# Tricking Archimede's principle

• jumpjack
In summary: If there is net upward force the object will rise (until it reaches the surface), if the net force is zero the object will float, if it is negative it will sink (until it reaches the bottom).

#### jumpjack

What would it happen if...

...you get half a wooden sphere, put it into a bucket filled with water, making the flat part of the half-sphere touching the bucket bottom?

Will the half-sphere come up floating?
Maybe not, 'cause water has no way to push it up?

A video woukd be cool.

jumpjack said:
What would it happen if...

...you get half a wooden sphere, put it into a bucket filled with water, making the flat part of the half-sphere touching the bucket bottom?

Will the half-sphere come up floating?
Maybe not, 'cause water has no way to push it up?

A video woukd be cool.

If there is no pressure acting on the bottom of the half-sphere it will not float.

However, with this type of experiment you'll likely find that there isn't a practical way to remove the water from the underneath side of the half-sphere once you submerge it in the bucket.

CS

The situation you described is extremely unstable. The slightest jitter of the half-sphere would allow water to get underneath and push it up.

stewartcs said:
If there is no pressure acting on the bottom of the half-sphere it will not float.

However, with this type of experiment you'll likely find that there isn't a practical way to remove the water from the underneath side of the half-sphere once you submerge it in the bucket.

CS
then if I place it on the bucket bottom and THEN I fill the bucket, it should stay there...?

What would be more suitable? a less-than-half sphere, or a... gothic-arc-shaped-wood-piece? (It's designed to carry all the weight it receives from above...)

jumpjack said:
then if I place it on the bucket bottom and THEN I fill the bucket, it should stay there...?

What would be more suitable? a less-than-half sphere, or a... gothic-arc-shaped-wood-piece? (It's designed to carry all the weight it receives from above...)

jumpjack said:
then if I place it on the bucket bottom and THEN I fill the bucket, it should stay there...?

If the water is prevent from entering the underneath side (e.g. a seal) then yes.

jumpjack said:
What would be more suitable? a less-than-half sphere, or a... gothic-arc-shaped-wood-piece? (It's designed to carry all the weight it receives from above...)

I don't know what a gothic-arc-shaped-wood-piece looks like. However, stability of floating bodies is determined the same way.

Take a look here for more information on that: http://www.coastal.udel.edu/faculty/jpuleo/CIEG305/stability_floating_body.pdf

CS

http://en.wikipedia.org/wiki/Suction_cup" [Broken]?

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gothic arc:

And what about a swing-shaped object?

It has more surface available for pressure from above than it has for "floating force" coming from below...
Would it flow or sink? (if static).

jumpjack said:
gothic arc:

And what about a swing-shaped object?

It has more surface available for pressure from above than it has for "floating force" coming from below...
Would it flow or sink? (if static).

The resultant force is what determines if it will float or sink (that and the weight of the object of course). Archimedes Principle will work just fine in order to determine if the object will float or not when placed in water.

CS

stewartcs said:
The resultant force is what determines if it will float or sink (that and the weight of the object of course). Archimedes Principle will work just fine in order to determine if the object will float or not when placed in water.

CS
yes, but I can't understand WHICH forces try balancing in archimedes' principle: gravity and...?
I don't think it is the attraction among H2O molecules (which causes a hole in the water to close by itself) can push over an object submerged into the water, trying to "expell" it... unless you're in outer space(*).
No, I think archimede's principle depends on gravity: but if gravity only pulls down, how do H2O molecules act on a body to "reverse" gravity effect?

(*)
what does it happen to half-a-sphere submerged in water if there is no gravity? I guess a wooden sphere would remain inside the water sphere (!) where you put it. But if you have a wooden HALF sphere... will it start moving?!? :uhh:

jumpjack said:
yes, but I can't understand WHICH forces try balancing in archimedes' principle: gravity and...?

The force generated by the hydrostatic pressure acting over the surface area of the object.

Draw a FBD and sum the forces. If there is net upward force the object will rise (until it reaches the surface), if the net force is zero the object will float, if it is negative it will sink (until it reaches the bottom).

CS

It sounds like the OP is trying to balance bouyancy and the film pressure:

## 1. How can you trick Archimede's principle?

There are a few ways to trick Archimede's principle, but the most common method is by creating a hollow object that appears to have a certain mass but is actually filled with air or another low-density substance. This makes the object seem to have a lower weight than it actually does, which can be confusing and tricky to measure accurately.

## 2. Why is it important to understand Archimede's principle?

Understanding Archimede's principle is important for many reasons. It can help us determine the buoyancy of objects and understand why some objects float while others sink. It is also crucial in fields such as shipbuilding, engineering, and even weather forecasting.

## 3. Can Archimede's principle be disproven?

No, Archimede's principle has been proven through various experiments and is a fundamental law of physics. However, it can be tricked or manipulated in certain situations, which can lead to confusion or misunderstandings.

## 4. How does Archimede's principle apply to real-life situations?

Archimede's principle is applicable in many real-life situations, such as understanding why ships float, why helium balloons rise, and why icebergs float in water. It is also used in industries such as oil and gas, where understanding buoyancy is crucial for drilling and extracting resources from the sea floor.

## 5. What are some common misconceptions about Archimede's principle?

One common misconception is that Archimede's principle only applies to objects in water. In reality, it applies to any fluid, including air. Another misconception is that the weight of an object affects its buoyancy. In fact, it is the density of the object compared to the fluid it is in that determines its buoyancy. Lastly, some people believe that Archimede's principle only applies to objects that are fully submerged in a fluid, when in fact it also applies to partially submerged objects.