 = 7.82 m/s [E]

I don't get why the air resistance is subtracted from Work done at t=7.0s. Please help explain part d to me...thank you!