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Recently, I was met with the following question:

When observing water flow around a bridge pylon, of the following statements, which one(s) are true:

  • a) The bridge pylon applies force to the water
  • b) The bridge pylon causes the energy of the water flow to drop
  • c) Because of friction between the water and pylon, there is an exchange of mechanical energy between the two

I assume a) has to be correct, as the water flowing around it doesn't cause it to move and, according to Newton's third law, the pylon has to apply an equal and opposite force to the water flow.

Likewise, I assume b) is correct. The wording of the statement is a bit ambiguous on the type of energy, but when looking at total energy, because of friction between the pylon and water, some energy is converted into thermal energy, some of which dissipates to the pylon, thus causing a drop in overall energy in the water flow.

And if I were to guess, c) is not true. Mechanical energy is the sum of kinetic and potential energy. Since there is no change in potential energy in either pylon or water flow, and the pylon stays stationary, its kinetic energy is constant.

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  • $\begingroup$ Welcome New contributor WeedGrindeR! It isn't clear what you're asking. You've stated your reasoning but you haven't asked a question that I can find. $\endgroup$ Aug 19 '19 at 19:29
  • $\begingroup$ So my question is which of the statements are true. I'm pretty sure a is true, but I'm unsure about b and c. $\endgroup$ Aug 20 '19 at 6:43
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I agree that a) is true. There is certainly a force applied by the water to the pylon. This is because the water stagnates at the front of the pylon (velocity = zero, maximum pressure) and the boundary layer separates from the sides of the pylon at the back. This causes a pressure differential between the front and back of the pylon. The pylon therefore exerts an equal and opposite force on the water, by virtue of Newton's Third Law.

I believe b) is not true. Energy is equal to force multiplied by distance moved (power equals force multiplied by velocity). Presumably, the bridge pylon is not moving. It is stationary whilst the force from the water acts upon it. Therefore, there cannot be any (mechanical) energy being transferred between the water and the pylon.

For c), refer to the answer to b). This is also not true. Yes, there will be a frictional force acting between the pylon and the water. However, again, the pylon is not moving, so this frictional force cannot be causing any energy transfer. A key point to bear in mind here is the no-slip condition where the water meets the pylon. This means that, at the surface of the pylon, the water is also not moving, so there is no energy being transferred between the two due to friction.

If you want to be pedantic, one could argue that there will be some heat energy generated in the water close to the pylon, due to turbulence. In this case, it is possible there could be a very small amount of heat energy transferred to the pylon from the water; however, that is heat energy, not 'mechanical energy'.

I hope this helps.

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