physics question

[Guinness]

If your 10 lb arm is suspended, the weight of the tank would be the same because your arm would have a 10 lb force down, but your shoulder would have a 10 lb force up. On the other hand, it wouldn't take 10 lbs of upward force to suspend your arm in a denser fluid than air, so I have no idea.
Guinness

 

[Option4]

Let's se if I can remember what they taught me in Submarine School...

An object which floats will displace its exact weight of water. The floating object's volume is greater than the volume of water displaced.

An object that has neutral bouyancy displaces its exact weight of water. The volume of the suspended object equals the volume of of water displaced.

An object that sinks will displace less water weight than its own weight. The volume of the object is the same as the volume of water displaced.

So, the density of the water (or liquid) and object needs to be known.

The arm has a certain density and volume. If the density of the arm is greater the the density of the water then an upward force needs to be applied to keep it suspended. It will displace a given volume of water. So the amount of weight increase in the tank of water would be the weight of water displaced by the arm less the upward force to keep the arm suspended. (Weight is a force, mass X acceleration of gravity)


Conversely, if the arm is less dense than the water, a downward force must be applied to the arm to keep it suspended. So the weight increase of the tank will be the weight of the volume of water displaced plus the downward force...

So, to sum it all up:



Objects that float have more volume that the displaced water and the floating objects density is less than the liquid.

Suspended Objects (neutral bouyancy) both object and liquid have the same density.

Sinking objects have equal volumes of displacement and the objects density is greater than the liquids.



Clear as mud. Right?

 

[Lil_Stinker]

Let's se if I can remember what they taught me in Submarine School...

An object which floats will displace its exact weight of water. The floating object's volume is greater than the volume of water displaced.

An object that has neutral bouyancy displaces its exact weight of water. The volume of the suspended object equals the volume of of water displaced.

An object that sinks will displace less water weight than its own weight. The volume of the object is the same as the volume of water displaced.

So, the density of the water (or liquid) and object needs to be known.

The arm has a certain density and volume. If the density of the arm is greater the the density of the water then an upward force needs to be applied to keep it suspended. It will displace a given volume of water. So the amount of weight increase in the tank of water would be the weight of water displaced by the arm less the upward force to keep the arm suspended. (Weight is a force, mass X acceleration of gravity)


Conversely, if the arm is less dense than the water, a downward force must be applied to the arm to keep it suspended. So the weight increase of the tank will be the weight of the volume of water displaced plus the downward force...

So, to sum it all up:



Objects that float have more volume that the displaced water and the floating objects density is less than the liquid.

Suspended Objects (neutral bouyancy) both object and liquid have the same density.

Sinking objects have equal volumes of displacement and the objects density is greater than the liquids.



Clear as mud. Right?

Not bad at all.

If I put a ball on the surface that weighs 8 oz (1/2 lb) then the overall weight will be 100.5 lbs. If I push the ball lower (I still don't touch the water) teh the "weight n the scale" will increase in proportion to the force exerted on the ball.

If I put a 100 lb weight to the botom then the weight is 200...

(Let's assume the volume of the weight is 1000cc) Pick up the weight off the bottom (using a device of nominal weight & volume), the "scale weight" will drop (nearly to 100) perhaps to 102.2(ish) you will need to exert an upward force in the neighborhood of 97.8 lbs.

I think that's close .. it's been a while.

 

[Option4]

(Let's assume the volume of the weight is 1000cc) Pick up the weight off the bottom (using a device of nominal weight & volume), the "scale weight" will drop (nearly to 100) perhaps to 102.2(ish) you will need to exert an upward force in the neighborhood of 97.8 lbs.

I think that's close .. it's been a while.

That's what my calculator says...good job!