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In my fluid mechanics class, I've learned that a fluid traveling in a pipe will generate a force when the exit area is smaller then the entrance area. Suppose a pipe is attached to a car that will use the kinetic energy of the air passing over the vehicle. Assuming that the car is moving at a constant velocity and the pipe is straight and the exit area is 1/4 the size as the frontal area, is it feasible for a moving vehicle to generate enough force from the air to reduce energy use?

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    $\begingroup$ sign up for thermodynamics class next $\endgroup$
    – agentp
    Mar 2 '17 at 5:19
  • $\begingroup$ Here ya go. britishlandsailing.org.uk/class-3.html $\endgroup$ Mar 2 '17 at 10:51
  • $\begingroup$ Or just apply Newton's third law. Whenever some "sourceless" force appears, you need to find the equal and opposite force somewhere. In this case, it will be air resistance against the pipe as it's being compressed. $\endgroup$
    – SF.
    Mar 2 '17 at 10:55
  • $\begingroup$ Isn't that how steam powered vehicles worked? $\endgroup$ Mar 2 '17 at 12:22
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    $\begingroup$ @4LPH4NUM3R1C No. Steam power involves dumpin high-pressure gas into a volume with a moveable wall (piston head) and forcing the volume to expand, with concomitent loss of pressure and temperature of the gas. $\endgroup$ Mar 2 '17 at 14:32
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No, you can not create energy or a force from nothing. It would break the laws of thermodynamics.

Specifically addressing your question in a simplified way; the large area on the front multiplied velocity equals the small area on the back multiplied by the higher velocity. In practice, the losses are quite high and it would waste lots of energy.

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Imagine holding a cup out the window with a small hole cut in the bottom. With the top of the cup facing the direction of travel, the air traveling into the cup would act to force the cup rearward, so it would actually increase the drag on the vehicle. In your example, the force acting on the pipe would add to the drag not decrease it.

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