Understanding Buoyant Force Measurement with a Regular Scale | Explained

[parkcman]

Hi everyone this is my first post so forgive me if i leave out details
A friend of mine tried to measure buoyant force by the following means
Using a regular scale where the setup is sitting on top of it

1) Using a scale, he measured the object and found the scale to give x[grams]
2) Then on the scale, he put a beaker with water, then tared it to give 0[grams] then put his object in the water and recorded the measurement as y[grams]
3) x-y = 0.005g (just for argument sake)

But intuitively, i know that the 0.005g is the error from the machine
And also that the generation of the buoyant force pushing the object up generates an equal and opposite force that pushes down and as far as the machine is concerned there is no buoyant force thus the experiment is wrong and should actually give x-y=0g // x=y[gram]

Could someone identify what the opposite force is?
Or tell me if i am wrong.

Also I cannot calculate the theoretical buoyant force as the object is too small for volume measurements


Spent some time googling but no luck

 

[Doc Al]

But intuitively, i know that the 0.005g is the error from the machine
And also that the generation of the buoyant force pushing the object up generates an equal and opposite force that pushes down and as far as the machine is concerned there is no buoyant force thus the experiment is wrong and should actually give x-y=0g // x=y[gram]

I think you've got it right. Sure there's a buoyant force acting on the object, but this set up won't measure it. If you drop something that weighs X into the beaker, the scale reading will go up by X.

To measure the buoyant force you'd want to suspend the object from a thread and then dunk it in the water. That way the scale reading would go up by an amount equal to the buoyant force.

Could someone identify what the opposite force is?

The buoyant force pushes up on the object and the object pushes down on the water.

 

[parkcman]

Hey thanks for the fast response
So then we agree that the scale will change by Y in water and Y = X
So then what would the summation on the forces be AFTER the tare
such that the net force the scale reads will be Y
Weight of Object + Buoyant Force - (___________) = Y <-- which the scale reads
We know Weight of Object = X and we know X = Y
X + Buoyant Force - (___________) = Y
Y + Buoyant Force - (___________) = Y
Buoyant Force = (__________)
So what is this mysterious (___________)
Or maybe my force balance is all messed up
Thanks for your help again

 

[Doc Al]

Hey thanks for the fast response
So then we agree that the scale will change by Y in water and Y = X
So then what would the summation on the forces be AFTER the tare
such that the net force the scale reads will be Y
Weight of Object + Buoyant Force - (___________) = Y <-- which the scale reads
We know Weight of Object = X and we know X = Y
X + Buoyant Force - (___________) = Y
Y + Buoyant Force - (___________) = Y
Buoyant Force = (__________)
So what is this mysterious (___________)
Or maybe my force balance is all messed up
Thanks for your help again

Rather than go into all that detail, let me state the following:
The weight of the object is X.
If you drop the object into the beaker of water, the scale reading (of the beaker of water + object) will increase by X. I hope that makes sense.
The buoyant force is a force between the water and the object, thus plays no role in the scale reading. It is internal to the system being weighed.

If that's not enough I can try to answer your questions. (For some reason, you count the upward buoyant force on the object, but neglect the equal downward force on the water. There's no net force on the system, thus the scale just reads the total weight of everything.)

 

[pallidin]

To measure the buoyant force you'd want to suspend the object from a thread and then dunk it in the water. That way the scale reading would go up by an amount equal to the buoyant force.

I like that! By suspending the object from a thread you remove the objects mass influence on the weight scale. Nice.

 

[Buckleymanor]

Originally Posted by Doc Al
To measure the buoyant force you'd want to suspend the object from a thread and then dunk it in the water. That way the scale reading would go up by an amount equal to the buoyant force.

I like that! By suspending the object from a thread you remove the objects mass influence on the weight scale. Nice.

What if the object floated won't the scale read zero.
Don't you have to measure the weight of water displaced by an object to get the buoyant force of a floating object.
It should also work for dunked objects.

 

[Doc Al]

What if the object floated won't the scale read zero.

If you drop an object that weighs X into the beaker of water and it floats, the scale reading will increase by X. When an object floats, its buoyant force equals its weight.

Don't you have to measure the weight of water displaced by an object to get the buoyant force of a floating object.
It should also work for dunked objects.

Measuring the displaced water is one way to find the buoyant force, but not the only way.

 

[Buckleymanor]

If you drop an object that weighs X into the beaker of water and it floats, the scale reading will increase by X. When an object floats, its buoyant force equals its weight.

Measuring the displaced water is one way to find the buoyant force, but not the only way.

I don't know what type of scales you are using.Spring scales give a reading of zero when an object floats it is in effect weightless.
Imagine a wood ball on the end of a thread .
Lower it into water, once the ball is floating the thread goes slack, there is nothing to measure.
There is the force of gravity pulling on the ball up to the point of floatation but how do you decide when the ball is floating to take any meaningfull measurements before that point.

 

[Doc Al]

I don't know what type of scales you are using.Spring scales give a reading of zero when an object floats it is in effect weightless.

In this thread, the water filled beaker is sitting on a scale. (See post #1.) That's the scale reading we are talking about.

Imagine a wood ball on the end of a thread .
Lower it into water, once the ball is floating the thread goes slack, there is nothing to measure.
There is the force of gravity pulling on the ball up to the point of floatation but how do you decide when the ball is floating to take any meaningfull measurements before that point.

You are talking about measuring the tension in the thread using a scale. If the object floats, that tension would be zero. Just let it go slack.

 

[Buckleymanor]

In this thread, the water filled beaker is sitting on a scale. (See post #1.) That's the scale reading we are talking about.

You are talking about measuring the tension in the thread using a scale. If the object floats, that tension would be zero. Just let it go slack.

Got it.
So the set up in post 1 is perfectly valid for floating objects but not for ones that don't.

 

[Doc Al]

So the set up in post 1 is perfectly valid for floating objects but not for ones that don't.

Valid for what purpose? As I've stated, the setup in post #1 will measure the weight of the object whether or not it floats. It does not directly measure the buoyant force, if that's what you mean. (Of course, when the object floats the buoyant force equals its weight.)