# Relationship between bridge pylon and water flow

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.

• 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. Aug 19, 2019 at 19:29
• 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. Aug 20, 2019 at 6:43