# Tag Info

### 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 ...
• 6,464
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 ...
• 782
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 ...
• 74.1k
Accepted

### 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 ...
• 74.7k

### 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 ...
• 41.4k

### 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 ...
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 $E$ ...
• 51.4k

### 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

### 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
Accepted

### Does A Pivot Exert A Force

Imagine the rod has no pivot and the rod is free to translate or rotate naturally of the perfectly frictionless surface. Lets say after the collision the end of the rod where the impact occurred moves ...
• 6,464
1 vote

### Does A Pivot Exert A Force

Before the collision, the ball moves to the right, as per the arrow. Assume it has speed v0. After the collision, the ball could have various different velocities but if the stick has inertia, they ...
• 111
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 ...
• 54.7k
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 ...
• 35.8k
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 ...
• 51.6k

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