Finding downward force on immersed object

[member 731016]

It does not say anything about water; it just says downward force.
(Now, if the object is less dense than water the net will indeed be exerting a downward force, but, from the equation Fi = Fg - B, Fi is measured positive up, so I assume all the objects naturally sink.)
It could be that the prof made a mistake and wrote downward instead of upward, or equivalently, "on the object" instead of "from the object". Either way, it is quite clear from the algebra that Fi is supposed to be the force the net exerts on the object.

It is also clear that this is not quite the same as the force recorded by the sensor, since that force (T?) includes the weight of the net (less any buoyant force on that). This makes me think the TA did not say, or did not mean, that T=0; merely that T is only very slightly different from Fi, so use the sensor reading as Fi and don't worry about T.

Thank you for you reply @haruspex!

I had a chat with the Professors today. Apparently, Fi is defined to be the upward force exerted by the force sensor and it not actually the downward force exerted by the water. They are going to change the lab script so that the definition of Fi will no longer say 'downward force on immersed object' which they agreed could imply the downward force from the water above the object. You are correct that $B ≠ F_j - F_i$.

$F_i + B - mg = 0$ and rearranging gives their expression.

Thank you and @kuruman very much for your help!

 

[member 731016]

The professor's statement that $F_i = F_g - B$ proves that $B \neq F_j - F_i$.

Thank you for you help @haruspex!

 

[malawi_glenn]

Thank you for your reply @kuruman!

Thank you for you help @haruspex!

What did we say about making posts just to say "thanks"?