Energy is a quantity which gives an overview of the amount of work doable by the system.
0
votes
5answers
172 views
Why is momentum conserved (or rather what makes an object carry on moving infinitely)?
I know this is an incredibly simple question, but I am trying to find a very simple explanation to this other than the simple logic that energy is conserved when two items impact and bounce off each ...
2
votes
1answer
53 views
Work done by gravity on Water
Now according to me we would see change in potential energy of system and equate it to the work done by gravity.
But when we see this the first column lowers by $H/2$ and right one rises by $H/2$ ...
2
votes
2answers
109 views
Elastic collision and spring
Bodies $A$ and $B$ are moving in the same direction in a straight line with a constant velocities on a frictionless surface. The mass and the velocity of $A$ are $2 \text{kg}$ and $10 \text{m/s}$. ...
2
votes
2answers
171 views
Increase thermal efficiency of combustion engine by using heat of coolant/exhaust?
I can't be the only one who's ever thought of this, but obviously it hasn't caught on:
In terms of energy density, fossil fuels are the best thing around short of enriched uranium (and, flammability ...
1
vote
1answer
56 views
White Dwarf radius
So I've been reading this about white dwarves, and various other sites about white dwarves. In all of them, they say that we can find the radius of a white dwarf by minimizing its total energy. I know ...
4
votes
1answer
82 views
(Re-)use of a space elevator (basic mechanics and potential energy source)
It's said that if a space elevator were made then it would be much more efficient to put objects in orbit. I've always wondered about the durability of a space elevator though. I don't mean the ...
-1
votes
2answers
59 views
Changing Direction in a Vaccum [closed]
If a mass of one kilogram is traveling at one meter per second at 90 degrees, how much energy is required to get it to travel going 180 Degrees?
17
votes
3answers
603 views
What's the real fundamental definition of energy?
Some physical quantities like position, velocity, momentum and force, have precise definition even on basic textbooks, however energy is a little confusing for me. My point here is: using our ...
0
votes
1answer
39 views
hurdles in creating (close to) infinite images
Let's put an object(hypothetical superman) inside a "well sealed" box containing only mirrors. Is it possible to create number of images that will be close to infinity, assuming that resolution of our ...
0
votes
1answer
45 views
Gravitational potential energy
Consider two places next to each other: Place 1, where there is a gravitational field whereas Place 2 - there's no field.
Now if we lifted a box in place 1, it gains potential energy. Then, we move ...
0
votes
1answer
150 views
Definition of energy
What is the definition of energy $E$ given a dispersion relation $\omega=\omega(k)$ where $k=|\vec k|$ and $\omega$ is not necessarily linearly proportional to $k$? What about momentum $\vec p$?
This ...
2
votes
1answer
141 views
How to understand the work-energy theorem?
How to understand the work-energy theorem?
I took a short lecture on physics for engineering last week. The lecturer emphasized that the work done on an object will cause the kinetic energy change as ...
0
votes
1answer
69 views
Why work to change velocity from 0 to 20 km/h is less then from 20 to 40? [duplicate]
Imagine spaceship in vacuum with mass = 1. At beginning, it has velocity 0, and kinetic energy 0.
$$W_1 = 0$$
Then, it turns on its engine, and changes velocity from 0 to 20 (delta v = 20). It's ...
0
votes
2answers
69 views
Kinetic energy with no velocity
When a body is displaced against the gravitational field of force it gains potential energy. When we drop the body it begins to move downward with a certain amount of acceleration, and the potential ...
2
votes
1answer
133 views
The relation between Hamiltonian and Energy
I know Hamiltonian can be energy and be a constant of motion if and only if:
Lagrangian be time-independent,
potential be independent of velocity,
coordinate be time independent.
Otherwise
...
3
votes
4answers
276 views
Why does higher acceleration minimize a car's fuel consumption?
I generally try to optimize my car's fuel consumption when driving, using my car's real-time MPG gauge and average-trip MPG indicator.
Until recently, I believed the slower the acceleration, the ...
-1
votes
2answers
227 views
Why perpetual motion wouldn't be possible if we are so technological advanced?
Why perpetual motion wouldn't be possible if we are so technological advanced?
It is just a thing that I was wondering for too long. I mean, we are able to create so powerful permanent magnets, like ...
2
votes
1answer
116 views
Calculating engine starter’s energy use
During a discussion on start-and-stop vehicle technology some bloke began pushing the point that re-starting the car uses stored energy from the battery, which needs to be replenished by increased ...
1
vote
1answer
46 views
kinetic energy of the stone
Suppose we have a man traveling in an open car (roof open) with speed $v$ towards right (man faces right). He throws a stone (mass $m$) towards right, in his frame-forward with speed $V$.
In the ...
1
vote
2answers
77 views
Sound as a conserved quantity
From the first man to the present day men, all of them have made some sound. Sound is an energy, it can neither be created nor can it be destroyed. Therefore, every word spoken by each human that came ...
1
vote
3answers
86 views
What limits the velocity of ships such as voyager 1?
Voyager 1 travels at a small fraction of light speed. I've read it's fueled by hydrazine, which is a cheap combustive. Questions:
What factors limit the speed of voyager 1 and similar rockets? Are ...
1
vote
1answer
42 views
What happens to this energy of the capacitor?
Let's say you have a charged, isolated parallel plate capacitor with a certain voltage. My book says that if you insert a dielectric, the voltage drops. However, that would require the energy between ...
1
vote
3answers
153 views
Integration by parts to derive relativistic kinetic energy
I have come across a weird integration during derivation of relativistic kinetic energy. Our professor states that i can get RHS out of LHS using integration by parts:
$$
\int\limits_0^x \! ...
5
votes
4answers
225 views
Can you create mass with $E=mc^2$?
If you use the equation $E=mc^2$ could you make matter by dividing the $c^2$? I'm sorry if this is a really stupid sounding question or if it shouldn't be asked here.
0
votes
3answers
56 views
How is does the solar sailing concept work?
This wiki page states the following about solar sailing, "a form of spacecraft propulsion using a combination of light and high speed ejected gasses from a star to push large ultra-thin mirrors to ...
3
votes
2answers
125 views
A partial differential equation for kinetic energy
The kinetic energy of a point particle of mass $m$ and speed $v$ is $K = \frac{1}{2}mv^2$. An elementary mathematics textbook I saw asked one to show that
$$ \frac{\partial K}{\partial ...
0
votes
0answers
92 views
Calculating the Heat Losses of Convection and Radiation [closed]
A water pipe for central heating with length $L=10m$ and diameter $25 mm$ is running over a cold attic with $T_{air} = 278 K$. The radiation temperature of the surroundings is usually lower than the ...
1
vote
1answer
92 views
Can negative energy explain blackholes?
I think black holes are peculiar and I want to know if there is a relation to negative energy. Does negative energy have positive mass and therefore can explain the nature of black holes? IS there any ...
0
votes
2answers
85 views
Can there be energy with no force or energy with no power?
I think that both force (number of newtons) and power (p=ui(?)) implies that there is energy so we can't have force without energy and we can't have power without energy(?)
But can there be energy ...
1
vote
1answer
259 views
Nuclear reaction: creation of deuterium
A proton p collides with a neutron(at rest) n at relatively low-energies and creates a 'deuterium-core' d:
p+n->d+γ
Find the wavelength for the photon as a function of the proton's momentum and the ...
0
votes
0answers
19 views
Tube light chokes away from the tube
I am going to fix 150 tubelights in a 17 X 12 ft room. To avoid heat generated warming of the room can I fix the chokes outside the rooms and connect to the tubes in the room? it will be almost25 ft ...
3
votes
2answers
247 views
Where does the energy of a lightning strike go?
Lightning contains a lot of energy, so where does this energy go after lightning has hit the ground?
Does it travel all the way to the core? What happens after that?
3
votes
1answer
231 views
How much energy would it take to blow up the earth?
There is a common statement running around that we as a species has enough nuclear weapons to blow up the earth several times over. What I want to know is: by how many orders of magnitude is that a ...
0
votes
1answer
157 views
Experiment to measure initial speed of high speed tennis ball?
I want to devise a way to measure the initial speed of a tennis ball fired from a tennis ball cannon, but without using any speed-measuring devices. Just plan distance-measuring and physics formulas.
...
0
votes
1answer
122 views
Where do planets get energy to revolve around sun?
We know that every planet in our solar system revolve's around the sun in a particular orbit. But were to they get the energy to revolve around the sun. And why do they not drop into the sun there is ...
3
votes
3answers
162 views
Why is electrical energy so difficult to store?
Does anyone know a general answer to these questions? (I've asked them together because they're all pretty related, it seems.)
Why is it that we find electrical energy so difficult to store? Do we ...
0
votes
0answers
43 views
Work, energy and friction [closed]
A car loaded with bricks has a total mass of 18kg and is pulled at a constant speed by a rope. the rope is inclined 30 degrees with above the horizontal, and the cart moves 20.0 m on a horizontal ...
0
votes
1answer
50 views
How hot would a disc have to be to create strong drafts that lift it upward?
I read this article on xkcd.xom, from its spin-off blog "What If?" and that answer made me wonder.
It was mentioned in that article that a very hot but indestructible box would eventually be able to ...
2
votes
1answer
72 views
Is energy always proportional to frequency?
Google has no results found for "energy not proportional to frequency" and many results for E=hf. Is there an example of an energy that is not proportional to frequency?
0
votes
1answer
50 views
calories in ice
Food contains calories, which is nothing but a measure for energy. 1 calorie = 4.18400 joules
if I am not mistaken. Does this mean that food contains less calories when it is colder?
Obviously I'm ...
1
vote
1answer
58 views
Static Friction work and Energy [closed]
I have this problem:
A 4.0 kg block is given an initial speed of 8.0 m/s at the bottom of a 20° incline. The
frictional force that retards its motion is 15.0 N. (a) If the block is ...
14
votes
6answers
1k views
Why cannot we store light in form of light?
We can store cold (ice),heat (i.e. hot water bag) and electrical charge (batteries). We can even "store" a magnetic field in a magnete. We can convert light into energy and then, if we want, back to ...
0
votes
1answer
117 views
Work being done without distance
I have this issue:
If you push a 40.0 kg crate at a constant speed of 1.40 m/s across a horizontal floor
(µk=0.25 ), at what rate (a) is work being done on the crate by you and (b) is the
energy ...
0
votes
2answers
144 views
Could arc reactors eventually be real?
Could arc reactors eventually be real? I have wondered about this, there is not much about it online. If they could how would they work?
0
votes
0answers
26 views
How did scientists come up with Work? What was it fundamentally defined as? [duplicate]
Now before I get into the questions I want to make a couple of things clear, I know that there were similar questions like this and I've been through (what i think is) all of them and none of the ...
0
votes
1answer
51 views
Potential energy and average force? [closed]
A body of mass $15\,kg$ originally at rest $5\,m$ above the ground falls and penetrates $15\,cm$ into soft earth.
Determine the loss of potential energy
Determine the average
resistance of the ...
4
votes
3answers
143 views
If an electric car were to drive without having to stop, would the range be greatly affected by the speed at which the vehicle is moving?
Of course aerodynamics factors into this question, and the faster you are moving the more air you have to push out of your way, the more energy you use. But would the difference be only a small ...
6
votes
2answers
269 views
Where does energy go in destructive interference? [duplicate]
I have read that when two light waves interfere destructively, the energy contained within is transferred to other parts of the wave which have interfered constructively. However, I am having some ...
1
vote
2answers
137 views
Finding maximum speed in a work-energy problem
I have the following problem:
The Royal Gorge bridge over the Arkansas River is $310\text{ m}$ above
the river. A $57\text{ kg}$ bungee jumper has an elastic cord with an
unstressed length of ...
0
votes
4answers
147 views
Where do photons get their energy from?
If energy required to accelerate a particle to the speed of light is infinite then where do they get it from?
But first if photon's are massless, then why do they collide to some other thing and get ...
