Do Floating Objects Have 'Weight'?

[Pauly854]

I will admit that I do not know a whole heck of a lot about physics, but there is something that I DO know about, and that is....MYTHBUSTERS!!! The plane took off.

 

[Walter]

Similarly, if you float a ten lbs block of wood in, the system would weigh 110 lbs., but if the tank were full to the brim and some water ran out when the wood was placed in then then it would weigh 100 lbs, since 10 lbs of water would have overflowed the tank.

If the block of wood floats (very likely), it would displace less than 10 lbs of water.

That depends on how you think of it. "Pound" is generic.

Not really. The pound is a measurement of force/weight, not mass. The imperial unit of mass is the slug. A gram is a metric measurement of mass while the newton is the metric measurement of force/weight. All scales measure force/weight. They can be calibrated in kilos (mass) if you know where the scale will be used (ie the surface of the earth), but the scale is not truly measuring mass. A scale calibrated in kilos (mass) for use on earth would be wrong on the moon, but a scale calibrated in pounds (weight) would still be correct on the moon. Weight changes depending on the gravitational field, mass does not.

Slugs and newtons are used so seldom that most folks have never heard of them.

 

[String]

Say for example, you've got an aquarium that weighs 100 pounds when full of water. If you were to float a 10 pound object inside of that aquarium, does the whole system now weigh 110???

Yes.

(provided the water doesnt slosh out of the tank when the object is placed in there).

 

[2 Big 2 Fail]

Originally Posted by 2 Big 2 Fail
That depends on how you think of it. "Pound" is generic.

Not really. The pound is a measurement of force/weight, not mass. The imperial unit of mass is the slug.

"Pound" does indeed have two formal usages.

Look up "pound-mass," Imperial measure of mass (usually defined in terms of kilograms), as opposed to "pound-force," the engineering measure of mass being affected by gravity. Both are spoken of as "pound," and both are commonly abbreviated as "lb," although sometimes as lb_m and lb_f.

When we talk about something weighing so many pounds on the moon (or at some other altitude, as in my joking answer), we mean pound-force. We don't have to be picky about which one we mean when we talk about weight on Earth, because 1 gravity works on 1 pound-mass to produce 1 pound-force.

I was going to say I would like to see this scale calibrated in pounds-force (weight) that would be correct on the moon. But the space station astronaut weighing chair device I think is best described as using the astronaut's mass against known force.

 

[Walter]

Interesting.

Slug

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Slug

 

[inventor]

If the block of wood floats (very likely), it would displace less than 10 lbs of water.

A bouyant object must displace at least its weight in water to be bouyant at that depth. Be it positively bouyant on the surface or at depth. Or to be neutrally bouyant at depth.

A 10 pound block of wood, that is floating at the surface, displaces 10 pounds of water. Even if the block is only partially submerged.

"Archimedes Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object."

Bouyancy: Archimedes Principle

 

[2 Big 2 Fail]

Interesting.

Slug

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Slug

You learn the darnedest things while writing a response. I had not known of the "slugette," AKA "snail" until I ran across it in a mass article. The only problem now is that it's just darned hard to find a way to use those newly acquired terms in office conversation.

 

[kanonfodr]

A scale calibrated in kilos (mass) for use on earth would be wrong on the moon, but a scale calibrated in pounds (weight) would still be correct on the moon. Weight changes depending on the gravitational field, mass does not.

You just contradicted yourself, Walter...

Peace,
Greg

 

[kanonfodr]

The plane's source of power is the prop or jets and has nothing to do with the wheels. Therefore the conveyor speed has little impact on the plane. So the plane will get up to take-off speed, but the wheels will just be turning faster.

If you would refer to RJP's post (and possibly this article) lift is generated by airflow over an airfoil (wing), how fast your wheels are spinning has no direct relation to what is going on with the airflow over the wing.

So if the conveyor belt were to be exactly contradicting the speed of the aircraft relative to the conveyor belt the plane would be staying in place relative to the rest of the world, there would not be enough airflow over the wings to generate lift, and the plane would not take off, no matter how fast the wheels were spinning (even at an equivalent to 200 mph). However, I have yet to see a conveyor belt that fast .

Peace,
Greg

 

[knowone]

Pound :boxing: Slug
for
VTOL
as the Hawk flies​