# Tag Info

51

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 ...

46

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, ...

36

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

12

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 :)

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 ...

9

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: ...

9

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 ...

7

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 ...

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

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

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

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 ...

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 ...

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

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 ...

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 ...

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 you fill the cockpit with water, the pilot will feel a buoyant force. Humans have about the same density as water, so ignoring the scuba suit, the pilot will feel a buoyant force about equal to his own weight. The plane's maneuvers don't change this result much. By the equivalence principle, when the plane accelerates, the water in the cockpit and the ...

4

An object floats if its upthrust (buoyancy) is in equilibrium with its downwards gravitational force. In other words (as stated by the wiki page), $$F_net = 0 = mg - \rho_f V_{disp} g$$ (where all the constants are pretty self-explanatory.) Clearly then, the properties of the object that determine whether/how it floats are its mass and volume. More ...

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 ...

4

The comparison is viable, here's why: Let's choose the positive $x$-direction to point upward, perpendicular to the water's surface. By Archimedes' principle, the magnitude of the buoyant force on an object of volume $V$ equals the weight of the displaced water; $F_B = \rho_w V g$ where $\rho_w$ here denotes the density of water. The buoyant force ...

3

Can you take an empty plastic water bottle, make a mark on the side of it, and press it down to that mark in your larger container? Then can you show that the water that's displaced is 1) enough to fill the bottle up to the mark, and 2) it weighs as much as its bouyancy before (because when you fill the bottle up to that line, and again immerse it down in to ...

3

In the most common cases, it's based on density. An object that is denser than the liquid will sink, and one that is less dense will float. To go into a little more detail: when an object is placed in a liquid, the liquid exerts an upward force on it, called the buoyant force. This force is equal in magnitude to the weight of the liquid displaced by the ...

3

The only parameter that matters is the density: the ratio between the weight and the volume. If the density is higher than the surrounding medium (let's say water, whose density 1000 kg/m³) is will sink and it will float if the density is lower (you have the special where they are both equal). Objects float due to Archimedes' principle: basically, and ...

3

The question Basic buoyancy question: Man in a boat with a stone is an exact duplicate. The answer is that if the thing you throw from the boat has a density greater than water the level falls. If the object's density is less than water the level rises. Oops: Ah, yes, as Alan points out in his comment, in Basic buoyancy question: Man in a boat with a stone ...

3

It is the result of a dependence of the pressure with growing depth, due to the gravitational field (i.e. the weight of the water). You may do an easy calculation with some simple geometrical form, e.g. a cylinder totally submerged in water, to quickly understand how it works. The force due to pressure in each surface element of the curved wall of the ...

3

Good answers, but let me try to make it intuitive. Water weighs 1 gram per cubic centimeter (or 1 kilogram per Liter, a cube 10 centimeters on a side, if you prefer). So if you have a tube 1 centimeter square, stopped up at the bottom, and you fill it with water to a height of N centimeters, then you know how much pressure there is at the bottom. It is ...

3

I don't think this can work, even with your clarification that the balloon is tethered to the ground. Let's identify all the forces. The balloon has a buoyancy, pulling it upwards. I will call that $F_b$ (b for balloon or buoyancy). The balloon pulls the pulley up. I will call the tension in the rope tethering the pulley to the ground $F_s$ (for stake). That ...

Only top voted, non community-wiki answers of a minimum length are eligible