101
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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 ...
- 90.1k
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 ...
- 3,335
78
votes
Accepted
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 ...
- 11.5k
46
votes
Accepted
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 ...
- 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 ...
- 26.5k
35
votes
Accepted
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 ...
- 56.6k
29
votes
Accepted
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 ...
- 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 ...
- 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 ...
- 101k
26
votes
Accepted
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 ...
- 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 ...
- 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 ...
- 25.4k
21
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Accepted
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 ...
- 336
20
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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 ...
- 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 ...
- 22.8k
17
votes
Accepted
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 ...
- 23.8k
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 ...
- 1,552
15
votes
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 ...
- 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 ...
- 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 ...
- 168
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) ...
- 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 ...
- 196k
12
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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 ...
- 101k
11
votes
Accepted
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 ...
- 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)...
- 11.7k
11
votes
Accepted
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 ...
- 23.8k
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 ...
- 3,141
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