Questions tagged [work]

The product of the force on an object and the displacement the object undergoes along the direction of the force.

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3 votes
5 answers
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Help me understand the derivation of the kinetic energy formula please

In my physics textbook, kinetic energy is defined as $W$Net $=$ $\int m\frac {dv}{dt}$ $dx$ This makes sense to me just fine. The book goes on to rearrange the integral to say the following: $W$Net $=...
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4 answers
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Confusion regarding work done by external force

Suppose a ball from outer space is moving towards earth. The force of gravity attracts the ball more and more strongly as it moves closer and closer to the earth. Now, let me apply an external force ...
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Orientation of gravity force in energy [closed]

My apologies in advance if this is a stupid question. A 0.20 kg rocket in a fireworks display is launched from rest and follows an erratic flight path to reach the point P, which is 29 meters above ...
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Find the external work for getting the wire from $\infty$ to the current location. (the potential in $\infty$ is $0$)

A ball of radius $R$, with charge density: $$\rho(r)=\beta r,\quad\beta > 0$$ A thin wire, with charge density: $\lambda[\frac{c}{m}]$ and length $R$, located between $x=2R$ and $x=3R$. My ...
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Relationship between work and potential energy

Ultimately what I am trying to do here is convince myself that the following relationship holds: $$F = -\frac{dU}{dx}$$ for a force $F$ and a potential function $U$. We have that the work $W$ can be ...
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What happens if I rewrite the acceleration in work formula this way?

Work is known as \begin{equation} W=\vec{F}.\vec{q}, \end{equation} thus for a gravitational force $\vec{F}_g=m\vec{g}$ we have \begin{equation} W=mgh. \end{equation} My derivation is as follows: \...
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Can magnetic forces do work on a moving, radiating charged particle?

I am not sure if this a repeat. Also, I am a high school student, so I'm saying this from what I understood of my teacher's explanation of Maxwell's law. Imagine a particle moving in a magnetic field. ...
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How to prove that work due to conservative forces are independent of path?

The Wikipedia article on conservative forces says, A force field F, defined everywhere in space (or within a simply-connected volume of space), is called a conservative force or conservative vector ...
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4 answers
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Does one "burn calories" by unloading a truck? [duplicate]

If I exercise by weightlifting, I can "burn calories". The way I understand the process is as follows: when I'm moving the weight up, the energy from chemical bonds turns into potential ...
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What work does a microwave oven do? [duplicate]

I learned that when energy is transfered it either produces work or it becomes thermal energy (heat). Work implies a force that acts on an object producing changes in its position. I'm learning these ...
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What is the work done in lifting the bucket? [closed]

A bucket is tied to a well with a chain 20 meters long to lift water. If the weight of the chain is 2 kg per meter and the weight of the bucket filled with water is 20 kg, what is the work done in ...
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1 answer
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Virtual work method in pulley [closed]

The problem illustrates two pulleys whose two ends are going downward with velocity $v$. The problem was about finding the velocity of the point $X$ i.e the point where the strings of the two pulleys ...
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Does gravitational potential energy exist?

If an object is taken at a height $h$ above the ground, its potential energy is $mgh$. When it is then released from there, it falls freely due to the force of gravity and its kinetic energy increases....
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Work done on person standing on an escalator

Say a person stands on the escalator which is going downwards. The normal force by the escalator does negative work, and the gravity does positive work. The person apparently loses some potential ...
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Work and energy in an elevator

I was reading some notes that offer the standard definition of energy, which is the ability to do work. Consider an elevator that is going up. That elevator is doing work because it is exerting force ...
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Where is energy for work performed by unpowered objects coming from?

Let's consider two simple effects for which equivalent everyday examples are easy to find: A helium balloon lifts a piece of rock from the ground into the air. A permanent magnet lifts a nail off ...
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Work done by me and Kinetic friction

What will be value of work in following cases. I move a box on a rough floor in a straight line for a distance 'd' from A to B then B to A back. 1- Is work done by me= 0? as displacement=0 (is that ...
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What is meaning of work done without doing net work on object gets stored as potential energy? There are similar questions but some doubts are still

In a conservative system positive Work done by external force or negative work done by conservative force without accelerating or giving Kinetic energy to the object, gets stored as potential energy ...
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1 answer
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Magnetic force is a no work force. Why is it doing work here?

Here the rod is projected with a velocity $v_0$ . Due to change in flux in the loop, we have an induced emf in the loop, which induces a current $i$ that exerts a force $Bil$ where $B$ is the magnetic ...
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1 answer
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Work on system with deformable boundaries

Background I am self-studying the book Elements of Gasdynamics by H.W. Liepmann and A. Roshko. In Chapter 1: Concepts from Thermodynamics, the authors discuss the First Law of thermodynamics and a way ...
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Conditions of work-energy theorem

The work-energy theorem states that the net work done by all forces on a particle is equal to the change in kinetic energy of the particle. I have a few queries on this theorem: Do the timing of the ...
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"Algebraic" view of energy/work concepts for High School teaching [closed]

I'm trying to describe to high school students the concepts behind Work and Energy. I need a sinple approach to this topic, and I thought to follow this path: Work is nothing but Energy transfer If ...
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Question on work done and frame of reference [closed]

While solving a question which is..... If an object is kept in a non inertial frame moving up with acceleration 'A' find the work done by pseudo force on the object taking a person standing on ground ...
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Work done by non-continuous force

How work done is really understood? I know that $W=F\cdot d$. I am interested in the meaning of force here i.e. Is it a continuous force applied till displacement? like the case of pulling trolley ...
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Deriving energy in Dielectrics

Note: I am working in the Lorentz-Heaviside system and all the integrals are over the whole space. Definitions: $$\vec E= \vec E_f+\vec E_b$$ $$\phi=\phi_f+\phi_b$$ $$\vec D=\vec E+\vec P$$ $$\rho=\...
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Spring compressed by external constant force,pot.energy stored over it is half of the work done by force, where does half gone [duplicate]

I have a doubt that for an ideal (massless)spring if apply an constant force over it until equilibrium the energy stored over it is ( kx^2)/2 and the work done by force is kx^2 so where the half of ...
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'Distanceless' force for aircraft seems to break laws of physics

When an jet is flying, its engines produce thrust. I assume that the thrust and fuel consumption is about the same at all aircraft speeds. However, when the airplane is going faster, it produces more ...
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1 answer
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Work done by gravitation on a system of two-body particles

Suppose we have two planets of masses $m_1$ and $m_2$ and of same radius $r$ kept a distance $d$.Owing to their gravitational force they both move and collide with each other. The system has undergone ...
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What is the status of the Work-Energy Theorem? [closed]

All the 'proofs' of the Work–Energy Theorem that I have seen show that the work done by the resultant force acting on a body is equal to $\Delta \left(\tfrac 12 m v^2)\right)$ for that body. [It's ...
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Average friction force for an object going up a slope

What is the difference between frictional force and average force of friction? I know we calculate frictional force in a typical slope problem by finding out the forces that act in opposition, most ...
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1 answer
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Joules do not makes sense to me [closed]

This is a fairly basic question that I am extremely confused about. I have a pretty bad physics teacher that cannot answer this question and is not even willing to, so here I am. The definition of a ...
-1 votes
3 answers
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Definition on type of work [closed]

A man carries a bag hanging it in his hand and he moves horizontally. The bag does not move up or down. What is the work done on the bag? The man gets tired after some time of the movement. Why?
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2 answers
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How to show that a radially symmetric central force is conservative?

Let $U\subseteq \mathbb{R}^3$ be open and $f:U\to\mathbb{R}^3$ be a radially symmetric central force, that is, a force field such that $$f(p) = -g(r)u_r$$ where $r=|p|$ and $u_r$ is the unit vector ...
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Goldstein: derivation of work-energy theorem

I am reading "Classical Mechanics-Third Edition; Herbert Goldstein, Charles P. Poole, John L. Safko" and in the first chapter I came across the work-energy theorem (paraphrased) as follows: ...
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Relation between rotational equilibrium and principle of virtual work

I am studying Analytical Mechanics from A Student's Guide to Lagrangians and Hamiltonians and I bumped into the principle of virtual work which states that: Virtual work is zero in equilibrium. This ...
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3 answers
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Why magnetic field doesn't do any work on moving charge?

As we know the Lorentz force $F=qv\times B$ never does work on the particle with charge $q$. But it does work on a dipole. My question is, doesn't a moving charge behaves like a dipole? I mean an ...
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Are principle of virtual work and principle of minimum potential energy same? And how is it related to Calculus of variation?

I am studying Finite element method and Classical Mechanics. I have come across three important terms Principle of virtual work (found in Classical Mechanics) Principle of minimum potential energy (...
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Why doesn't $dU=nC_{v}\,dT$ hold for all substances?

Consider the following proof for change in internal energy of real gases, liquids and solids(assuming Non-$PV$ work $=0$): Let X denote real gases, liquids, and solids The First law of thermodynamics ...
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Why is the force being the differential of a potential equivalent to it being a conservative force?

I was reading Goldstein's book on mechanics and came across this theorem: $F(r) = - \nabla V(r)$ is a necessary and sufficient condition of the force field being conservative. So far, I have ...
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Can a human being do work on themselves to increase their kinetic energy

BACKGROUND: A few days ago, I came across this question in the book "200 puzzling problems in physics". As the name suggests there are some puzzling problems in it :) The problem I was going ...
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Elementary work in magnetic matter

In section 5.16 Jackson$^1$ derives energy density in magnetic. With that in mind, he derives the expression of elementary work to build the configuration $$\delta W=\int\delta\boldsymbol{A}\cdot\...
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What does it mean when "work done cannot be regained"?

This question came in the Rajshahi University admission exam in 2014-15 Q) Which one is true in the case of conservative force? (A) Energy is wasted (B) The work done cannot be regained (C) The law of ...
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7 answers
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Does work differ between inertial frames?

I have a question regarding the frame dependence of both kinetic energy and work. I was able to prove that if I have a system of particles with masses $m_i$ and momenta $p_i$, then the change in ...
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2 answers
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Solving an inexact differential

So we have something called an inexact differential which is when a function is path dependent meaning I can't just subtract the initial and final states to get an answer. Take for example work which ...
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Does taking the $\log$ of a quantity change the unit of the quantity?

\begin{align} &= c [P - b \ln(P+b)]_1^{500} \\ &= 0.125 \frac{\text{cm}^3}{\text{g}} \times \left[\begin{aligned}&500 \text{ bar} - 2700 \text{ bar} \times \ln(500 \text{ bar} + 2700 \text{...
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Why do internal forces in a rigid body do no work? [duplicate]

Why is the work done by the internal forces in a rigid body zero? Most proof's for that sort of thing seem to assume that the internal forces in the rigid body are in the line attaching the points in ...
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1 answer
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Can a block set on a inclined plane fall vertically downwards? [closed]

At the top of a wedge of inclination $\theta$, a block is kept. There are no external forces acting on this block in horizontal direction. Is is possible for this block to fall vertically downwards? (...
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1 answer
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Why does a spring have potential energy compared to a rod?

From what I understand, a solid body or rod should not add work to a system. This is because these bodies internal forces are passed through touch: If the action and reaction forces are applied at the ...
1 vote
2 answers
65 views

Change in kinetic energy when force is perpendicular to velocity

I am a high school student and a have a fundamental doubt. It is said that when force is applied perpendicular to velocity, there is no change in kinetic energy since there is no change in speed. For ...
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What is constant in an isobaric process, internal pressure or external pressure?

I was taking a course in thermodynamics, where the instructor started to derive the formula for work in case of isobaric process for an ideal gas It stated W = -∫ P_ext dV And then equated it with -∫...

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