Hot answers tagged

67

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 ...


62

The work you need to do (to insert the log) against the pressure of the fluid at that depth is equal to the work done by the fluid to get the log up to the height you desire. If you consider a log of volume $V$ and a tank of depth $h$, the pressure at that depth would be $\rho gh$, where $\rho$ is the density of the fluid, and $g$ the acceleration due to ...


53

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, ...


45

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


39

The balloon has a very small mass and friction is large (large surface area), so the oscillation is very damped.


38

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, ...


37

The buoyant force on a body immersed in a fluid is equal to the weight of the fluid it displaces. In other words, $$ F_B = \rho_{\text{fluid}} V_{\text{body}} ~g $$ The force of gravity on the body is equal to $$ F_g = m_{\rm body} ~g $$ The apparent weight of this body will therefore be equal to the sum of these two forces. $$ W_{\rm app} = \rho_{\rm ...


36

Here is an explanation that needs no explicit equations. 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 ...


35

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' principle 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} = ...


28

It's a combination of two effects: buoyancy and adhesion. Buoyancy lifts the cork up as much as possible, until it displaces its own weight of water (Archimedes' principle). For this reason, the cork will seek the highest point of the water level. Because of adhesion between the water molecules and the glass, the water level is highest at the edges (the ...


24

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 :)


22

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 ...


22

Tied balloons do behave like a pendulum, you only need really massive ones: You can see it live in a video. Hot air balloons have such a big amount of..erm...hot air that during the start you can expect oscillations because while the surface area is big, the mass inside is so big that the dampening is low enough. They are not exactly like a pendulum ...


21

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, ...


20

What seems to be happening is that capillary effects in the presence of gravity create a situation in which the cork being maximally decentralized in the glass corresponds to a minimum energy configuration. My guess is that the cork is non-wetting, and therefore surrounded by a water surface that bends down in the proximity of the cork, thereby creating a ...


19

If you have a "slack" balloon (one with no elasticity, like is used for some extreme altitude work like the one Felix Baumgartner used for the highest free-fall) then the pressure inside is the same as the pressure outside, and the balloon will not find an equilibrium position due to pressure (the volume of air displaced will change with altitude, and the ...


18

Average human body volume = 0.0664 $m^3$ (seems low to me, but that's according to Wolfram Alpha: http://www.wolframalpha.com/input/?i=volume+human+body). Density of air depends on temperature and pressure, but is about 1.2 or 1.3 $kg/m^3$. That means we displace, on average, about 80 g of air, giving us a buoyancy of about 0.8 N (about 1/6 lb). The ...


15

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 ...


14

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 ...


13

I looked up the answer to this one in a book published in 1914 - you don't get many citations 99 years old! For the interested, the book is "A Textbook of Physics Vol 1" by J. H. Poynting and J. J. Thompson, page 188 in my copy. Incidentally that's the same J. J. Thompson who discovered the electron - Poynting has a vector named after him though only ...


13

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.


12

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: ...


12

1) Technically while you push it down it will cause an increase in pressure (so the weight will change) but assuming the box has a hole and the air can equalise then the weight of your initial and end state will be the same. This is because the air is equalised within the box at all stages and at the end state the air that was in the box is now above it. ...


12

This is because that the cork floats to the highest point it can. The water is not flat in the glass - it curves up a the edges so the cork gets higher by going to the edge. When the glass is full, really completely full just above the level of the rim of the glass, the water will be bowed a bit so that it goes down at the edges and is highest in the ...


11

Buoyancy is dependent on the density difference between the object and the surrounding fluid. Though the pressure goes up in the ocean with increasing depth, the density remains more or less constant because water is almost incompressible. The pressure in water increases by about $100\,{\rm kPa}$ for every $10\,{\rm m}$ of depth, and the Marianas trench is ...


10

The reason behind buoyancy is the pressure difference in fluids. More specifically, the difference in hydrostatic pressure on different levels, since the hydrostatic pressure of water increases with depth ($p=\rho gh$ wher $h$ is the distance below the surface). The part of the body which is subject to higher hydrostatic pressure will be pushed more upwards ...


10

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 ...


9

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 ...


8

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 ...



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