Simple mechanics question: will reading of the scale change?

[tonnijames]

Homework Statement

A beaker filled with water is placed on the scale.
A weight denser than water is tied to one end of a string; the other end of the string is tied to a stick. The weight is then placed in water with the stick supported over the rim of the beaker. Total weight at this time is recorded. If the string is suddenly cut and the weight sinks at a constant velocity, will the reading of the scale change? Assuming stick and string are massless.

Any help is appreciated. Thank you!

Relevant Equations

Ft=Fg - F(buoyant) = F(stick on beaker)
Fnet final = 0 on the weight.

My reasoning is that
1)at initial condition, net force of stick on the beaker is equal to the weight of the weight minus the buoyant force applied by the water to the weight, which is the tension force in the string. The recorded weight should be equal to mass of beaker + water + net force of stick on beaker.
2)when string is cut, as it sinks at constant velocity Fnet=0 for the weight. Thus the weight is "canceled out" by the buoyant force and net forced applied by stick on the beaker is 0. I suspect that the final reading will be less, but I could not be sure.
Below I have drew a diagram.

 

[andrewkirk]

The final reading - after the marble has come to rest on the bottom of the beaker - must be the same as before cutting the string, because the mass of the beaker + water + marble + string + stick has not changed, and all parts are at rest in the end, as they were in the beginning. Since the weight of a stationary object depends only on its mass, the weight must also be the same.

There will be a slight reduction in weight immediately after the string is cut, as the marble accelerates to its constant descent velocity, returning to the normal weight as soon as that velocity is reached. That is mirrored by a slight increase in weight between when the marble first contacts the floor and when it comes to rest. That increase in weight relates to the deceleration.

The 'blips' of decreased and increased weight are similar in some ways to the lightness you feel in a lift (US elevator) when it starts to descend and the increased weight when as it slows to stop at a lower floor.

Often in physics problems it is assumed that the descent or ascent velocity is infinitesimally slow, in order to make these blip variations as the item accelerates to and decelerates from the ascension/descent speed small enough to ignore.

 

[tonnijames]

The final reading - after the marble has come to rest on the bottom of the beaker - must be the same as before cutting the string, because the mass of the beaker + water + marble + string + stick has not changed, and all parts are at rest in the end, as they were in the beginning. Since the weight of a stationary object depends only on its mass, the weight must also be the same.

There will be a slight reduction in weight immediately after the string is cut, as the marble accelerates to its constant descent velocity, returning to the normal weight as soon as that velocity is reached. That is mirrored by a slight increase in weight between when the marble first contacts the floor and when it comes to rest. That increase in weight relates to the deceleration.

The 'blips' of decreased and increased weight are similar in some ways to the lightness you feel in a lift (US elevator) when it starts to descend and the increased weight when as it slows to stop at a lower floor.

Often in physics problems it is assumed that the descent or ascent velocity is infinitesimally slow, in order to make these blip variations as the item accelerates to and decelerates from the ascension/descent speed small enough to ignore.

Thank you for the explanation. In my original post I made a mistake that the weight of the marble will be canceled by the buoyancy of water, due to marble descending at constant velocity. Now I see that regardless of the motion of marble, as long as it's stationary and the mass does not change, reading would not change.

 

[SammyS]

There is another effect which may be worth considering.

The object (a marble or whatever) is denser than water.

What is true of the center of mass of the system as the object sinks?

 

[jbriggs444]

There is another effect which may be worth considering.

The object (a marble or whatever) is denser than water.

What is true of the center of mass of the system as the object sinks?

That effect is already contemplated in the response by @andrewkirk. Yes, the center of mass of the system is moving downward. But what can be said about its downward velocity?

 

[SammyS]

That effect is already contemplated in the response by @andrewkirk. Yes, the center of mass of the system is moving downward. But what can be said about its downward velocity?

Oh my!

Well yes, it is constant. Damn you Sir I. Newton !