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59

It travels forwards instead of backwards in an accelerating car for the same reason that a helium balloon travels upwards instead of downwards under the influence of gravity. Why is that? In an accelerating car, for all intents and purposes the acceleration can be considered a change in the amount and direction of gravity, from pointing straight down to ...


47

The balls are entering the water well below the surface. The pressure there is much higher than at the surface. The work needed to push the balls into the water at this depth cancels the work gained when they float back up. We can ignore the gravitational force on the balls since gravity pulls down as much as up as you traverse the loop. Mathematically, ...


40

When your car accelerates forward, the air inside moves back relative to the car. This creates a slightly high pressure in the rear of the vehicle and a low pressure up front. Since helium is lighter than air, it moves away from the region of high pressure. A similar balloon filled with $CO_2$ would move back, since it is heavier than the surrounding air


34

Here is a free body diagram of the balls: … and one of the water volume: The four balance equations are $$ \begin{align} B_1 - T_1 - m_1 g & =0 \\ B_2 + T_2 - m_2 g & = 0 \\ F_1 + T_1 - B_1 - M g & = 0 \\ F_2 - B_2 - M g & = 0 \end{align} $$ where $\color{magenta}{B_1}$,$\color{magenta}{B_2}$ are the buoyancy forces, ...


30

Good question. Assume we have one cube of ice in a glass of water. The ice displaces some of that water, raising the height of the water by an amount we will call $h$. Archimedes principles states that the weight of water displaced will equal the upward buoyancy force provided by that water. In this case, $$\text{Weight of water displaced} = ...


24

Here is an explanation that needs no calculations. Consider the following diagram, in which part1 and part2 represent the ice. The displaced water volume equals part2 volume and has as much mass as (part1+part2) Now look at what happens when both part1 and part2 melt: their mass does not change, it is (part1+part2) it becomes water. And we just said ...


19

The weight on the left bowl would be the weight of the water plus vase plus ping-pong ball (plus thread, ignored). The weight on the right bowl would be the weight of the water plus vase plus the buoyancy of the steel ball (plus the buoyancy of the submerged thread, ignored). That buoyancy is the weight of an equivalent volume of water. Since the ping-pong ...


17

A Thought Experiment We can arrive at an intuitive explanation without any special knowledge of physics. The strategy is to re-create the setup as closely as possible while keeping the two sides in balance. Imagine that you start with two identical beakers, filled with the same amount of water, no balls. Placed on the scale, they balance. On the left, ...


17

Fun question. Here's my "me-too" answer. Suppose the car has just emerged from a river, so there's a lot of water in it, and the balloon is tied to the floor. Then you drive away. The air in the car is just like a bunch of water :)


12

The energy needed to submerge a ball is equal to the energy gain from other ball to emerge from the water on the other side, so any waste on friction drives the process impossible.


11

As the comments above indicate, factors like density, pressure and temperature are important for a Jupiter submariner. Of course nobody yet has the exact details of Jupiter's interior structure, but there's a diagram in this LASP page[WebCite archived version] that indicates the following: ...


11

It acts precisely like water in a cup. Or, more specifically, like the air in the cup. Since the helium is a much lower density than the nitrogen and other gasses in your car, it can be visualized like an air bubble in a bottle. The container for the helium(the balloon) has negligible mass. When you accelerate forward, the water in a bottle will move ...


10

This diagram is my attempt to show the situation first when the rock is in the boat and secondly when you've chucked the rock over the side. The mass of the boat of $M$ and the mass of the rock is $m$. The density of water is $\rho_w$ and the density of the rock is $\rho_r$. In the first case Archimedes' principle tells us that the volume of water ...


9

What is probably being referred to here indirectly is the fact that air with moisture in it is less dense that dry air. The question becomes, is the buoyancy force of an empty egg with the optimal moisture content of air sufficient to overcome it's weight? Searching around I see that water vapor has a density of 0.804g/Land dry air has a density of 1.27 ...


8

The problem is in the seal. The amount of work to move the seal against the water pressure is the same amount of energy that is gained by the balls when they are pushed up by the water. Even if we remove the seal and we imagine a magic "one-way pass-through" wall, the ball would still need to displace the same volume of water as itself in order to get into ...


7

Trivially the density of the water increases identically to the object and so bouyancy is maintained...


6

Submerging objects in a liquid does not change the mass of those objects. It does effect the weight they would register on a scale, though. The bouyant force a fluid exerts upwards on a body submerged in it, $$F=\rho Vg$$ where $\rho$ is the density of the fluid, $V$ is the volume of the fluid displaced, and $g$ is the acceleration due to gravity. The ...


6

For a given volume, light things float and heavy things sink. The cup sinks when you fill it with water because it becomes heavier, and therefore more dense. When the cup becomes more dense than water, it sinks. The cup would sink just as well if you filled it with rocks, lead, etc. The condition for the cup to sink is that its weight must be greater ...


6

The Costa Concordia wasn't sunken, it was aground. Which meant that part of it was still above the water. The ping pong solution can only be used if the ship is completely underwater. Otherwise, it will have an opposite effect. The buoyant force (force by which a fluid pushes up on a body, thus keeping it afloat), is proportional to the mass of the fluid ...


6

Balloons are buoyant because the air pushes on them. The air doesn't know what's in the balloon, though. It pushes on everything the same, so the buoyant force is the same on all balloons of the same size. If the "balloon" is just a lump of air with an imaginary boundary, then the lump won't go anywhere because the air isn't moving on average. So the ...


6

Would hot hydrogen (in the same sense as hot air) be able to lift even more mass? Yes. Though I suppose the fire danger goes up, and you certainly can't use a propane burner to warm it... Would a higher or lower density of hydrogen in a ballon lift more? Lower density always means higher buoyancy. If you could have a balloon which had ...


6

Just a tad about how bouyancy works. Any fluid in a gravitational field possesses a pressure gradient, (which if the gas/liquid is in equilibrium) counterbalances the effect of gravity. Gravity acting on such a fluid creates this pressure, which is referred to a hydrostatic pressure. To make a long story short, the external pressure (of the air) is greater ...


6

Actually, the answer is a bit more subtle than just density. The principle that is behind floating objects is Archimedes' principle: A fluid (liquid or gas) exerts a buoyant force, opposite apparent gravity (i.e. gravity + acceleration of fluid) on an immersed object that is equal to the weight of the displaced fluid. Thus, if you have an object fully ...


5

DENSITY It is because of densities of the object that is floating and the liquid in which it is floating. If an object have density lower than a fluid it will float otherwise it will sink. Density of entire object [mass / volume] should be taken into account and not merely the density of material it is made up of. A ship made up of iron floats ...


5

Helium balloons are pulled by gravity, as are all objects with mass. The reason they don't fall is that there is another force acting on them, a buoyant force from air pressure that is equal to the weight of the air displaced by the balloon. The reason you don't float is that the weight of the air you displace is quite a bit less than your weight (a person ...


5

dmckee's answer is a great not-too-technical description of buoyancy. Read that first. But in case you're interested, I thought I would go into some more detail. The buoyant force on a submerged object (e.g. a balloon submerged in air) is equal to the weight of the displaced fluid, $$F_b = \rho_f g V$$ as dmckee said. The physical origin of this force is ...


5

I assume you are asking why we are not drawing air out of a balloon like container so as to create the lower density that helium or hot air gives us. The answer is that it is hard to maintain a vacuum with a thin enough, so as to be almost weightless, rigid contaning surface. A balloon with gas inside equalizing the atmospheric pressure with the gas ...


4

I am not good at explaining things but here it goes try to understand it this way assume the rectangle to be you car, now the dotted area is filled with normal air and their is a helium balloon in between So when you accelerate everything in the car tries move backward with respect to the car even the helium balloon, so then why the balloon goes ...


4

Assuming they are filled to the same volume, and the air surrounding them is of the same density, the buoyant force acting on the balloons will be the same. Buoyant force is simply equal to the weight of the amount of surrounding fluid that would occupy the space filled by the balloon, if the balloon were not there. It has nothing to do with the contents or ...


4

If the object floats: water level stays the same If the object sinks: water level decreases Consider the force balance. The Earth exerts an upward force on the lake. Anything floating on the water is included in the weight of the lake. Since water is constant density, the upward force on the lake is a direct function of the water level - a higher level ...



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