# Damping Force in a piston: Ideal Gas versus a Fluid

• Wavefunction
In summary, the conversation discussed the different types of friction and how they relate to the movement of a piston in a fluid. It was noted that for a gas-filled piston, the dampening force is proportional to the square of the velocity, while for fluids it is simply proportional to the velocity. The presence of intermolecular forces in liquids may play a role in this difference. The conversation also mentioned other types of friction, including dry, viscous, and drag force friction.
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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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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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## 1. What is damping force in a piston?

Damping force in a piston is the resistance or counterforce that acts against the movement of the piston in a closed chamber. It is caused by the friction between the piston and the walls of the chamber, as well as the resistance of the fluid or gas inside the chamber.

## 2. How does the damping force differ between an ideal gas and a fluid?

The damping force in a piston is dependent on the properties of the fluid or gas inside the chamber. In an ideal gas, the damping force is mainly caused by the collisions between gas molecules and the walls of the chamber. In a fluid, the damping force is also affected by the viscosity of the fluid, which is the measure of its resistance to deformation.

## 3. How does the damping force affect the movement of the piston?

The damping force acts as a resistance to the movement of the piston, slowing it down and reducing its amplitude. This can result in a smoother and more controlled movement of the piston, preventing it from slamming or oscillating excessively.

## 4. Can the damping force be adjusted in a piston system?

Yes, the damping force can be adjusted in a piston system by changing the properties of the fluid or gas inside the chamber. For example, increasing the viscosity of a fluid can increase the damping force, while decreasing the pressure of an ideal gas can decrease the damping force.

## 5. How does the damping force impact the overall performance of a piston system?

The damping force plays a crucial role in the performance of a piston system. It helps to control the movement of the piston and prevent excessive vibrations, which can cause damage to the system. A properly adjusted damping force can improve the efficiency and accuracy of the piston system.

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