Tagged Questions
38
votes
9answers
3k views
What makes running so much less energy-efficient than bicycling?
Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why?
According to the law of conservation of energy, bicycling should be more intensive because you have to ...
2
votes
2answers
112 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 $10 \text{m/s}$. ...
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 ...
2
votes
1answer
144 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 ...
2
votes
1answer
134 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
...
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
123 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 ...
1
vote
2answers
142 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 ...
5
votes
3answers
371 views
Kinetic energy with respect to different reference frames
I'm having problems understanding the following situation. Suppose two 1-tonne cars are going with the same orientations but opposite senses, each 50 km/h with respect to the road. Then the total ...
7
votes
3answers
551 views
Is the normal force a conservative force?
Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative?
For example, consider ...
3
votes
3answers
196 views
Potential energy in $E_f^2=(mc^2)^2+(pc)^2$?
Let's consider
$$E_f^2=(mc^2)^2+(pc)^2$$
where the $mc^2$ is the rest energy due to the rest mass -- in Finnish "lepomassa".
$$ \sqrt{(mc^2)^2+(pc)^2} - mc^2~=~(\gamma-1)mc^2$$
is the kinetic ...
0
votes
2answers
106 views
Elastic potential
I have a doubt:
elastic potential energy is given by: $U=\frac{k}{2}x^2+K$
but does elastic potential exist? (for example: potential gravitational energy is given by $U=mgz+K$ and gravitational ...
10
votes
2answers
287 views
How does rest mass become energy?
I know that there's a difference between relativistic rest mass. Relativistic mass is "acquired" when an object is moving at speeds comparable to the speed of light.Rest mass is the inherent mass that ...
4
votes
2answers
979 views
How efficient is the human body?
This question sort of comes to mind when hearing how efficient an internal combustion engine is turning chemical energy in mechanical energy (something like 20-40%) with lots of excess heat. As an ...
0
votes
1answer
58 views
Not working brakes: just another energy conservation problem
A car is driving down a mountain ($v=90 km/h=25 m/s$, when the driver realizes that brakes aren't working. He try to lose velocity going up an inclined ($20°$) plane, with a friction coefficient of ...
0
votes
2answers
92 views
Energy problem: What's wrong here?
A car ($m=540\,\text{kg}$) engine, has a power of $60\,\text{kW}$. The static friction coefficient between wheels and road is $k=0.6$. How long does it take to reach the speed of ...
4
votes
3answers
329 views
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.
...
0
votes
1answer
211 views
How should I apply conservation of energy to this problem?
A block rests on a table. Then the block is pushed by a spring and slides across the table until it falls to the ground. The mass of the block is $1.30\text{ kg}$, the spring constant $550\text{ ...
0
votes
0answers
50 views
Energy can't be created or destoryed? [duplicate]
Possible Duplicate:
Conservation law of energy and Big Bang?
If energy can't be created or destroyed then how did the big bang create energy? and if energy can't be created then does that ...
6
votes
3answers
447 views
Is the total energy of the universe constant?
If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
1
vote
5answers
810 views
Is the law of conservation of energy still valid?
Is the law of conservation of energy still valid or have there been experiments showing that energy could be created or lost?
2
votes
1answer
161 views
Would the Moon drift away from the Earth due to extraction of tidal energy? [duplicate]
Possible Duplicate:
Why does the moon drift away from earth?
It seems to me that, due to conservation of energy, the moon would drift away from the Earth if humans began extracting large ...
6
votes
4answers
156 views
Is energy extensivity necessary in thermodynamics?
Given a partition of a system into two smaller systems, the energy $U$ is devided into $U_1$ and $U_2$, with
$$U=\mathcal{P}(U_1,U_2):=U_1+U_2,$$
so that $U_2$ is given by $U-U_1$. Here the ...
3
votes
2answers
1k views
Are there devices which convert thermal energy to electric energy?
Converting optical energy to electric energy is a huge business based on the photovoltaic effect. Is there an analogous effect for phonons? Are ther devices which convert phonon energy to electric ...
2
votes
1answer
134 views
Why don't cancelling wavefunctions for two different particles give zero total wavefunction?
Let $\left|a\right>=e^{i(kx-\omega t)}$， $\left|b\right>=-e^{i(kx-\omega t)}$ be two neutral particles in the 1D free space without any interaction. Then ...
2
votes
1answer
147 views
Man on treadmill and energy/mass conservation
Suppose we put a very obese man inside a large air tight isolated room with a treadmill and he runs on the treadmill for 24 hours and at the end of the day he sees his weight has come down (mostly ...
0
votes
2answers
99 views
Calculating kinetic energy?
Would this be a valid equation to calculate kinetic energy created from a drop from a height:
$$E_{kinetic} ~=~ v_{vertical}tmg$$
Velocity multiplied by time gives distance. Distance multiplied by ...
1
vote
1answer
119 views
Is energy “destroyed” when walking?
Conservation of energy states energy can't be destroyed, but isn't energy used up when walking in a straight line? If your not walking up a slope, kinetic energy isn't converted to gravitational ...
2
votes
1answer
352 views
Conservation of Energy in Special Relativity
In classical Newtonian mechanics, from what I understand, conservation of energy stems from the fact that all known forces are conservative forces,
and vector calculus tells us that they can be ...
1
vote
2answers
117 views
Matter - Antimatter Reactory Practicality
With current technology, would the energy released by a matter-antimatter annihilation be more than the energy needed to created the antimatter in the first place? Would it be worth it? Just curious, ...
0
votes
2answers
1k views
Rotational kinetic energy during vertical circular motion of a particle
Why is it not necessary to take into account rotational kinetic energy when using the Law of Conservation of Mechanical Energy to solve vertical circular motion problems? After all, the particle is ...
24
votes
10answers
3k views
What happens to the energy when waves perfectly cancel each other?
What happens to the energy when waves perfectly cancel each other (destructive interference)? It appears that the energy "disappear" but the law of conservation of energy states that it can't be ...
0
votes
1answer
283 views
How did the inflaton field “add” energy to the universe?
How did inflation add energy to the universe? What mechanism did this occur by? In other words, where did that energy come from? Was it due to the quantum fluctuation (or that scalar field rolling ...
4
votes
3answers
339 views
Conservation of Energy and Momentum Regarding Forces - clarification needed
The other day, my teacher stated something along the lines of, "Conservation of momentum is not violated by the actions of internal forces, but the conservation of energy is violated. Energy is ...
-4
votes
1answer
182 views
Tricky Conservation of Momentum problem: find the ratio of the carts by mass percentage lost [closed]
A wagon is coasting at a speed $v_A$ along a straight and level road. When 42.5% of the wagon's mass is thrown off the wagon, parallel to the ground and in the forward direction, the wagon is brought ...
6
votes
3answers
755 views
On what basis do we trust Conservation of Energy?
I'm happy to accept and use conservation of energy when I'm solving problems at Uni, but I'm curious about it to. For all of my adult life, and most of my childhood I've been told this law must hold ...
6
votes
2answers
310 views
Have red shifted photons lost energy and where did it go?
I think the title says it. Did expansion of the universe steal the energy somehow?
0
votes
3answers
144 views
Equation $H(q,p)=E$ is the equation of motion or energy-conservation law?
I do not completely understand, why do we consider Hamilton–Jacobi equation $H(q,p)=E$ as equation of motion, whereas it is looks like energy-conservation law?
1
vote
1answer
113 views
Will a photon emitted from something moving quickly have a shorter wavelength?
If a photon is emitted from a light source moving at any speed, the photon will nonetheless always move at c (assuming it is emitted in a vacuum.)
If the speed of a photon's emitter cannot influence ...
3
votes
2answers
909 views
Conservation of Momentum/Energy collision Problem
I'm working on a physics problem in preparation for the MCAT and there's this particular problem that's troubling me. I don't know if it's a bad question or if I'm not understanding some sort of ...
2
votes
3answers
82 views
Certain material heating water in a recipient
I don't know how to resolve a problem, but I don't want the answer since I'm almost going to have it resolved.
What the problem says is we have 85 liters of water at 7ºC in an iron pot of 29kg. We ...
12
votes
2answers
4k views
Why can't energy be created or destroyed?
My physics instructor told the class, when lecturing about energy, and that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
2
votes
1answer
211 views
Thermal energy generated due to loss in kinetic energy when observed from two different frames of reference
A body is moving with a velocity $v$ with respect to a frame of reference $S_1$.
It bumps into a very heavy object and comes to rest instantaneously, its kinetic energy
$$\frac{1}{2}mv^2$$
as ...
2
votes
4answers
2k views
Where do electrons get their ever-lasting circulating energy?
We all know (or maybe know) that to move, we need to spend energy. If you want to drive a car, you gotta spend gasoline.
We also know that energy can't be created (first law of thermodynamics, and ...
2
votes
1answer
180 views
What “I” should use in Rotational Energy formula $(I \omega^2)/2$
$\text{Rotational Energy} = \frac{1}{2} I \omega^2$. What $I$ should be used?
$I$ as a inertia tensor matrix = stepRotation * inverse moment of inertia * inverse stepRotation;
Or I as moment of ...
2
votes
3answers
224 views
Is there a measure of internal energy flow?
A system might have internal energy and/or kinetic energy. Kinetic energy in classical mechanics is a form of energy the object has, only because of its relative movement to other objects.
If you ...
2
votes
0answers
374 views
Bungee jump physics
Question:
A bungee jumper jumps from a bridge. The length of the loose rope is 30 m. When the jumper reach the lowest point possible, the rope stretches 10 m. What is the final stretch of the rope, ...
4
votes
3answers
2k views
Force as gradient of scalar potential energy
My text book reads
If a particle is acted upon by the forces which are conservative; that is, if the forces are derivable from a scalar potential energy function in manner $ F=-\nabla V $.
I ...
2
votes
2answers
288 views
6
votes
2answers
325 views
What is changing in latest light bulb technologies?
I'm confused with the latest home lightning bulbs.
Understanding filament bulbs was easy. For example take 220V, 100W filament bulb:
Power = $V^2/R$ Filament gets heated and emits energy in the form ...
