Apparent weight in buoyancy vs. elevator

[Kaydubz]

So take an object that weighs 120 N. If we put this object on an elevator that has a net downward force of 70 N, then the apparent weight of the object will be 50 N. Fgrav + Fnormal = Fnet, where the Fnormal is negative and upwards and the apparent weight.

Take the same object and put it in water. Once again, lets assume that the object is moving downwards with a net force of 70 N. In this case, with the same weight of an object and same net force, the apparent weight is 70 N, with the buoyant force equalling 50 N. Fbuoyant + Fnet = Fgrav, where Fbuoyant is upwards and negative but Fnet is the apparent weight.

Is this just an odd coincidence from the nature of buoyant force as opposed to the elevator's normal force, or am I missing something?

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[sexyman]

My head hurts, your one crazy cat kaydubz

 

[DrDuber]

So take an object that weighs 120 N. If we put this object on an elevator that has a net downward force of 70 N, then the apparent weight of the object will be 50 N. Fgrav + Fnormal = Fnet, where the Fnormal is negative and upwards and the apparent weight.

Ok first, don't think of this problem in terms of force of the elevator. You should be looking at this in terms of acceleration and reference frames. If the elevator is accelerating (note not just maintaining speed) downward, then the relative acceleration of the object to the scale determines what force is read by the scale (the difference in acceleration experienced by the body and the acceleration of the elevator).

Take the same object and put it in water. Once again, lets assume that the object is moving downwards with a net force of 70 N. In this case, with the same weight of an object and same net force, the apparent weight is 70 N, with the buoyant force equalling 50 N. Fbuoyant + Fnet = Fgrav, where Fbuoyant is upwards and negative but Fnet is the apparent weight.

Is this just an odd coincidence from the nature of buoyant force as opposed to the elevator's normal force, or am I missing something?

I am not sure I understand your question. The object is traveling in water with a net force of 70 N, therefor any scale measuring the weight of the object will read 70 N. In this case the net force is -70=-120(acceleration due to gravity) + 50 (drag due to the water) using the sign convention of a negative force acts down.

I think the bggest difference between these two situations is your frame of reference. In the first part, we are viewing the weight of the object from the frame of reference of the elevator, ignoring the rest of the universe. In the second case we are viewing from an external reference frame, essentially standing still as the object goes by and then measuring. I think this accounts for the discrepancy you are seeing.

 

[majik1213]

So take an object that weighs 120 N. If we put this object on an elevator that has a net downward force of 70 N, then the apparent weight of the object will be 50 N. Fgrav + Fnormal = Fnet, where the Fnormal is negative and upwards and the apparent weight.

I think your algebraic expression should read that Fnet=Fgrav - Fnormal, not plus (+). After all, the normal force, which points up, opposes the direction of both the net force and the gravitational force, which both point down.

Also, the net downward force of which you speak is not a property of the elevator but rather of the system containing both the elevator AND the object that sits on it; that is, rather than saying, "If we put this object on an elevator that has [i.e. exerts] a net downward force of 70 N [...]," you might wish to say, "If we consider this object as part of a system that experiences a net downward force of 70 N...." When I first read your post, at least, I thought the object was being pushed by the ceiling of the elevator due to an accelerating downward descent rate.

Last, I do not understand your question. Could you rephrase it please?

 

[Kaydubz]

I've included some diagrams (lol). The comments about the frames of references helped.

In the case with the elevator, the scale is also accelerating downwards with the net force along with the object, while in the case of the water, the scale is probably fixed on an immobile surface above the water. So even though net force and f_grav are equal, the scales measure different things. The first measures the normal force, and the second measures the net force.

Think that's right