- #1
ariana0923
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Homework Statement
A 2.1 x 10^3 kg car starts from rest at the top of a driveway that's sloped at an angle of 20.0 degrees with the horizontal. An avg. friction force of 4.0 x 10^3 impedes the cars motion so that the cars speed at the bottom of the driveway is 3.8m/s. What is the length of the driveway?
Homework Equations
Wnet = change in KE
KE=1/2mv^2
(so you can substitute it)
Wnet = Fdcos(angle)
The Attempt at a Solution
I'm racking my brain here! I don't know what I'm doing wrong...
Wnet = (Fnet)(d)(cos angle)
Wnet = change in KE
Wnet = (KE f) - (KE i)
Wnet = KEf - 0
Wnet = 1/2mvf^2
*****note mvf is mass(final velocity)^2*****
1/2mvf^2 = Fdcos(angle)
1/2(2.1 x 10^3)(14.44) = 4.0 x 10^3(cos 20) (d)
15,162 = 3758.77d
d=4.03 m
**The book's answer is 5.1 m
Now, I'm not sure how to handle the fact that friction is acting in the opposite direction that the work is being done, so i don't know how to account for that negative work, since the answer is not negative.
But other than that, i don't know how to get 5.1
Any help is appreciated!