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Work of a car's engine as it moves up a slope

  • Thread starter v0rtexza
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  • #1
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Homework Statement


A car of mass 900 kg accelerates up a slope. The velocity of the car at the bottom of the slope is 5 m/s. By the time the car reaches the top of the slope, its velocity is 15 m/s. The slope is 8m high from the ground and has a length (radius - angled length - not base) of 500m. The average frictional force experienced by the car between A and B is 50N.

Calculate: The total work done by the car's engine between the bottom and top of the slope.


Homework Equations


F = ma
m = mass in kilograms
a = acceleration in m/s
F = force in newtons
W = F x s
s = distance
W = work

The Attempt at a Solution



I am quite at a loss with this question but this is my attempt.
Fbottom = m x a = 900 x 5 = 4500N

Work (bottom) = f x s
= 4500 x 500 (slope is 500m)
= 2250000J

Ftop = F x ma
= 900 x 15
= 13500N
Work (top) = f x s
= 135000 x 500
= 6750000J

W (top) - W (bottom) = 450000J

I am sure that my answer is incorrect but I am unsure as to how to solve it and the memo is missing the answer, any help would be appreciated. Sorry for the n00b question! :)
 
Last edited:

Answers and Replies

  • #2
CAF123
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its acceleration is 15 m/s
There is a typo here somewhere, you mean its velocity is 15m/s?
 
  • #3
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Yeah, sorry about that!
 
  • #4
CAF123
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88
It is difficult to follow your solution because you have inputted velocities in place of accelerations.
I think this is meant to be solved using the work-energy theorem, but also incorporating the effects of an external force, ie friction, so; [tex] W = \frac{1}{2}mv_f^2 - \frac{1}{2}mv_i^2 + F_f d = ΔK.E + F_f d, [/tex] where [itex] F_f [/itex] is the force of friction.
 
  • #5
CWatters
Science Advisor
Homework Helper
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Don't forget the gain in PE as well as KE?
 
  • #6
CAF123
Gold Member
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88
In taking into account the change in potential energy, i believe we add on, to the eqn above;
[tex] W = \vec{F}.\vec{d} = mgdcos(90 +θ) = -mgdsinθ [/tex]

Apologies for forgetting that earlier, I must have done the problem ignoring the incline!
 

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