57

You are correct if your boat will only travel in a straight line. In real life the motion of the boat will often have a yaw angle, so that it is moving slightly "sideways" relative to the water. For example it is impossible to make a turn and avoid this situation. If the front is too sharp, the result will be that the flow can not "get round ...


57

Certainly, the mechanism is called cavitation and it works like this: As an object moves through water, a pressure distribution gets built up around the walls of the object and depending on its shape, it is possible for pressures at some points to be lower than ambient pressure- the best example of this is on the backwards face of a rotating propeller blade. ...


54

Suppose your cup was full of a lump of stuff that had no friction. Then as you rotate your cup there is no force between the cup and the stuff, and so it would not rotate. In the case of coffee - the friction is from the viscosity of the coffee. But, as your cup pulls some of the coffee around with it right at the edge, the rest of the coffee slips on that ...


43

What you are describing is called a tidal bore. Quoted from Wikipedia - Tidal bore - Description: Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than 6 meters (20 ft) between high and low tide) and where incoming tides are funneled into a shallow, narrowing river or lake via a broad bay. The ...


38

That simplified form of continuity equation assumes that the fluid is incompressible. That is only a valid assumption at low Mach numbers. I think a typical “rule of thumb” is that a Mach number less than 0.3 is required for the assumption to hold. So for the continuity equation to hold in that form requires a speed which is much less than the speed of sound ...


31

There are three (that I can think of) ways for water to supply an upwards force on something. Buoyancy Surface tension "Thrust" by pushing off of the water Buoyancy is out. The buoyant force is equal to the weight of the water displaced by the object. Since we are looking at moving across the water's surface, this means that not much water is ...


30

The sand particles interact on a macroscopic level different from water. The edges can lock together and more efficiently distribute force. Water molecules, being part of a liquid, do not distribute force this way. They tend to move out of the way instead. Mr Wizard has a great demonstration of this, using a plunger and salt rather than bullets and sand. ...


29

Any speculation about what shape might be best is meaningless without specifying the flow conditions. For the keel on a boat, the main one is the Reynolds Number, a parameter that is proportional to the the length multiplied by the speed. In most low-speed applications, a sharp leading-edge is not the best. With any incidence, the flow will tend to separate ...


26

You can check your hypothesis by closing the mouth of a pipe (from which water is flowing out) to different degrees. You will find that the flow velocity increases initially as the outlet area is decreased, reaches a maximum and then falls off to zero as you proceed to completely close the outlet. This is because, when the outlet area is very small, viscous ...


25

The phenomenon which you describe is known as a tidal bore. To understand it, one has to go beyond the strictly idealized linear theory for waves as taught in most courses of physics and consider waves that reach to a finite depth under the water surface. This is because the tug of the Moon on the water affects the entire ocean, from the surface to its very ...


25

The low viscosity of the coffee means that you can rotate the cup without significantly moving the liquid it contains: there's simply not enough friction to 'drag' the liquid by the cup's wall. It would be a different picture with a viscous liquid like oil or runny honey. It's useful to remind us what Newtonian viscosity $\mu$ really is. (Source) For $\mu \...


25

The soap bubbles are a side-effect of the cleaning process. It is the mixing of air with the soapy water, and the film stability of the resulting bubble walls, that generates and maintains the bubbles. (Note that soap bubble liquid contains glycerine, which is a powerful film stabilizer that makes the bubbles last as long as possible). Note also that it is ...


24

that's because the objective is not to blow air into the flute, it's to excite a resonant mode in the air contained within the flute tube. When you blow across the flute opening, you are producing a burst of white noise at the flute opening, and the flute then filters out the frequencies (and overtones) at which it is resonant with that particular set of ...


23

I just can't wrap my head around why pressure decreases as velocity increases This is a classic misunderstanding of Bernoulli's equation. What Bernoulli's equation actually says is that the velocity will increase in the direction of decreasing pressure: $P_2-P_1=-\frac12\rho(v_2^2-v_1^2)$. This makes sense: if the pressure is higher on the left than on the ...


22

You are right. From continuity of the incompressible fluid you have $$A_1 v_1 = A_2 v_2.$$ So obviously the velocity is changing. Thus the fluid is accelerated, and therefore there must be a force causing this acceleration. In this case the force comes from the pressure difference between the wide and the narrow part of the pipe. (image from ResearchGate - ...


18

I’d like to answer by expanding the analogy made by @Charlie. A theory of everything would be like knowing the rules of chess. We could understand all the rules, the pieces, and their moves and interactions. But there would remain many deep mathematical problems: e.g., what’s the perfect strategy in chess? It seems unlikely that this will be solved in our ...


18

Yes this is theoretically possible, and it is also observed in practice. Gases, including oxygen and carbon dioxide certainly, and probably also nitrogen, are absorbed in solution by water from the air (if oxygen were not absorbed fish would not be able to breathe underwater). Once in solution, the distribution of dissolved particles diffuses naturally ...


16

By conservation of volume, you must have the same flow rate at the surface as the flow rate through the tube/exit. If the tank is at least 10 times the diameter then it has at least 100 times the area. That means the exit velocity is at least 100 times faster and the velocity term in the bernoulli equation is 10000 times bigger at the exit than the velocity ...


16

Of course the answers like 'it is because of low viscosity' are good, but it is also nice to overcomplicate this problem. You are not rotating the cup The cookie does rotate/move in some sense but only slightly. The reason that the cookie is not rotating a lot is that you are actually not rotating the cup. Instead, you are giving the cup a short twist, which ...


15

Is this possible; due to some effect of turbulence? Sure. I'd consider that video plausible, at the very least. Can the "suction force" due to a moving vehicle propel an object, faster than the speed of the vehicle itself? If so, from where does the extra kinetic energy of the box come from? First, keep in mind that the box already has enough ...


14

I did some calculations based on the this article referenced earlier. (If you still can't open the link in Firefox, it's a bug) The main concern appears to be the required power output. To stay above the water, you need, roughly $ m g=1/3\rho A v^2 $ where $A$ is the area of your foot step, $v$ is the speed of your foot kicking the water, $\rho$ is water ...


13

Yes, some human sized animals can run for a few seconds over water - but not humans nor moose. As other answers pointed out, the animal would need fins and a large power output - that is, large muscles arranged in a way that a large part of its power can be directed to efficiently move the fins. Dolphins have both, and dolphin running over water (that is ...


13

I think #1 was answered very well by https://physics.stackexchange.com/a/610277/110549 so to answer #2: Is there any plausible reason why a soap with foam can do better cleaning than the same soap without any foam? Time spent rubbing hands. Foam/bubbles are fun. If it takes time to produce foam then it increases the amount of time a person takes rubbing ...


12

Usually, Bernoulli's principle is derived from the conservation law of energy by realising that pressure $P$ is not only a force per area, $F/A$, but also an energy per volume, $\frac{E}{V}=\frac{F\cdot s}{A\cdot s}$. Although this derivative is perfectly fine, I agree that it does not build any intuition. Therefore, I'd like to take a different approach: ...


11

First of all, let's analyse the effect of the shaft in a particular circular cross-section. When the shaft of the motor rotates, it causes the water surrounding it to revolve around the shaft in a circular path. The flow rate of water near the shaft is more than that away from it. This is a consequence of viscosity of water. According to the Bernoulli's ...


11

That is one version of the continuity equation for sure. But that is not the most general one. As a matter of fact, that condition can be deduced from the incompressibility of a fluid as in a limit of the Navier-Stokes equation: $$\rho(\partial_t + v \cdot \nabla)v + \nabla p = 0,$$ where $\rho$ is the density, $p$ is the pressure and $v$ is the velocity ...


10

The fluid has to speed up as it enters the narrower region. That means that the bit of fluid just entering the region has to be being pushed from behind. So the pressure behind it must be larger than in front.


10

i just can't wrap my head around why pressure decreases as velocity increases.. When velocity increases then you obviously have acceleration. Now what is causing this acceleration? As always (according to Newton's second law, $\vec{F}=m\vec{a}$) acceleration is caused by a force. In this case the force acting on a piece of fluid comes from the pressure ...


10

Molecules of soap are composed by one hydrophobic and one hydrophile end. They clean because the hydrophobic end sticks to dirt stuff that is normally greasy, while the hydrophile end allows the product (soap + dirt) be washed up with water. When a thin layer of water has 2 layers of soap molecules, (one at each side), it is possible for all the hydrophile ...


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