Apparent weight in fluids

[mozdef]

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ok, so I just want to make sure the apparent weight of an immersed object in a fluid is the difference between it's actual weight mg and the buoyant force acting on the object..right?

but the apparent weight of an object that's sitting on a scale in an accelerating lift is the normal force acting on the object by the scale?

i always get confused because in the second case, it's the upward force thats the apparent weight of the object and in the first case its the net force thats the apparent weight of the object. is this all correct?

 

[habman6]

Ya thats right. Remember though if the accelerating lift is pushing up on the person (normal force), the person is also pushing down on the lift (apparent weight). You deal with the normal force so you can calculate the magnitude

 

[mozdef]

thanks, and also one more question if you don't mind. If the apparent weight increases, then g increases as well? And if the apparent weight decreases because of downward acceleration, g decreases...since mass is constant?

 

[misterex]

I like to think of someone standing on an elevator in this situation. If the elevator is accelerating down, the person feels light because his apparent weight is smaller. However, if the elevator is accelerating up, the person feels heavier because his apparent weight is higher. Hopefully that helps someone.

 

[habman6]

thanks, and also one more question if you don't mind. If the apparent weight increases, then g increases as well? And if the apparent weight decreases because of downward acceleration, g decreases...since mass is constant?

No g is not increasing. Its called apparent weight because its not a true weight, but its a force that acts in the same way, hence adding to or subtracting from the true weight.

The true weight will ALWAYS be mg. "g" is determined by Gm1/r², and will not change with an accelerating object (unless the radius is changing also.....or mass.....but thats getting into quantum, which doesnt follow these gravitational laws).

The apparent weight is simply the TRUE weight, plus or minus some other force counteracting/acting with it. For example, in a lift accelerating upwards, the weight is determined by the normal force, which is composed of the action/reaction forces between the true weight, and the accelerating elevator.

True weight = downward force due to our mass. Always = mg
Apparent weight = upward force, due to the normal force.

 

[xlr8]

its called "apparent weight" because usually the normal force is used to give you a weight reading. however in fluids, where there is another "upward" force, if you were to measure the additional force required for equilibrium with like a rope, you will find the tension is "apparently" less.

 

[Atomic13]

An underwater scale would give you the apparent weight, which would still be measured by the normal force. The same scale, out of the water would give the regular weight.