10
votes
Accepted
Is the work I do on the object always equal in magnitude but opposite in sign to the work the object does on me?
I think there is a difficulty in answering this question because it is not always possible to locate energy in one body or the other when they are interacting. Some of the energy (and it might even be ...
- 59.6k
9
votes
Does life erode Earth's mass over time?
Remember that nearly all of life's energy ultimately comes from the sun. Life uses energy that is received from the sun, "mixes" it with matter from the earth, and eventually that energy ...
- 11.1k
8
votes
Does a rocket moving in a circle expel exhaust at a greater velocity?
This is an interesting question, because when it is moving in a circle, the magnitude of its tangential velocity is constant and its angular velocity is also constant. Therefore the total kinetic ...
- 6,102
7
votes
Accepted
Where does the energy in a fundamental interaction come from?
In your electron example, there are two problems:
Although independently both electrons gain energy, you would never find two electrons close to one another to begin with, because such repulsion ...
- 689
7
votes
Is the work I do on the object always equal in magnitude but opposite in sign to the work the object does on me?
A mass $m$ which starts from rest has constant force $F$ acting on it for a time $\Delta t$.
The change in position of the mass is $\frac 12 \frac F m (\Delta t)^2$ and this can be used to determine ...
- 97.9k
7
votes
Accepted
Spring potential energy, conversion
The problem in your book assumes an ideal (lossless) and massless spring. That is typical of entry level physics problems.
Without the ball you would need to consider, at a minimum, that the spring ...
- 73.7k
5
votes
Does a rocket moving in a circle expel exhaust at a greater velocity?
The energy of the fuel does become the kinetic energy of the exhaust.
Let us ignore the fact that the rocket becomes lighter as it burns fuel, and that it will eventually run out of fuel. The rocket ...
- 41.1k
5
votes
Spring potential energy, conversion
In the case where there is no ball (and assuming there is no friction/other dissipative forces), the spring will expand beyond its natural length due to the momentum it gains while unstretching. At ...
- 936
5
votes
Is the work I do on the object always equal in magnitude but opposite in sign to the work the object does on me?
Newton's third law holds
If we deal with a system that can be described by the laws of classical mechanics, third law is a principle of such a theory and thus it holds. So, to answer your question
...
- 10.7k
5
votes
Validity of $\mbox{d}H/\mbox{d}t=\partial H/\partial t$ for dissipative systems
In principle you can use Hamiltonian mechanics also for dissipative systems, or fon systems which cannot be completely described by a Lagrangian.
When a Lagrangian is provided, the Legendre ...
- 73.8k
4
votes
Does life erode Earth's mass over time?
Good question! No, life does not reduce the mass of Earth. Life is cycling elements such as C, O, N and H. It is fuelled nearly completely by solar energy. This is converted to heat, which at the end ...
- 25.3k
4
votes
Is the work I do on the object always equal in magnitude but opposite in sign to the work the object does on me?
In other words, are the 'work I do on the object' and the 'work the
object does on me' equal in magnitude but opposite in sign?
Sometimes yes, sometimes no.
For the work to be equal and opposite the ...
- 73.7k
2
votes
Accepted
Do off-centre forces create additional energy?
If the line of action of the force is not through the centre of mass then body can be thought of as being acted on by a force whose line of action is through the centre of mass which produces a linear ...
- 97.9k
2
votes
Accepted
"Friction at a contact point", rubber-band experiment
Yes, there will be some heat produced as a result of friction between the rubber band and the board, and also some heat produced as a result of internal friction within the rubber band itself, as it ...
- 56.4k
2
votes
Is it possible there can be a non-Fourier model of string vibration? Is there an exact solution?
A vibrating string can support a very large number of vibrational modes simultaneously. This is because waves on strings superimpose linearly. Note here that an electric guitar string struck hard with ...
- 95.5k
2
votes
Accepted
Strogatz's condition on definition of energy
If $E$ does not depend on $x(t)$, then $\frac{dE}{dt}=0$ doesn't give you any information about $x(t)$. Since the entire goal of classical mechanics is to solve for $x(t)$, this means such an $E$ ...
- 51.2k
1
vote
Why particle is certain to escape the conservative field when the mechanical energy is positive?
From the comments:
By @naturallyInconsistent
For convenience, we chose to put the conventional dividing line for this thing so that zero is the limiting case. i.e. it is a choice, a very natural ...
1
vote
Energy and momentum conservation for photon absorption
As you point out, not all of the energy of the photon goes into exciting the electron, Similarly when stationary atom emits a photon, some of the electon level enenrgy goes into the recoil kinetic ...
- 54.5k
1
vote
Validity of $\mbox{d}H/\mbox{d}t=\partial H/\partial t$ for dissipative systems
I'll split my answer in some paragraphs: 1. Newton's mechanics in strong and weak forms; 2. Lagrangian mechanics; 3. Hamiltonian mechanics.
1.a Newton's mechanics: strong form. Newton's second ...
- 10.7k
1
vote
Does a rocket moving in a circle expel exhaust at a greater velocity?
Energy is frame dependent.
In a inertial frame that is momentarily comoving with the rocket, its kinetic energy is zero, and the kinetic energy of the gas is the same for both cases, if we suppose the ...
- 16.7k
1
vote
Energy Transfer and Work in Action-Reaction Forces
Because doing work on something doesn't make you gain kinetic energy.
Now I have said this multiple times on this site, and will continue:
Bringing the human body into physics never, ever, helps. It ...
- 35.4k
1
vote
Do off-centre forces create additional energy?
There is a standard demonstration in which an off-centre force is provided by a string wrapped around a cylinder on a air-table. Another string is connected to the c-of-mass of an otherwise ...
- 54.5k
1
vote
Is the work I do on the object always equal in magnitude but opposite in sign to the work the object does on me?
Does Newton's third law also hold in the work-energy theorem? In other words, are the 'work I do on the object' and the 'work the object does on me' equal in magnitude but opposite in sign?
Sometimes ...
- 39k
1
vote
Does a small domino toppling a large domino violate conservation of energy?
There is no paradox at all. With a small activation energy it is possible to initiate a process that releases a lot of energy that was stored in a metastable state.
Combustion is an example: with a ...
- 5,078
1
vote
What are the forces that do work when a spring between two masses pulls inwards?
There is no kinetic energy change in the second system (1,2, spring). So there is no net work done, so no need to ask which external forces that do the work.
The constituent particles of the system do ...
- 51.4k
Only top scored, non community-wiki answers of a minimum length are eligible