Tag Info

Why do rain drops fall with a constant velocity?

Here is a slightly different way to think of this. If the net force is zero, the acceleration of the droplet is zero- even though its velocity is not zero. With the acceleration zero, the velocity ...
• 96.1k
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Why is it easier to handle a cup upside down on the finger tip?

Take a look at this picture of a cup slightly out-of-balance : In case (A), generated torque is directed out of your reference axis and in case (B) - towards your reference axis. So in case A), you ...
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Why do things cool down?

You exchange heat with the objects around you. If the objects around you are hotter than you, you'll heat up. If the objects around you are cooler than you (neglecting the heat you're generating due ...
• 28.2k
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Why (does/we assume) gas exert same pressure everywhere in a closed container?

An imbalance of pressure would itself cause an internal flow in the gas. So if the gas has reached equilibrium the pressure must be the same everywhere. The above is for a gas in ordinary ...
• 59.8k
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Is thermodynamics only applicable to systems in equilibrium?

It entirely depends on what you think "thermodynamics" is. The traditional idea of thermodynamics dealing with systems whose macrostate can be fully described by e.g. temperature, pressure and volume ...
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Why do rain drops fall with a constant velocity?

If the forces on an object add to zero (in an inertial reference frame) it experiences zero acceleration and hence moves with a constant velocity (but not necessarily zero velocity). Thus, this is a ...
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How is quantum mechanics consistent with statistical mechanics?

The question doesn't make sense, as not every system has to be in thermal equilibrium. The argument you're making works for classical mechanics too. "Consider a ball lying on the ground." "That's ...
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How can I prove that a state of equilibrium is unstable?

In the centre of a bowl there is equilibrium. Put a ping pong ball in it. If this ball is ever shaken slightly away from equilibrium, it will immediately roll back. A "shake-proof" equilibrium is ...
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Earth as a perfect sphere and an object trying to stand still

Indeed, you need friction. Otherwise this object will slide to equator just as you expect. In fact, all the objects will slide there to form equatorial bulge and eventually the sphere will be turned ...
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Why do rain drops fall with a constant velocity?

This is Newton's first law: if the force vanishes the velocity is constant. Constant but not necessarily zero. The resistive force increase as long as speed increases. When it is equal to the ...
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Why are thermodynamic potentials minimised?

You can trace energy minimization (e.g., internal energy minimization for a closed system at constant volume, enthalpy minimization for a closed system at constant pressure, Helmholtz free energy ...
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A paradox while explaining the equilibrium of books

You are correct, in a scenario with an even number of books there will be no friction acting between the pair in the middle. To see this, take the simplest case - two books. The force $F$ applied by ...
Accepted

Is ergodic hypothesis in contradiction with the notion of equilibrium?

You have to be careful to distinguish between microstates and macrostates. Thermodynamic equilibrium is a macrostate which consists of a mixture of all possible microstates of energy $E$ weighted by ...
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How are curved soap films stable?

Note that in your diagram you take a 2D slice in, let's say, the $xz$-plane where $z$ is the axis of symmetry and $x$ is the other axis you have chosen. This intersects the 2D soap film on a 1D line ...
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Why is it easier to handle a cup upside down on the finger tip?

Maybe because when the cup is the right way up, it’s centre-of-mass is above the point on your finger meaning that as your finger tries to balance the cup any small motion will generate a torque about ...
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Why (does/we assume) gas exert same pressure everywhere in a closed container?

It depends on the resolution of your measuring device. A gas contains on the order of $10^{22}$ molecules zipping about. The pressure on a wall of the container is due to the tiny force applied by ...

Objects falling from table

If the centre of gravity of the object is vertically above the edge of the table then the object is in equilibrium. However, this equilibrium position is unstable (like a pencil balanced on its point) ...
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How does this tensegrity table work?

Stable mechanical equilibrium broadly means that any movement would incur a net energy penalty. This is intuitive when we see a ball at rest in a dip, for example; it’s clear that any rolling would ...
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How are the Lagrange points determined?

Sketched proof of all possible Lagrange points: Consider first the 2-body problem. Deduce that possible Lagrange points must lie in the orbital plane (because a probe will always be gravitationally ...
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Facing a paradox: Earnshaw's theorem in one dimension

Your example does not contradict Earnshaw's theorem for electrostatics, because it rules out stable equilibrium in a region without charge, possibly containing fields made by charges outside that ...
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How does this tensegrity table work?

What's confusing here is probably that the lifting is dose via the tensile force of the rubber band, which due to the placement on the arms pulls down the bottom and up the top part. Mechanically, ...
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Why do things cool down?

Everything that is not 0 Kelvin radiates electromagnetic energy. In vacuum, this is the only relevant form of heat transfer. The hotter you are, the more energy you radiate (I believe the relevant ...
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Crystals and Earnshaw's theorem

As mentioned by @tippy2tina, the Pauli exclusion principle (a quantum phenomenon) is one reason, and the other is the discrete nature of electron states in a potential well (another quantum phenomenon)...
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Difference between Reversible and Irreversible processes in Physics vs. Chemistry

Is it possible that reversible physical (thermodynamic) systems are not related to reversible chemical reactions? Correct. No real process is thermodynamically reversible; entropy is generated ...
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