
#1
Apr708, 05:08 PM

P: 90

1. The problem statement, all variables and given/known data
a 2.1*10^3 kg car starts from rest at the top of a driveway that is sloped at an angle of 20 with the horizontal. An average friction force of 4.0*10^3 N impedes the car's motion so that the car's speed at the bottom of the driveway is 3.8m/s., What is the length of the driveway? 2. Relevant equations 3. The attempt at a solution Ff=4000N 2100 Kg X=20 degrees Vf=3.8 m/s Vi=0 m/s Wnet=1/2MVf^21/2MVi^2 Wnet= 1/2*2000*3.8^2 Wnet= 15162 N Wnet = FfdcosX 15162=4000cos20D D=4.03m but the book says 5.1m? i cant figure out what i did wrong thx for the help. 



#2
Apr708, 05:13 PM

Mentor
P: 40,875

You forgot about gravity. Friction just slows the car; it's gravity which pulls it down the slope.
To find the net work you must consider all the forces: gravity and friction. (Don't forget that they act in different directions.) 



#3
Apr708, 05:14 PM

P: 90

i see so the force applied would be Fgravity*sin20 which would give me 7045.96 and i would subtract the Ff to get the total force. right?




#5
Apr708, 05:56 PM

P: 90

thxxx



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