Damping Force in a piston: Ideal Gas versus a Fluid

by Wavefunction
Tags: damping, fluid, force, ideal, piston, versus
Wavefunction is offline
Feb23-14, 05:52 PM
P: 44
I was in my physics of the human body class and we got on the topic of pistons. It was given to me that the dampening force pertaining to such a piston filled with a gas is proportional to the square of the velocity of the piston; however, for fluids it was given as simply proportional to the velocity. So my question is why is this true? I suspect the fact that liquids have intermolecular forces present and ideal gases don't (in real gases these forces are very weak) has something to do with this but I'm not sure how. Thanks in advance for your input guys and girls!

To clarify what I'm asking about: I'm asking why the dampening force associated with a gas filled piston is proportional to v^2 rather than v.
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256bits is offline
Feb24-14, 02:20 AM
P: 1,260
A fluid can be either a gas or a liquid. Be sure to use these words correctly.

There are several types of friction that you are familiar with.
1. Dry friction, or Coulomb friction, is known as static and kinetic friction. The friction force is proportional to force keeping the parts in contact.
2. Viscous friction is where the fluid is in between two parts moving relative to one another, or the fluid is being squeezed through a space such as an orifice or that between two surfaces. The force of friction is proportional to the velocity. This is the usual dashpot dampening that one finds in shock-absorbers. The fluid can be either a gas or liquid
3. Drag force friction occurs when an object moves through a viscous fluid. The force of resistance is proportional to velocity squared. Fluid can be either gas or liquid.

So when your piston moves in a cylinder, to act as a dashpot, the fluid either has to flow between the space between the piston and cylinder or through an orifice, to give a damping force proportional to the velocity of the piston.

If the piston is moving in the fluid with no cylinder containment, drag forces from the fluid will cause a friction force proportional to the square of the velocity of the piston.

PS, also add rolling friction to the list.

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