# Tagged Questions

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

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### Work in circular motions

Suppose that a satellite circles around a planet that exerts $2000N$ of gravitational force on the satellite. I understand the fact that since the circular motion and the centripetal force are ...
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### Work done by vibrating string

figure from http://webusers.physics.illinois.edu/~m-stone5/mma/notes/amaster.pdf The string has fixed ends, a mass per unit length of $\rho$, and is under tension $T$. This source claims that "the ...
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### How to reconcile the two definitions of work? (mechanical and thermodynamical)

When studying classical mechanics, work is defined as: $W_M=\int F_{tot} \hspace{2 mm} dx$. However, for thermodynamics, work is defined as: $W_T=\int -F_{ext} \hspace{2 mm} dx$. I'm having trouble ...
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### Work done by gravity on Water

Now according to me we would see change in potential energy of system and equate it to the work done by gravity. But when we see this the first column lowers by $H/2$ and right one rises by $H/2$ and ...
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### Area under a $pV$ diagram

What does the area under a Pressure volume diagram equal? I read in my textbook it equals 'external' work done, but why is this? First of all, what exactly is external work? Can you get it ...
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### Could $\int (\ddot\phi + \mu\dot\phi^2)d\phi$ ever be negative? How to show it?

I am working on the following problem: Given a mass $m$ at rest at the base of an incline surface shaped like 1/12th of a circle with radius $R$. The mass is pulled up the incline by a rope that ...
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### Work done by Lorentz Force in case of motional emf

In the classical example of the slidewire generator where the rod slides on a U-shaped conductor in a magnetic field, we get a charge separation due to the Lorentz force. The way the induced emf is ...
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### Thermodynamic cycles, when is the work negative/positive?

ever since I begun calculating thermodynamical cycles, I've had problems with determining the sign of the work along a particular bit of the cycle. Of course, I guess that an arbitrary cycle is 'bendy'...
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### How can magnetic fields have energy and yet be unable to do work with that energy? [duplicate]

Magnetic fields can't do work. However, we use the following equation to describe the energy density of a magnetic field. $u = \frac{B^2}{2\mu_0}$ The term energy density suggests that the magnetic ...
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### Line integral definition of work clarification

So I am kind of confused about the role of force when calculating work. Specifically, when defining work using a line integral. There is a paragraph in my calculus book that is really throwing me off ...
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### Is the coefficient of restitution frame independent and energy conservation?

In this question I am ignoring relativistic effects. The following statements I think are true: change in Kinetic energy is not invariant under change of frame Force is invariant under change in ...
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### How to calculate wasted energy

Suppose you are pulling a weight along a track at an angle (in the picture 45°). If the object is dislocated by a distance $D_{45}$ let's assume that the mechanical work done on/energy transmitted to ...
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### Energy of electron spinning in a magnetic field

When an electron travels in circles in a uniform magnetic field, it must lose energy because all accelerated charges radiate, and must therefore spiral down to the center. Is this energy compensated ...
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### work done by tension

The problem statement, all variables and given/known data Consider the following arrangement: Calculate the work done by tension on 2kg block during its motion on circular track from point $A$ to ...
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### Intuition behind Work

I have a doubt in understanding the intuition behind the concept of work. First of all, I think this isn't duplicate, I've searched on the site, and the closest thing I've found was this post which is ...
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### When work is performed solely by magnetism, is there an equivalent loss of energy from the magnetic field?

When two magnets are placed within appropriate proximity and released, the attractive force will perform work and bring them together. Work is performed overcoming friction. Can we measure a ...
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### Does moving something horizontally in gravity do no work?

Bill’s job is to lift bags of flour and place them in the back of a truck, which is parked next to him. Sally is loading the same bags of flour into a similar truck that is located 10 m away. ...
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### How much work can a single grain of rice do? [closed]

I found a website saying a grain of rice contains 1/10 kcal. I'm not a physicist and haven't done maths for a long time. But here's what I came up with: E = m * g * h One small calorie equals 4....
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### Centripetal Force Acceleration

In uniform circular motion, acceleration is $\frac{v^2}{r}$ and time which it acts $\rightarrow 0$. So $\Delta v = 0$, but then why/how does direction change, when the acceleration should be ...
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### Work Done to click a mouse?

Is there any good research done to find out the work done in clicking a mouse button. any link to that would be greatly appreciated. P.S. i am not too sure whether this question belongs here or not,...
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Is $Q=\Delta U+W$ for when the the work is done from the system while $\Delta U=Q+W$ is for when the work is done by the system? Will anybody explain this to me, please? When do we use $Q=\Delta U+... 5answers 2k views ### Is there a mathematical derivation of potential energy that is *not* rooted in the conservation of energy? For simplicity I'll consider only gravity, but in general this question only applies to conservative forces. As per my understanding, the way one gets to the equation for gravitational potential ... 3answers 987 views ### What is the work done against a force? Suppose a particle travels a path$\gamma : I\subset \mathbb{R}\to \mathbb{R}^3$subject to a force$\mathbf{F}: \mathbb{R}^3\to T\mathbb{R}^3$, then we know that we define the work done by the force ... 4answers 18k views ### What is the difference between dot and cross product? What is the difference between dot product and cross product? Why do we use cross product to find torque, why can't we use dot product? Also we use dot product to find work done and not cross product?... 3answers 131 views ### Work done by an ideal gas expressed as change in potential energy of fluid I get confused in problems where it is necessary to evaluate the work done by a gas that someway moves a mass. I'll make an example. Consider the tube containing mercury and an ideal gas in A (... 3answers 249 views ### Is there an intuitive explanation of the work formula? Upon learning calculus, I decided it was time to derive all of classical mechanics to give myself a good understanding of physics. What I found was that, while trying to do so, I would need some ... 4answers 1k views ### Does stopping the same bike and rider at the same velocity with the front brake require less energy than the back brake? It's the same body made by the rider and the bike moving at the same speed. So, even though braking on the front/back alters the normal forces on the opposite wheels thus creating more friction with ... 1answer 70 views ### Does the line integral definition of Work involve distance or displacement? My textbook reports the following definition of Work: where ds is the infinitesimal displacement. I know that an infinitesimal displacement is usually denoted by dr and I also know that the ... 4answers 1k views ### Is work done = change in KE, or change in mechanical energy? Starting from$F = ma$we get: $$F=m\frac{dv}{dt} = m\frac{ds}{dt}\frac{dv}{ds} = mv\frac{dv}{ds}$$ which leads to the work done: $$W = \int Fds = \int mvdv = m\frac{v^2}{2} - m\frac{u^2}{2} \... 4answers 221 views ### Why work W and heat Q are different concepts? I understand heat as the flow of energy (through radiation, convection or conduction) from one body to another. When I think about conduction (for example) I visualize particles that jiggle a lot ... 4answers 2k views ### Is work done by how much velocity is changed or how much displacement is done? A sailboat is moving at a constant velocity. Is work being done by a net external force acting on the boat? The answer key is "No" according to the work energy theorem about work is done when there ... 3answers 123 views ### How do I relate ds to dx? A block of mass 5\text{ kg} is kept on an inclined plane with angle of inclination 37°, attached to a spring with spring constant 10\text{ N/m} kept at the base of the incline. The ... 2answers 1k views ### Entropy dQ=TdS and Work dW = -pdV conditions? What are the conditions in order for the equations: Entropy dQ = TdS and Work dW = -p dV to work? I think for dQ = T dS, it must be a reversible process? But for dW = -p dV, shouldn't it ... 2answers 215 views ### Mechanical work to required battery power I have a very practical question where I've calculated the mechanical work needed by a simple mechanical system by solving the line integral W = \int_C \ F \ dx. However, since I have a black spot ... 2answers 325 views ### Work done by frictional force on a sliding block A block slides across a table horizontally with an initial velocity V. The frictional force F brings it to rest after its Centre of Mass covers distance S. What is the work done by the ... 2answers 93 views ### Lack of rigour in usual derivation of Work-Energy Theorem The derivation of the Work-Energy theorem usually goes as follows: You define the work done on a particle under net force \vec{F} as$$W=\int\limits_C \vec{F}\cdot\mathrm{d}\vec{r}$$And then you ... 5answers 3k views ### when an object is lifted (at a constant velocity) shouldn't the work done on the object be zero? [duplicate] When I lift an object from the ground (at a constant velocity) I'm applying force on the object equal to its weight and the earth is also pulling it downwards with equal amounts of force. So if the ... 2answers 6k views ### Work done against gravity [closed] The work done against gravity is mgh, well at least that's what my textbook says. I have a question: I can apply a force say 50N, so total work done = mgh + mah. Where ma = Force. But the truth ... 1answer 67 views ### Work done or not in this case? I have a very simple question. A motorboat directed upstream is seen to be at rest from the bank of a river. Is the engine doing any work? Is it right to say that since it is not causing any ... 2answers 2k views ### Having trouble understanding the work energy principle intuitively I'm having trouble understanding the work energy principle intuitively. This is what I'm solid on so far: If you have a ball rolling down a hill, it loses potential energy and gains kinetic energy. ... 3answers 61 views ### Using thermal expansion for mechanical work Assume I could heat up a bar with zero heat loss to the environment. I could then use the thermal expansion to do some work, for example compress a gas. Where does the energy needed for the ... 4answers 204 views ### If velocity is constant, how can p = F\cdot v be non zero? If an airplane of mass m is flying at a constant speed v, the power of the airplane is$$P = m\cdot v\cdot g $$where g is the acceleration of gravity and therefore:$$ F = m\cdot g, $$but, ... 2answers 1k views ### Factors on which Coefficient of restitution depend What are the factors on which coefficient of restitution depend? What is the reason for more coefficient of restitution of two glass balls (0.95) than for two lead balls (0.20)? 1answer 189 views ### Is work is equal to mv^2 (without \frac{1}{2})? I was trying to come up with an equation for work that doesn't include time, because I don't know time. Here's what I did:$$ work = Fd = mad = m{v\over t}d = m{v\over\left({d\over v}\right)}d = mv^... 1answer 207 views ### Confusion with curl of Lorentz magnetic force Since the magnetic force is a no work force,$dW=\vec F\cdot d\vec r=0$for$\vec F(\vec r)=q(\vec v(\vec r) \times \vec B(\vec r))$, therefore$\oint \vec F \cdot d\vec r=0$by Stoke's theorem. ... 2answers 2k views ### Why do two equal-and-opposing forces each do work on a moving object but not on a stationary object? Consider a point-mass$m$having constant velocity but undergoing influence from two forces,$F_1$,$F_2$, having equal magnitude but opposite directions. Because the forces' magnitudes are equal, I ... 2answers 661 views ### Measuring energy when no work is done — comparing push-ups to planks tl;dr Is there any meaningful (physical) way to compare the energy expended in the exercise of doing$x$pushups in$t_1$seconds, vs the exercise of doing the plank for$t_2$seconds? I'm ... 2answers 6k views ### Does a force do work on an object with constant velocity? I know that a force does no work on an object if the object's displacement is zero, but if an object is moving at a constant velocity$\bar{v}$, and a force$\bar f$(let's say that$\bar f$and$\bar ...
Imagine spaceship in vacuum with mass = 1. At beginning, it has velocity 0, and kinetic energy 0. $$W_1 = 0$$ Then, it turns on its engine, and changes velocity from 0 to 20 (delta v = 20). It's ...