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-2 votes
3 answers
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Confusion regarding Newton's Third Law and Conservation of Energy

A specialized system consisting of two bodies A and B. A has 1 J of energy and can thus displace B 1 m by applying a force of 1 N. But at the same time, A also experiences a reaction force and is ...
1 vote
1 answer
19 views

Why is the direction of friction not being considered in this problem for calculating the work done?

The solution to the problem happens to be $mg(h+kl)$ because the work done by force $F$ has to be equal to the sum of the work done by friction and gravitational force. But how is the work done by ...
0 votes
4 answers
150 views

Is Energy stored work? [closed]

Energy is the ability to do work therefore Energy should be the primitive form of work. But I cant find any conformation, if wrong can anyone explain to me the relationships between Work, energy, and ...
7 votes
6 answers
3k views

Can there be acceleration without work?

Since $W=Fs$, $F=\frac{W}{s}$. When you substitute this in the formula for acceleration, $a=\frac{F}{m}$, you will get that $a=\frac{W}{ms}$. Then, when work equals zero, acceleration will be zero.
1 vote
2 answers
88 views

Is Energy stored Force?

I am currently studying the relationships between energy work and force. I am trying to conceptualize them into one concept of causality. When a force is applied work is done, and energy is ...
1 vote
1 answer
27 views

Work done on a body and work done by the body

I want to know that, if we suppose that, if a ball is falling freely then work would be done on the ball by gravity so energy would be given to the ball? and when the ball rebounds after striking the ...
1 vote
2 answers
119 views

Proof Of Conservation Of Energy

Yesterday I was trying to prove The law of conservation of energy, But in a different approach: Using the concept of vector field here is how it went: I assumed a force field $\mathbf{F}=f(x,y,z){\...
1 vote
1 answer
48 views

Does Accelerating to Double the Speed Require Quadruple the Fuel Consumption, or Double the Fuel Consumption?

I'm writing a piece on the efficiency of trains versus trucks, and one of the points I was making is that trains can take advantage of not having to stop and start multiple times. This is obviously ...
0 votes
1 answer
45 views

Is a velocity-dependent force $\vec{F}$ that doesn't do any work on an object a conservative force? [duplicate]

Let's consider a point like object with mass $m$ upon which acts a force $\vec{F} = \vec{c} \times \vec{v}$ ($\vec{c}$ is supposed to be a constant vector). Given that $\vec{F}$ is perpendicular to $\...
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1 vote
0 answers
25 views

Aren't the virtual work/virtual power principles in mechanics simply solving for the vector differential equation of motion in a preferred direction?

My conceptual understanding of the virtual work/virtual power principles is that, by hypothesizing "virtual displacements"/"virtual velocities", one can solve the equations of ...
  • 165
7 votes
6 answers
2k views

How can a system have kinetic energy if the centre of mass of the system is stationary?

(I'm doing high school physics so please go easy with the crazy calculus) From what I understand, a system has translational kinetic energy if the system of mass is moving at a velocity but the ...
  • 181
1 vote
3 answers
98 views

A man is rowing a boat upstream is at rest with respect to shore is he doing any work

Is the man doing any work or not when is rowing a boat upstream? I m confused That Okay he's at rest with respect to Shore but at some sites it can be seen that Yes he is doing work then how he is ...
-1 votes
2 answers
68 views

Motivation for definition of work [closed]

Why do we take the dot product in the work energy theorem? Consider the integral $$\int\vert\vec F\vert\vert d\vec r\vert$$ Why don't we define this to be work done for example, instead of $\int\vec F\...
0 votes
1 answer
120 views

How much more fuel to double the speed?

Suppose it requires a certain amount of fuel to accelerate a car (or a spaceship) from 0 to a speed $v_1$ (in some initial frame of reference). Let's ignore all friction forces, and only consider ...
  • 109
0 votes
2 answers
78 views

Is the topic of gravitational potential energy convoluted and unnecesary? [closed]

If we throw an object upwards from the surface of the Earth, we do some work on it to give it kinetic energy. Now as it travels up, Gravity of the Earth does negative work on it till it loses all the ...
0 votes
2 answers
57 views

How is work done by Tension zero in this case?

How is the work done by tension zero in this case. There is no horizontal force acting so the centre of mass won't displace in the horizontal direction as it will be in vertical. But the work done by ...
1 vote
2 answers
73 views

How can we jump when the ground does no work on us? [duplicate]

This may seem like a silly question, but how can we jump when the ground does no work on us? When we jump off the ground, the ground does not move. This means the force from the ground on us does zero ...
0 votes
3 answers
62 views

Why energy of a system equals to sum of its mechanical energy and its internal energy? [closed]

My books said that energy is a quantity that: Its variation equals to work done on system (1) It equals to sum of its mechanical energy and its internal energy (2) So, can (2) be inferred from (1)? ...
0 votes
1 answer
47 views

What should I take as virtual displacement?

A bicycle rider has M. The wheels of the bicycle are at distance l apart and horizontal distance between the seat and the rear wheel is d. Use principle of virtual work to find the normal reactions on ...
1 vote
1 answer
72 views

How can the Joule be the unit of both work and energy?

Say a person applies 1 N to a box with a mass of 1 kg, displacing 1 m. This is one Joule of work (1 N for 1 m). Now say the person applies 1 N to a box with double the mass, displacing 1 m as well. ...
5 votes
7 answers
846 views

Work-energy theorem and Newton's second law

I was going through a book in which it was written that 'Work-energy theorem for variable force, in general, does not incorporate the complete dynamics of Newton's 2nd law of motion as the theorem ...
  • 51
0 votes
1 answer
47 views

Distance a block slides along a surface with friction given with an initial velocity [closed]

For this part (e) of this problem, Source: https://ocw.mit.edu/courses/8-01sc-...647ea989a352a972dc4b3dfe_MIT8_01F16_pset7.pdf The solutions are, However, I don't understand why they only used a ...
1 vote
1 answer
56 views

Why is force "accumulated" more at a higher speed?

I tried to understand why kinetic energy is proportional to the square of velocity. In this endeavor I stumbled upon a book "Emilie du Chatelet: Daring Genius of the Enlightenment" (ISBN 978-...
  • 47
3 votes
2 answers
291 views

What is energy and how it is classfied?

My physics book defines energy like this: "Everything that works has energy, energy $E$ of a system is a function of state and its variation $\Delta E = W$ ($W$ is work done on system)". ...
1 vote
0 answers
62 views

Work-Energy Theorem and First Law of Thermodynamics [duplicate]

I am little confused about the connection between these two concepts in Physics: First, in classical mechanics, a basic result is the work-energy theorem, which states $$\Delta T = -W.$$ This equation ...
1 vote
1 answer
51 views

How to derive equation for thermal energy ($E$ thermal = Friction force X Distance)

We are covering the energy principle in my physics course and so far we have been using the equation $$\Delta KE + \Delta E_{th} = W_{Ext}$$ which is change in kinetic energy plus change in thermal ...
1 vote
1 answer
58 views

Equilibrium and maximum velocity position of a person jumping on pogo stick (spring)?

Consider the following diagram: It is given that: A is the position of maximum compression of spring and the child is at rest(at that instant). At B, the child is having an upward velocity and the ...
2 votes
2 answers
41 views

Work done by ladder on the boy and work done by boy on ladder [closed]

I was stuck in a question: In a children's park there is a slide which has a total length of $10 \,\mathrm m$ and a height of $8 \,\mathrm m$. Vertical ladder is provided to reach the top. A boy ...
  • 21
0 votes
4 answers
77 views

Work done by vertical force

We know that work is defined by the dot product of force and displacement,which mathematically means $W=Fs\cos \theta$ in scaler. But let's suppose an object is moving in $x$ axis. It's vertical ...
  • 897
4 votes
3 answers
393 views

Work done by the force of gravity

My question is very similiar to this one: Work done by gravity on falling object does not seem to equal change in mechanical energy As I've understood it, work is only done on an object if the object ...
  • 51
0 votes
4 answers
61 views

Does a moving object with a constant velocity do any work?

Since Work=Force$\cdot$ Distance and the net force on a moving body is zero, does that mean that no work is done by a body moving with a constant speed? Also, the work done is the energy expended. If ...
  • 1
0 votes
6 answers
96 views

Importance of keeping constant velocity

In finding work done by a spring or gravity (i.e conservative forces), when an external agent does work, why is it done so that velocity i.e kinetic energy remains constant? I know from Work energy ...
  • 897
0 votes
1 answer
66 views

What is the derivative of potential energy with respect to $x$? [duplicate]

I was reading an article which was proving the conservation of energy with calculus, but they did not explain why the derivative of potential energy with respect to $x$ is the negative force. $${dU\...
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0 votes
1 answer
116 views

When is the work done minimum?

Work $W$ is defined as the dot product of the vectors force $F$ and displacement $s$. The dot product can be written as the products of the magnitude of the force, magnitude of the displacement and ...
  • 1
0 votes
1 answer
20 views

Work done and deriving elastic collisions

Using Newton's second law, it is possible to write the work done on a path going from position A to B as: $$W_{AB}=\int^B_Am\frac{d\vec{v}}{dt}\cdot d\vec{r} =m\int^B_A(\frac{d\vec{v}}{dt}\cdot \vec{v}...
user avatar
-1 votes
1 answer
26 views

Find the speed of the particle when it has covered a displacement of magnitude $L$ [closed]

This is my first question on this site. A particle of mass $m$ is constrained to move along $x-\textrm{axis}$. The only force acting on the particle is given by $$F=F_0\cos\left(2\pi\frac{x}{\lambda}\...
  • 101
0 votes
1 answer
36 views

Work power energy related question [closed]

If work done by conservative force = $-\Delta U$ always (even if there is air friction), and Work-energy theorem is applicable anywhere, and we derive law of conservation of mechanical energy using ...
  • 1
2 votes
1 answer
41 views

Change of velocity without doing work

Body of mass $1\rm\, kg$ is moving towards left side at velocity of $-2\rm\,m/s$ and is slowing down due to the constant external force until it stops. After that, it's speeding up towards the right ...
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1 vote
2 answers
160 views

Why does small work done mean $dw=f.ds$ and why not $dw=df.ds$ and why not $dw=s.df$? [duplicate]

Work, power and energy questions. Why does small work done mean: $$dw=f.ds$$ and why not: $$dw=df.ds$$ and why not: $$dw=s.df \ \ ?$$
2 votes
2 answers
306 views

Work Integral and its derivation

The work integral is something I saw long time ago and in completely understood it. \begin{align} W_{12} & =\int F(x)dx=m\int^{t_2}_{t_1}adx=m\int\left(\frac{dv}{dt}\right)dx=m\int\left(\frac{dv}{...
4 votes
4 answers
1k views

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 $=...
0 votes
4 answers
65 views

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 ...
0 votes
2 answers
41 views

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 ...
  • 137
0 votes
2 answers
80 views

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 ...
  • 1,362
2 votes
1 answer
105 views

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: \...
user avatar
0 votes
0 answers
41 views

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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1 vote
0 answers
51 views

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 ...
1 vote
1 answer
87 views

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 ...
  • 897
0 votes
3 answers
54 views

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....
0 votes
1 answer
42 views

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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