# Tag Info

1 vote
Accepted

### Confusion in definition of Potential Energy

Without writing the explicit meaning of formulae, comments, and answers may only increase the original confusion. The work done by a force ${\bf F}$ when a point-like body is displaced along any path ...

### Confusion in definition of Potential Energy

Your definition of potential energy is wrong. Yes, $ΔPE_{AB}=-W_{AB}$, but $W_{AB}$ is not the work done by the conservative force from $A$ to $B$. It is the work done by an external force (against ...
Accepted

### How can power be defined as the derivative of work with respect to time if work is not a function?

My two cents. I think that's because sometimes it's possible to convert inexact differential to the exact one using integrating factors. For example, if the process is quasi-static adiabatic , then ...

### How can power be defined as the derivative of work with respect to time if work is not a function?

If so, why do I usually find power defined as the derivative of work with respect to time if work is not a function Just because work is shown as an inexact differential in the differential form of ...

### What if the point of application of force doesn't move but the force itself moves, does it count as work?

You should understand that Force is a vector quantity which has a property of 'translation', force aren't fixed in position, most of the problems that you've came across have forces that translate ...

### How to mathmematically direction of friction of spool?

We use the concepts of Impulse and Angular impulse. If the force is applied below the center of gravity in forward direction then, frictional force will act in backward direction because the contact ...

### How to mathmematically direction of friction of spool?

With friction, the bottom doesn't slide. It matches the speed of the table, which is $0$. The spool rolls. Suppose the spool was sitting on ice, so F was the only force. How would the spool accelerate?...
1 vote

### Why isn't work $Fd \sec \theta$?

I assume you mean why work is not $Fd\sec\theta$. Only work done by a force perpendicular to displacement is zero. In your case the other component is not perpendicular and contributes to work

### How can the energy of a system increase even if net work done on it is zero?

This will result in increased kinetic energy of the blocks, thus result in an increase in the total energy of the system. is a correct statement but must include an extra word, $\dots$ increase in the ...
1 vote
Accepted

### How can the energy of a system increase even if net work done on it is zero?

In $W=\vec F \cdot \vec s$ the $\vec F$ is a force acting on the system and $\vec s$ is the displacement of the material of the system where the force is acting. So in your example both forces are ...

### How can the energy of a system increase even if net work done on it is zero?

Either the pair of blocks are one thing, in which case the one thing's kinetic energy is the kinetic energy associated with the translation of the center of mass of the pair of blocks, which doesn't ...

### Can any irreversible work source be simulated by a reversible work source?

It briefly explains the reason; what the work source does is simply to apply force to the piston, and therefore it does not matter how the force is applied, whether or not it is applied by an ...
1 vote

### Why is Power in an electric circuit equal to $VI$?

Very simply, The voltage (potential difference) between two points measures how much energy is required to move a unit charge from one point to the other. And current on a path between two points is ...

### Why is Power in an electric circuit equal to $VI$?

Heuristically (without diving into full microscopic theory): Currents are made by moving charges, the power in classical dynamics is given by the $P= F v$ where $F$ is the force and $v$ is the speed ...

### Why does force perpendicular to the velocity change only its direction; not the speed?

Most answers rightly point out that a force, in order to achieve what you are stipulating (it changes the direction of the object, not its speed), must be constantly (at each instant) perpendicular. ...

### Why does force perpendicular to the velocity change only its direction; not the speed?

It's really very simple, because both force and Velocity are vectors, not scalars. That is, they have a direction and not just a value. When you exert a force on an object, the resultant acceleration (...

### Why does force perpendicular to the velocity change only its direction; not the speed?

The force perpendicular to velocity does not change the speed of an object because it only affects the direction of motion, not the magnitude of the velocity. This is a consequence of the fundamental ...

### Can any irreversible work source be simulated by a reversible work source?

It briefly explains the reason; what the work source does is simply to apply force to the piston, and therefore it does not matter how the force is applied, whether or not it is applied by an ...

### Point forces doing work

This is intended to add one more perspective which is, I believe, consistent with the answers given by @BioPhysicist and @Farcher. Clearly the energy that raises the boy up the ladder comes from the ...
Accepted

### Newton's third law in thermodynamics

For question 1, when we do a force balance on a body, we include only the forces exerted by other bodies on that body, and not forces which it exerts on other bodies. So the two action-reaction ...

### Newton's third law in thermodynamics

Newton's 3rd law only describes what I consider to be the same force that acts on two objects, not two different forces. And forces not acting on the same object can never sum or cancel each other. ...

### Is Joule equivalent to Joule/radian?

A radian is not a unit in the same sense that a second or a metre is a unit. The last two are defined using special standardising items (e.g caesium light sources) whereas the radian is defined as the ...
1 vote

### Is Joule equivalent to Joule/radian?

A radian has no physical dimension, it's a pure number. So, if you do dimensional analysis, \begin{equation} \dfrac{[torque]}{[angle]} = \dfrac{[torque]}{1} = [torque] \ . \end{equation} Recalling ...

### Why does force perpendicular to the velocity change only its direction; not the speed?

I want to come at this from a slightly different perspective. If you were writing a computer program to simulate a ball on a string but you wanted to simulate it with forces in Cartesian coordinates, ...
Accepted

### Sign convention of work

But, if work output is taken as positive, please explain why do we use a negative sign to denote the work output in the equation above when it should be positive? If work is positive (work done by ...

### Why does force perpendicular to the velocity change only its direction; not the speed?

I want to highlight something you said: Even if we assume that at the given instant , the change caused by the acceleration along it does not contribute significantly and magnitude effectively ...

### Why does force perpendicular to the velocity change only its direction; not the speed?

If we represent the movement in a 2D Cartesian plan, the velocity vector at any instant is $\mathbf v = (v_x,v_y)$. The velocity modulus is $\sqrt{v_x^2 + v_y^2}$. Now suppose that the derivative of ...