# Tag Info

163

Needing any excuse to break away from the work I was doing, I immediately assumed the task of answering this question. Yes, ants can walk on mercury with no trouble at all. I bet it was even kind of fun, but this little punk just split as soon as possible. I got a video, but had to settle for a screenshot for this post.

134

Of course, by common sense, if you put together two objects with masses $m_1$ and $m_2$, and nothing comes out, then you end up with mass $m_1 + m_2$. Weights are a little more complicated because of buoyant forces. All objects on Earth continuously experience a buoyant force from the volume of the air they displace. This doesn't matter as long as volume ...

90

When submerged, the coin displaces as much water as it has volume (logical). When floating on the box, the coin displaces as much water as corresponds to its weight. As metal has a higher density than water, it means that the coin in the box displaces more water than when the coin is submerged.

87

When put in water, an objects sinks to the point where the volume of water it displaces has the same weight as the object. Archimedes was the one who discovered this. When you put lead in water, the weight of the lead is much greater than that of the same volume of water. Hence it sinks to the bottom. As ice only weighs about 90% of its volume of water, 90% ...

82

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 ...

64

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 ...

62

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} = m_\text{water ... 62 Whether or not a small animal/insect can walk on a liquid is determined much more by surface tension than by density. To see why this is consider a dense liquid without any surface tension. You would float in it very well, but if you tried to walk on it you would step right through the surface and fall over, sinking until you were sufficiently submerged to ... 58 The domestic chicken's egg shell has about 7000 pores that allow the embryo to breathe. When an egg rots the yolk and surrounding materials decompose and they give off gasses which can pass through the shell. This allows mass to leave the interior of the egg resulting in less density for the volume of the egg making it more buoyant. 54 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 54 Yes it floats. And it has displaced its "own weight of water" in the sense that if you had filled the container with water and only then lowered the ship into the container, nearly all that water would have been dispaced and is now sloshing around on the floor. 53 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 ... 52 I like to answer by reinterpreting your question: if you expect the ice to be completely atop the water because ice is less dense than water (as indicated in your left image), then you would also expect the ice to be completely below air because ice is more dense than air (in order for this to be true, think pushing your ice cube down into the water so that ... 50 The answer is no. Water, being a liquid, is nearly incompressible, meaning that the density changes very little with increasing pressure. In the very deep ocean, the pressure can approach 10^{8} Pa (about a thousand times greater than standard atmospheric pressure of 1.01\times10^{5} Pa). However, the bulk modulus B of liquid water (the reciprocal ... 43 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} $$where \color{magenta}{B_1},\color{magenta}{B_2} are the buoyancy forces, \color{red}{T_1},\... 39 The balloon has a very small mass and friction is large (large surface area), so the oscillation is very damped. 39 The buoyant force on a body immersed in a fluid is equal to the weight of the fluid it displaces. In other words,$$ 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 ...

39

The fluid does not really exert an upward force on a body. It exerts a force everywhere on the body normal to its surface. [Adapted from Hyperphysics] In a gravitational field, the pressure increases with depth, so those normal forces which would otherwise cancel, end up summing to an upward force vector. If the tank were placed on, say, a centrifuge, ...

38

The force required to push an object into water increases as the object submerges, i.e. as the amount of water the object displaces steadily increases. But I think if you do the experiment carefully you will find that, once the object is fully submerged, the force required should be almost constant. Thereafter, many objects get easier to push down with ...

37

This is less of a physics question and more of a neurophysiology question. Physically, as you have noted, both the proper acceleration (measured by an accelerometer) and the coordinate acceleration are the same in the two situations. So this is about the neural pathways involved in producing this specific illusion. There are four main senses involved in the ...

34

Yes, it will, but in a different way than one might imagine. In the Earth's oceans it is definitely impossible. From the phase diagram of water, one can see that assuming a constant temperature of 300 K, water becomes solid ice VI at about 1 GPa. This kind of pressure would be encountered at a depth of about 100 km (actually a bit less because of the ...

33

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 :)

32

@knzhou supplied a good answer. I’m going to offer a couple of other interpretations. The first has nothing to do with the fact that you’re mixing liquids—it’s just that there are difficulties in determining mass precisely by measuring weight. As already pointed out, there is the buoyancy of the air—that produces a mass error of about $-0.0013$ g/l at sea ...

32

It doesn't need to be rotten. When the egg is getting old, it evaporate water and looses mass while drying. Even if not rotten.

31

I think the real question you're asking here is: why do less-dense fluids completely stay above more dense fluids, whereas less-dense solids partly sink? If the ice covered the same area as the water below, then it would sit completely on top of the water. That's the case in a completely ice-covered pond.But because the ice does have a non-negligible weight,...

29

I want to supplement Emilio's intuitive answer discussing what would happen with some thoughts as to why what you propose in your second part cannot happen. What kind of object could fall out of the water and reach the ground before the water does? Let's assume the water is a single entity. In order for the object to accelerate faster than the water, the ...

28

The object would actually float exactly the same for both values of $g$. Let $V$ be the volume of the body, $d$ its relative density, and $V'$ be the volume inside water. Then for equilibrium of the body, $V \cdot d \cdot g=V' \cdot 1 \cdot g$ So, $V'/V$ is independent of acceleration due to gravity.

24

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 ...

23

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, ...

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