# Drag force

#### werehk

PHF Hall of Fame
When the initial velocity is much smaller than critical temperature, then the magnitude of force mainly depends on viscosity. However, when initial velocity is much larger than critical velocity, then the magnitude of force mainly depends on pressure.

Here's the problem, why the drag force would be related anything to pressure?

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#### topsquark

Forum Staff
When the initial velocity is much smaller than critical temperature, then the magnitude of force mainly depends on viscosity. However, when initial velocity is much larger than critical temperature, then the magnitude of force mainly depends on pressure.

Here's the problem, why the drag force would be related anything to pressure?
First, I am not quite certain exactly what kind of motion you are referring to. Second, the initial velocity can't be larger or smaller than a critical temperature as they are different quantities, so I'm not sure what you are talking about there, either.

-Dan

#### werehk

PHF Hall of Fame
Sorry, I type the wrong thing. That is the critical velocity. I am referring to movement of object inside liquid

#### topsquark

Forum Staff
Sorry, I type the wrong thing. That is the critical velocity. I am referring to movement of object inside liquid
My only thought is that for small speeds (compared to the critical speed) the motion of the object is constrained pretty much only by the liquid trying to get out of the way, so the motion is defined by the viscosity. When the speed of the object is large the fluid pretty much reacts as if there were a "shield" in the way of the object causing a resistive force. My best guess is that this kind of motion is Universal: it depends only on the Mechanics of the situation, not the specific nature of the fluid.

-Dan

• werehk