149 votes

When I walk down the stairs where does my potential energy go?

Make it simple. If a mass of your weight fell down the height of three flights of stairs through the air, when it landed where would the kinetic energy accumulated by falling go? moving the earth ...
anna v's user avatar
  • 230k
91 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 NoToPutinsWarInUkraine's user avatar
70 votes

Why can't we make a perpetual motion machine by using a magnet to pull up a piece of metal, then letting it fall back down?

Now, in my head, if you give the pendulum a little impulse, it will swing up in one direction and get attracted by the magnet just a tiny bit. You've neglected to account for the magnetic attraction ...
Emilio Pisanty's user avatar
55 votes
Accepted

When I walk down the stairs where does my potential energy go?

The heat is predominantly generated in your muscles. More direct conversion of potential energy to heat is when a person is sliding down a pole to get to a lower floor quickly. With sufficient ...
Cleonis's user avatar
  • 18.4k
37 votes

Will tsunami waves travel forever if there was no land?

The waves will not travel forever. Water particles moving against and around each other will have friction, and the friction will cause motion energy to be converted to heat (which will dissipate ...
Michael Teter's user avatar
32 votes
Accepted

What exactly makes a force conservative?

All fundamental forces are conservatives and I would say that this is a postulate. Fundamental physics is constructed in such way that there is a quantity called energy which can be assigned to every ...
Diracology's user avatar
28 votes
Accepted

How do non-conservative forces affect Lagrange equations?

More generally, Lagrange equations$^1$ read $$\begin{align} \frac{d}{dt}\frac{\partial (T-U)}{\partial \dot{q}^j}-\frac{\partial (T-U)}{\partial q^j}~=~&Q_j-\frac{\partial{\cal F}}{\partial\dot{q}^...
Qmechanic's user avatar
  • 187k
28 votes

What is the electrical difference between rolled and unrolled extension cables?

The reason it is better to unroll the cable is because it improves its ability to dissipate heat, which could be important for heavy loads, i.e., when the cable could potentially become hot. The role ...
V.F.'s user avatar
  • 12.1k
28 votes
Accepted

Is there a fundamental reason for the exponential dependence of the evolution of the temperature in an electronic deviced that is powered on?

This behavior is basically described by Newton cooling with heat generation, using the equation: $$MC\frac{dT}{dt}=G-k(T-T_{\infty})$$where T is the temperature, t is time, M is the mass, C is the ...
Chet Miller's user avatar
  • 31.3k
26 votes
Accepted

How do resistors generate different heat if we make the current fixed and changed the voltage and resistance? Notice the flow of charge is constant

Your initial circuit is like this: So you get 2A flowing and a power of 20W dissipated in the resistor. Then you double the voltage and the resistance: The two batteries add up to single source of ...
Oscar Bravo's user avatar
  • 4,377
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
  • 15.9k
25 votes

Why does critical damping return to equilibrium faster than overdamping?

There are good mathematical answers to this question. I'd like to give an intuitive one. Consider the following situation: a sphere falling through a viscous fluid: Equilibrium is when the sphere ...
Whiskeyjack's user avatar
24 votes
Accepted

Does fluid speed affect liquid cooling?

What I fail to see is how moving too quickly could also impair cooling performance as stated in a lot of online forums. One argument I clearly remember from reading about this a while back was: and: ...
Gert's user avatar
  • 34.9k
24 votes
Accepted

Can a water vortex (whirlpool) be used to efficiently store energy?

Yes, this would be horribly inefficient compared to pumped hydro or even a regular flywheel. With a rotating fluid, there's a lot of viscosity. This viscosity generates heat and slows the fluid down....
JMac's user avatar
  • 15.1k
22 votes

Will tsunami waves travel forever if there was no land?

To answer this, I would appeal to the general principle which we call the 2nd law of thermodynamics. One way of expressing it is that the entropy of an isolated system cannot decrease. This means that ...
Andrew Steane's user avatar
19 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
  • 6,127
19 votes

Does a fan rotating with a uniform angular velocity consume electrical energy?

when rotating at constant speed, the fan disc is continuously performing work on the air drawn through it by imparting momentum to it. To perform that work requires a constant input of energy from the ...
niels nielsen's user avatar
17 votes

What happens to half of the energy in a circuit with a capacitor?

At the moment the circuit is completed, the capacitor has zero voltage, while the supply has $V$. This voltage difference creates an electric field that accelerates charges. This acceleration sets ...
BowlOfRed's user avatar
  • 37.2k
16 votes

What exactly makes a force conservative?

As you know, energy is always conserved. When we talk of a force being non-conservative, it means that the force is operating within a system from which energy is allowed to escape. Perhaps the most ...
Dancrumb's user avatar
  • 1,016
16 votes
Accepted

What happens to the energy in an ocean wave after it breaks on a reef or the shore?

This is an interesting article with some numbers for the energy in waves A wave with a height of 2 m and a wavelength of 14 m breaking along 2 km of coastline (surface area = 32,000 m2) has ...
anna v's user avatar
  • 230k
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
  • 1,542
15 votes
Accepted

Calculating the total time elapsed until two pendulums "stop colliding" gives a divergent result

Your calculation is perfectly correct, under the standard idealizations in mechanics. From a mathematical point of view this isn't that surprising; divergent times are pretty common. For instance, ...
knzhou's user avatar
  • 98.6k
15 votes
Accepted

Would it be possible to hop on top of a bouncing ball?

Yes it is possible in principle, though the bear will have to do a lot work to keep the ball moving. Suppose the speed of the ball at its highest point when it hits the bear is $v$ and the mass of the ...
John Rennie's user avatar
15 votes

When I walk down the stairs where does my potential energy go?

Yes, once you are standing still at the bottom of the stairs then all of the potential energy you had at the top of the stairs has been transformed into heat (due to friction and heat produced by your ...
gandalf61's user avatar
  • 42.1k
14 votes

How much energy is wasted by a noisy refrigerator?

The answer by niels nielson is much more useful than my answer. But just in case you really do want a rough estimate of how much power is emitted as sound... According to [1] (references are listed ...
Chiral Anomaly's user avatar
14 votes
Accepted

Shouldn't resistance only affect the kinetic energy of an electron?

Your analogy is faulty. Think instead of the falling ball as having already reached a constant terminal velocity because of air drag when its gravitational potential energy is 10 J. Then since it’s ...
Bob D's user avatar
  • 64.4k
14 votes

Why does critical damping return to equilibrium faster than overdamping?

to compare the heavy damping path and the critical damping path you have to adjust the initial conditions you obtain $$x_C(t)=\left( c_{{1}}+c_{{2}}t \right) {{\rm e}^{-\beta\,t}}$$ and $$x_H(t)=w_1\,...
Eli's user avatar
  • 10.9k
14 votes
Accepted

Playing cymbals in vacuum

When you strike cymbals (or any other object) you give them a certain amount of energy. If the object is stiff and light (like cymbals) this energy will cause the object to vibrate. The vibrations ...
gandalf61's user avatar
  • 42.1k
14 votes
Accepted

Is David Tong incorrect in this remark about classical mechanics in his QM lectures?

Indeed, I think that the statement in Tong's book is quite ambiguous (though it is not definitely false as I discuss below). In principle there is no relation between the possibility of a Hamiltonian ...
Valter Moretti's user avatar

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