101 votes

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 ...
niels nielsen's user avatar
92 votes

Why does a system try to minimize its total energy?

The anthropomorphic formulation "tries to" is misleading. Under the effect of ambient noise, matter explores the possible configurations around its current state: e.g., two single hydrogen atoms ...
Joce's user avatar
  • 3,335
78 votes

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 ...
Agnius Vasiliauskas's user avatar
46 votes

Why do most office chairs have 5 wheels?

Consolidating some of the points made in the answers to the question you linked, and comments: When constructing a chair, 4 legs is easy when you use traditional (wooden) construction - 90 degree ...
Floris's user avatar
  • 118k
44 votes

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 ...
The Photon's user avatar
  • 26.5k
35 votes

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 ...
Andrew Steane's user avatar
29 votes

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 ...
ACuriousMind's user avatar
  • 122k
28 votes

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 ...
Roger V.'s user avatar
  • 57.5k
26 votes

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 ...
knzhou's user avatar
  • 101k
26 votes

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 ...
Steeven's user avatar
  • 50.3k
25 votes

Why does a system try to minimize its total energy?

This is a consequence of the second law of thermodynamics, which states that In a closed system with fixed internal energy (i.e. an isolated system), entropy is maximized at equilibrium. It can ...
valerio's user avatar
  • 16.1k
21 votes

Why doesn't an object, despite having a non-zero potential energy stored in it, fall by itself from the elevation?

It is wrong to think potential energy is stored in the object. The earth pulls the object down, but the object pulls the earth up. They share the potential energy. The object fails to fall down ...
Timaeus's user avatar
  • 25.4k
21 votes

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 ...
FiatLux's user avatar
  • 336
20 votes

Why does a system try to minimize its total energy?

This is really a statistical effect, as pretty much all of thermodynamics. You have two free hydrogen atoms. They tend to move around the space they have, and when conditions are favourable (there's ...
Luaan's user avatar
  • 6,296
19 votes

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 ...
my2cts's user avatar
  • 22.8k
17 votes

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 ...
Chemomechanics's user avatar
15 votes

Why does a system try to minimize its total energy?

I'm going to take a slightly different approach and say it's because we defined energy to make it so. In other words, systems "try" to find the lowest energy state because energy is a concept humans ...
thomij's user avatar
  • 1,552
15 votes

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 ...
tparker's user avatar
  • 46.6k
15 votes

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 ...
joseph h's user avatar
  • 29k
14 votes

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 ...
Phillip Petty's user avatar
13 votes

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) ...
gandalf61's user avatar
  • 47.4k
12 votes

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 ...
Qmechanic's user avatar
  • 196k
12 votes

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 ...
knzhou's user avatar
  • 101k
11 votes

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 ...
lvella's user avatar
  • 949
11 votes

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)...
Gilbert's user avatar
  • 11.7k
11 votes

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 ...
Chemomechanics's user avatar
10 votes

Does zero net force imply zero net torque?

does it also leads to ... No, it doesn't. A simple Counterexample: Consider the figure below (the bar $\textrm {AB}$ and forces $F$ are on a plane parallel to $\textrm {xy}$ plane) We have $\Sigma ...
lucas's user avatar
  • 3,141

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