Force vs Weight: Which is Heavier?

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SUMMARY

The discussion clarifies that the weight registered by a scale when a jar containing flies is placed on it remains constant regardless of whether the flies are sitting at the bottom or flying inside the jar. The total mass of the system, which includes the jar and the flies, does not change based on the flies' position. The forces exerted by the flies on the air and the jar contribute to the overall weight measured by the scale, leading to the conclusion that the weight remains the same in both scenarios unless the flies perform a collective motion that alters the forces acting on the scale.

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  • Understanding of basic physics concepts such as mass and weight.
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  • Basic principles of motion and acceleration as they relate to forces.
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This discussion is beneficial for physics students, educators, and anyone interested in understanding the principles of force and weight in dynamic systems. It provides insights into how forces interact within a closed environment, making it relevant for those studying mechanics or fluid dynamics.

havechanged
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Hey!

Okay, there are a bunch of flies in a capped jar. You place the jar on a scale. The scale will register the most weight when the flies are:

A. sitting on the bottom of the jar.

B. flying around inside the jar.

C. weight of the jar is the same in both cases.


I can't organize my thoughts! I can't explain this.

Any help will be greatly appreciated!
 
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C.

The flies are suspended in air, the air forces down on the jar. That's assuming the flies are not performing some collective motion. For example, if they all suddenly dive to the bottom, the jar will be momentarily lighter, regaining normal weight when they land.
 
Yeah, that is what I was thinking...it is just driving me crazy because I can't collect my thoughts as to the "scientific" reason this is so. Hmm...thanks though!
 
Well, for a scientific reason, think about this. The mass of the jar (which includes the flies inside of it) does not change depending on the location of the flies. Unless they perform some act like krab said.
 
Agreed. The answer, without special circumstances, is C.
 
Originally posted by havechanged
Hey!

Okay, there are a bunch of flies in a capped jar. You place the jar on a scale. The scale will register the most weight when the flies are:

A. sitting on the bottom of the jar.

B. flying around inside the jar.

C. weight of the jar is the same in both cases.


I can't organize my thoughts! I can't explain this.

Any help will be greatly appreciated!

First, if you consider the MASS "jar+flies", that is definitely a constant whatever the flies are doing: sitting or flying.

Now, if you consider the weight of the "jar+flies", that means you are looking at force that is exerced on the scale. There is there tree contribution to that force: from the jar (constant force), from the air in the jar and from the flies in the jar. Now do not forget that the flies rely on the air to be able to fly. Therefore, whether they sit on the bottom or they fly should not make any difference. In the air, the wings of the a fly exert on average a force on the air which is transmitted to the bottom of the jar and therefore to the scale. If there was no air (vaccum), the fly could simply not fly and would always rest at the bottom.

An easier picture would be to consider the jar filled with water whith a fish swiming in the jar. It would not matter if the fish float at the surface, sit a the bottom or swim in the middle: the fish weight applies a force on the water which is transmitted to the bottom of the jar.

Note: you may find that the force is not always constant in time and that the force is fluctuating during the measurement. That is because when the fish/flies move around they exert additional forces necessary to accelerate/decelerate their motions. However, these fluctuation should average to zero and the average weight in time should be "jar+air/water+flies/fish". Think that it is the same when you try to measure your own weight on a scale but you do not stay still...
 
If the flies are all accelerating upwards, the jar will register the highest value.
 
Originally posted by FZ+
If the flies are all accelerating upwards, the jar will register the highest value.

And if they are accelerating downwords - stooping ?
 

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