Anybody here good at physics?

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LCF
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Anybody here good at physics?

Post by LCF »

I have encountered a rather embarrassing confusion about a physics question, and I'd like to talk it through with somebody who's good with mechanics. Math, I know; physics . . . well, I still remember sweating bullets over trying to analyze the forces in a cross-braced gate on a final exam in college.
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Joshua Smith
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Re: Anybody here good at physics?

Post by Joshua Smith »

Now I'm curious. I can explain the compression, tension, load lines, and shear forces in a cross braced gate, but didn't go to collage to study no fisicks. Is it something mechanical, or are you pondering covalent bonds and the like?
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Re: Anybody here good at physics?

Post by Magoi »

With trepidation, I will say I know something about physics (being an engineer). Even better, my daughter is a third year engineering student at UW, and she just finished her physics and calculus. Between the two of us we should be able to figure it out.
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Re: Anybody here good at physics?

Post by kdupreez »

Lynne, I betcha AirSix (Ben Messenger) would be a logical person to approach too :)
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LCF
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Re: Anybody here good at physics?

Post by LCF »

It's a fluid dynamics problem.

I thought about asking Ben, because Ben seems to understand everything :)
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Re: Anybody here good at physics?

Post by Magoi »

Fluid dynamics - in a pipe or external flow? Gas or liquid?
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Re: Anybody here good at physics?

Post by LCF »

In the ocean . . . I really hate to confess what the question is, because I feel stupid that I can't answer it.

Is the speed at which a diver is moved by current affected by the mass or the profile of the diver? In other words, if you present a smaller cross-sectional area to the current, will you be moved slower? If you have more mass, will you be moved slower? I keep trying to think this through, and it seems to me that profile ought to have an effect, but I just can't sort out all the considerations. Just physics-challenged, as always.
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Re: Anybody here good at physics?

Post by Magoi »

The only stupid question is the one you don't ask.

The answer is both affect the speed you will travel due to current, at least until you reach terminal velocity. In general, the greater the mass, the acceleration created by the current will be slower, due to inertia. The more streamlined the cross section exposed to the direction of the current, the less drag will be created, reducing the acceleration created by the current. These issues are only relevant if you are moving at a speed lower than the current. Once you are moving with the current (at the same speed) neither mass nor profile has any effect - you are simply drifting at the speed (and direction) of the current. If you try to change your speed (or direction) relative to the current. profile is more important than mass, but both have an effect: the greater your mass, the more effort it will take to change direction. The more streamlined you are, the less effort it will take to oppose the current or to change direction.

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lamont
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Re: Anybody here good at physics?

Post by lamont »

"First, consider a spherical diver..."

Force on the diver is proportional to the velocity squared and the surface area of the diver (assuming density and drag held constant).

Power to overcome the force is proportional to the velocity cubed and the surface area of the diver (density and drag again constant). Mass does not appear so the power required to fight a current is the same if the cross sectional area of the diver is constant. That is an interesting result because it means that loading up a diver with scooters and stages must be changing surface area or the drag on the diver, top speed on a scooter won't depend on mass.

F = ma, so the acceleration on the diver will be lower if the divers mass is higher. If two divers who have the same profile (area and drag) are kicking into a current and stop kicking at the same time, then the lighter diver will be accelerated away faster by the force from the current. Of course in reality the lighter diver usually has less surface area so feels less force.
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Re: Anybody here good at physics?

Post by BASSMAN »

So where is the " X " in this equation?
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Re: Anybody here good at physics?

Post by Norris »

BASSMAN wrote:So where is the " X " in this equation?
eXam airsiX eXpose....those are the ones I found, and I am really not that good at physics...
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renoun
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Re: Anybody here good at physics?

Post by renoun »

lamont wrote:"First, consider a spherical diver..."
Would a spherical chicken be any different?
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Re: Anybody here good at physics?

Post by diverden »

renoun wrote:
lamont wrote:"First, consider a spherical diver..."
Would a spherical chicken be any different?
With or without feathers?
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Re: Anybody here good at physics?

Post by airsix »

Great responses by Magoi and Lamont - better than I could have articulated. I will add one thought that I hope will help make these concepts seem more logical if they don't already.

When you are immersed in any fluid which itself is not accelerating (be it air, water, etc.) your felt-velocity, if any, is only in relation to the fluid. Here's an example: If you hold a goldish in a bowl on your lap while riding in a car, what is the velocity of the fish? Relative to the fluid, the fish has no velocity. The fish is unaware it is traveling 60mph relative to the surface of the earth just as you are unaware the earth you stand on is traveling through space at 18 miles per second (and change). Likewise, a diver drifting with the current has no velocity with relation to the surrounding water. Size, mass, and profile only affect acceleration as Magoi and Lamont stated. So, if the boat is anchored in current and you jump in the water, your size/mass/profile only matters for the moment it takes the current to catch you and get you up to speed. Relative to the fluid, you only had velocity when you thought you were stationary. Once you are moving with the current you are, relative to your environment, stationary once again.

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Re: Anybody here good at physics?

Post by Jeremy »

lamont wrote:"First, consider a spherical diver..."
"and, neglecting friction, we find..."
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Re: Anybody here good at physics?

Post by ArcticDiver »

lamont wrote:"First, consider a spherical diver..."

...
Brings back memories of some of my last dive trips. All but a couple would come close to fitting this description "spherical diver". :)

I'm glad you included drag in the equation. Whenever, a diver, or any other body, is moving at a different velocity or direction than the prevailing current(wind or water or ?) drag is an important component.

Actually if a person wants some really simplified and easily understood instruction in this area pick up a book on navigation, either airplane or boat. This is a fairly routine calculation for either. Of course now the installed computer does it for the crew. But, I think crew still is taught the Hows and Whys.
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