- #1
raddian
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In my physics lecture, my professor "proved" that acceleration does not affect the amount of energy used in driving a car. Assume car is driven along flat road. In the short,
(assume air resistance is neglected, and the W done by the car's engine, [tex] W_{engine} [/tex] is non conservative, [tex] \Delta PE \Delta KE [/tex] are change in potential and change in kinetic energy, respectively )
[tex]
\Delta PE = 0 \\
\begin{align}
\Delta KE &= W_{engine}\\
&= 1/2 m {v_f}^2 - 1/2 m {v_o}^2
\end{align}
[/tex]
m is constant mass, vf and vi are final and initial velocity, respectively.
Proof done: change in kinetic energy does NOT depend on acceleration; only the mass and final velocity of the car.
Okay now let's add air resistance:
[tex]
W_{res} = \frac{ \rho D A v^2}{2}\\
\begin{align}
\Delta KE &= W_{engine} - W_{Res}\\
&= 1/2 m {v_f}^2 + \frac{\rho D A v^4}{8a}
\end{align}
[/tex]
where ## \rho ## is the air density (constant), D is coefficient of drag, and A is cross section.
Doesn't this mean that the energy used (in this case the gas in the car) depends on the acceleration of car, if there is air resistance?
Is friction done by the ground on the car affected by acceleration?
(assume air resistance is neglected, and the W done by the car's engine, [tex] W_{engine} [/tex] is non conservative, [tex] \Delta PE \Delta KE [/tex] are change in potential and change in kinetic energy, respectively )
[tex]
\Delta PE = 0 \\
\begin{align}
\Delta KE &= W_{engine}\\
&= 1/2 m {v_f}^2 - 1/2 m {v_o}^2
\end{align}
[/tex]
m is constant mass, vf and vi are final and initial velocity, respectively.
Proof done: change in kinetic energy does NOT depend on acceleration; only the mass and final velocity of the car.
Okay now let's add air resistance:
[tex]
W_{res} = \frac{ \rho D A v^2}{2}\\
\begin{align}
\Delta KE &= W_{engine} - W_{Res}\\
&= 1/2 m {v_f}^2 + \frac{\rho D A v^4}{8a}
\end{align}
[/tex]
where ## \rho ## is the air density (constant), D is coefficient of drag, and A is cross section.
Doesn't this mean that the energy used (in this case the gas in the car) depends on the acceleration of car, if there is air resistance?
Is friction done by the ground on the car affected by acceleration?
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