# Tagged Questions

Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding [tag:energy-conservation] if it is specifically about its conservation.

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### Finding Rotational Kinetic Energy Of A Clock

The problem I am working on is: Big Ben, the Parliament tower clock in London, has an hour hand 2.70 m long with a mass of 300 kg, and a minute hand 4.20 m long with a mass of 100 kg (see figure ...
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### How to get energy of collision if you know force of gravity of an object($m \rightarrow F=mg$)?

How to get energy of collision if you know force of gravity of an object($m \rightarrow F=mg$)? You get energy of collision by kinetic energy $E_k= \frac{1}{2}mv^2$, but if you use just force of ...
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### Thorium radioactivty vs Uranium radioactivty nuclear power

May i please open this question by asking that if you intended to answer this please could you provide links based on your answer. I have read ( and posted one ) on thorium and a lot of the answers ...
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### Is two cars colliding at 50mph the same as one car colliding into a wall at 100 mph?

I was watching a youtube video the other day where an economist said that he challenged his physics professor on this question back when he was in school. His professor said each scenario is the same, ...
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### What are the optimal conditions to fuel your car?

I was filling my car earlier today, and noticed a sticker posted on the pump. This pump dispenses fuel at a volumetric amount measured in standard gallons (231 cubic inches). It does not adjust ...
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### $E^2 = (mc^2)^2 + (pc)^2$: What units are used to measure $E$, $m$, $c$ and $p$?

$$E^2 = (mc^2)^2 + (pc)^2$$ If I am using this equation to figure out the energy of something, what units would I use? Would it be the metric system? I.e. kilograms ...
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### Intuitively Understanding Work and Energy

It is easy to understand the concepts of momentum and impulse. The formula $mv$ is simple, and easy to reason about. It has an obvious symmetry to it. The same cannot be said for kinetic energy, work,...
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### Does closing curtains 'make your home warmer'?

I mean, in the sense that the act of closing curtains would somehow reduce the amount of heat loss of the house to the outside, thus making it warmer for a given supply of heating.
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### Energy Question

Carl throws 100 g of Bohrium-262 (Half life: 0.10 seconds, undergoes alpha decay) to Sukhdeep with a initial horizontal velocity of 20.0 m/s at an initial height of 2.00 m above the ground. What is ...
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### How can kinetic energy be proportional to the square of velocity, when velocity is relative?

Let's start with kinetic energy (from los Wikipedias) The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a ...
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### Confused about unit of kilowatt hours

So I am a little confused on how to deal with the Kilowatt hours unit of power, I have only ever used Kilowatts and I have to design a residential fuel cell used as a backup generator for one day. ...
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### Elastic Collision Between Two Objects [duplicate]

Possible Duplicate: Elastic Collision And Momentum I've already asked this question, Elastic Collision And Momentum, but I didn't find the answer particularly helpful--sorry, I don't mean to ...
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### What is the formula for max kinetic and max potential energy of a spring?

What is the formula for max kinetic and max potential energy of a spring?
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### Activation energy and entropy

First assertion If a system is already in a high temperature, adding energy, will increment the entropy in a low amount (compared with a system in a lower temperature). Question (if assertion is ...
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### Why are we not using thorium for energy? [duplicate]

Possible Duplicate: What practical issues remain for the adoption of Thorium reactors? I have been reading and watching a bit on thorium TH 90 and cannot understand why this is not being used ...
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### A Lawnmower Problem

For my physics class we were just given an exam that asked the following: A 12Kg lawnmower is pushed with a constant force along the 30* handle with 80N. Break the Forces into its vector components....
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### Can I take heat from the air and convert it to electricity?

Its a summer day and the air in my house has been heated up. I could switch on my air conditioning, but then I'd be using energy from the grid in order to reduce the amount of energy in my house. ...
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### Positivity of Total Gravitational Energy in GR

I read the following statement in the introduction to an article: Over the last 30 years, one of the greatest achievements in classical general relativity has certainly been the proof of the ...
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### Dark Energy saving the world

Ok, my understanding of physics is still limited, hoping to change that in the following years, but I have a question. Dark energy is what provokes the expansion of the universe, so, in theory, it ...
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### Mass-Energy relation

Einstein mass- energy relation states $E=mc^2$. It means if energy of a paricle increases then mass also increases or vice-versa. My question is that what is the actual meaning of the statement "...
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### How to accurately explain evaporative cooling?

I am trying to clearly express in one or two sentences how increased evapotranspiration could cool a region. The audience is educated but non-scientific. Is it accurate to say that the water vapor ...
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### Could we use particle colliders as fusion generators?

So I know the basic gist is that fusion power's main issue is sustaining the fusion. I also know that there are two methods. The Torus method and the laser method. The torus magnetically contains ...
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### What arrangement of sound waves would be needed to heat air in a typical sized room?

From what I understand, sound is simply the jostling of the molecules that make up the air in a specific pattern, widely known as waves. I also know that these are longitudinal waves. If we were to ...
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### What are the properties and impediments of a liquid air fueled engine?

I recently came across a very interesting article that suggested the possibility of using liquified gases like air, nitrogen, or oxygen as a power source for cars. It appears that this company is ...
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### When Hamiltonian and the total energy are the same

In which condition, the Hamiltonian is the same as the total energy of the system, or say $H=T+V$?
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### What would jumping into a pool and feeling cold be called? Conduction, or convection?

This was another question from my son's workbook. It said: ...
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### What are the challenges to achieving cold fusion?

I am an absolute neophyte regarding physics. What are the challenges to achieving cold fusion? I'm not sure this is a duplicate of Why is cold fusion considered bogus?, because that question is ...
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### Kinetic Energy of Stone

If a stone is launched upward, of which is subject to gravity and air resistance, which of the following will have a greater kinetic energy? The stone at a point on its way up, or the stone at the ...
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### Maximum Kinetic energy of a spring

The block in the figure below lies on a horizontal frictionless surface and is attached to the free end of the spring, with a spring constant of 35 N/m. Initially, the spring is at its relaxed ...
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### Is the Schwarzschild black hole unphysical?

To obtain the Schwarzschild metric from Einstein equations of general relativity, we suppose that the energy density is a distribution : $$\rho (\vec{r}) = M \delta(\vec{r})$$ The Schwarzschild ...
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