How can I calculate velocity from drag force and other variables?

In summary, the conversation is about finding the formula to calculate velocity from a given drag force value. The individual is having trouble finding resources for this specific calculation, but has found a website that may have the formula for a fee. They provide context for their question, explaining that they want to use a force sensing cell to measure speed based on the drag force exerted by water on the sensing cell. There is some confusion about the terminology and the formula being proportional to acceleration rather than velocity. The individual clarifies their question and states that they only need the basic formula to calculate velocity from the given parameters.
  • #1
reality1980
2
0
Hi, I'm looking for the formula to get the velocity value from a drag value. I found plenty of resources to do it the other way around (velocity to drag) but no luck to get it the way I need it.

This website seems to have it but they don't provide the rule and they actually want money to enable the calculate button... http://www.lmnoeng.com/Force/DragForce.htm

So basically I have the drag force value, the fluid density, area, force and drag coeff., I want to get the velocity from that.

For some context, I want to use a force sensing cell with a known area facing water towards the heading direction and get my speed value from the force exerted by the water on the sensing cell.

Could anybody help me geopardize the equation to get it right?
 
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  • #2
If I remember fluid mechanics properly

then


[tex]F_{drag}= \frac{1}{2} \rho v^2 AC_d[/tex]
 
  • #3
"Drag" is a force which is proportional to acceleration, not velocity. Do you mean the "terminal speed" when drag opposes some outside force?

(rock.freak667 gives a formula for drag as a function of speed.)
 
  • #4
Ok yes, maybe I poorly explained what I wanted. Drag is proportional to acceleration, and it's not quite what I want. I'm not sure of the term to use here as I started searching for frontal resistance and wikipedia pushed me towards the drag term.

Let's use an example here... If you're on the road going 20MPH and stick your hand out the window, your hand will be pushed by the air rushing in (which I believe is named frontal resistance). This effect is proportional to speed, not to acceleration and will be constant at a given speed (no, there is no wind in this perfect theory world!). The faster you go, the more pressure is exerted on your hand.

I have a very similar setup; a load cell plate installed on the front of an aquatic vehicle measures the force at which the water pushes on it. I can get a force value with this measurement. Now I need to get a speed value from the force value, the area of the sensing plate and the fluid (water) density.

I'm aware that density changes with temperature but I can compensate for that. All I need is the basic formula to get my velocity.

Thanks :)
 
  • #5
That's a different matter. That's not a matter of the speed being a function of drag, it a matter of drag being a function of speed! If you have such a function then there must be one value of the drag for each speed but not necessarily a value of speed for each value of the drag, which is what you are asking for.
 

FAQ: How can I calculate velocity from drag force and other variables?

What is drag force and how does it affect velocity?

Drag force is a resistive force that acts opposite to the direction of motion of an object through a fluid (such as air or water). It is caused by the friction between the object and the fluid. The greater the drag force, the more it will slow down the object and decrease its velocity.

What factors influence the drag force acting on an object?

The drag force depends on the size, shape, and speed of the object, as well as the density and viscosity of the fluid it is moving through. The drag force also increases with the cross-sectional area of the object and the fluid's velocity relative to the object.

Can drag force ever be beneficial to an object's velocity?

Yes, in certain situations drag force can actually increase an object's velocity. This is known as the Magnus effect, where a spinning object experiences lift force from the surrounding fluid, causing it to accelerate in the direction of the spin.

How does the drag coefficient affect the relationship between drag force and velocity?

The drag coefficient is a dimensionless quantity that represents the amount of drag force experienced by an object in a fluid. It varies depending on the shape and size of the object, as well as the fluid properties. A higher drag coefficient means the object will experience a greater drag force at a given velocity.

How can I calculate the drag force acting on an object at a certain velocity?

The drag force can be calculated using the formula Fd = 1/2 * ρ * v^2 * Cd * A, where ρ is the density of the fluid, v is the velocity, Cd is the drag coefficient, and A is the cross-sectional area of the object. This formula applies to objects moving at subsonic speeds in a fluid.

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