# All Questions

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### Find the resultant force in a motion problem with a loop

In the approximate diagram, the block with mass $m$ starts without movement in the position $P$ in the top of a hill with height $5R$. After that, the block falls down through the hill, and reaches a ...
35 views

### Question about Helmholtz's paper “ON THE CONSERVATION OF FORCE”

Below follows the exact extract from Helmholtz's paper "On the conservation of force". Let us now imagine, instead of the system $A$, a single material point $a$, it follows from what has been just ...
202 views

### Elastic potential energy and springs

The formula for elastic potential energy(for a spring) has been derived by assuming the following things: 1.Work done by a deforming force on a spring from a relaxed state (state where spring is not ...
114 views

### Proof of total energy is constant in a central force field

I saw many proofs of total energy is constant in a central force field. But all the proofs end up showing this formula $$m[{\dot r}^2 + r ^2{\dot \theta}^2 ]+ \int f(r)dr = E$$ is constant. But ...
994 views

### Why Pascal's Law is true and what is the mechanism for force amplification at molecular level?

"A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid" "A pressure change occurring anywhere in a confined incompressible fluid ...
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### When finding the force of a falling raindrop, how to account for not knowing the collision time?

So my question is to find the force of impact on the ground of a raindrop of mass $m$ that falls at velocity $v$ right as it hits the ground. So what I did was find the momentum of the object, which ...
82 views

### Where does the energy come from when a magnet moves a compass? [duplicate]

My very limited understanding of magnetism is that it is essentially stored energy. However what I'm confused on is about the following situation: If you have a compass and you move a magnet close to ...
46 views

### Force, work and the apparent disappearance of Mechanical Energy [duplicate]

A man exerts force on a wall of bricks. The man must have consumed the energy he possessed (mechanical energy?) to exert the force. The man sweats and tires himself out but the wall does not move. ...
34 views

### Forces From Opposite Sides On A Block

Suppose we have an indestructible box in space , and we apply equal forces on two opposite side of box , the box wont move but then where will the energy go?(Conservation of energy)
69 views

### Why does exerting force sometimes cost energy and sometimes not?

Inspired by these similar questions, which have some very good answer to what they are, but they all focus on what is happening. None of them explains why it only applies sometimes and not others. I'...
43 views

### Same amount of calories burned for half the reps and double the weight? [duplicate]

So I was looking at the work= force x distance equation and was just wondering if for example I normally do 100 dead lifts at 100 pounds. Would I get the same calories burned if I did 50 dead lifts ...
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### How does $W=0$ for the following equation and question on conservative forces?

We have $W = fd\cos(\theta)$ = Kinetic Energy + Potential Energy. Our example is a $10$kg ball is falling from a height of $10$m. I can see why Kinetic Energy and Potential Energy cancel out to become ...
197 views

### Forces involved in annihilation event

Combustion can create energy by breaking/creating bonds between atoms via the electromagnetic force to produce a resultant lower energy system. Nuclear fusion and fission processes can create energy ...
303 views

### Work done by force on block against gravity and friction up irregular rough incline?

I recently found a question which I think is from Irodov which asked about the work done by a force $F$ on a body of mass $M$ up a rough hill. At each point in the path of the body the force is ...
88 views

### When is kinetic energy linear?

If we were to plot kinetic energy and time, what would the circumstances be in order for kinetic energy to increase or decrease linearly? The question I had in class showed a body with some velocity ...
442 views

### Doubt on an answer to a mechanics problem [closed]

Question: A block of Mass m is connected to another block of mass M by a massless spring of spring constant k . The blockes are kept on a smooth horizontal plane and are at rest. The spring is ...
120 views

### Difference between force and energy equilibrium

I want to find out how far a spring stretches. If I use the force equilibrium, I get $kx=mg \to x=mg/k$. If I use the conservation of energy, I get $0.5kx^2=mgx \to x=2mg/k$. How can I choose ...
3k views

### Why do we ignore normal forces when applying conservation of energy to roller coasters?

Consider this setup to study conservation of energy: It is usually taught that the potential energy at any point on this roller coaster is given by: $$U =mgh$$ Where $h$ is the height above the ...
119 views

### Identifying Kinetic energy in Work energy thorem

I am new to this topic and I can't find a way to solve the questions even though I identify the forces which do the work....I am more confused at the Kinetic energy part. Can you tell me how to ...
386 views

### How bosons do not violate conservation of energy?

As far as I understand bosons are energy packets which carry forces: e.g. Higgs bosons carry gravity. What I don't understand is, for example if we have an isolated object which constantly releases ...
128 views

### Concept of Work Done [closed]

If one of the forces acting on a particle is conservative, will its work equals the change in Kinetic Energy? True or False Well, according to the formula $\text{W} = \Delta\text{K.E}$ the statement ...
209 views

### Normal force - 3 things I don't understand

So i have a couple of (lengthy) questions about the normal force. Let's say i have a setup where a box falls from the air onto a rough surface, with a horizontal velocity. Let's neglect air ...
264 views

### Validation of Work-Energy Theorem

Is the work-energy theorem valid when there's an impulsive force during motion of considered body? For eg: Consider a man jumping from some height into a swimming pool of certain depth, if we apply ...
825 views

### How is energy conserved when a man climbs a stair?

Suppose a man of mass 50 kg climbs a stair of height 10 m.Clearly it gains 5000 J of potential energy.But since work done by normal reaction on the man is zero does'nt it violates the principle of ...
793 views

### Potential Energy of a System and work–kinetic energy theorem

If you lift a book from the floor to a shelf, $K_f-K_i=0$,because the final and initial speed are 0. So from the work–kinetic energy theorem the work done by external forces should be $0$, so there ...
323 views

### Nature of Work done by Forces

I am confused about the possible nature of work that a conservative and non conservative force can do. Do Non conservative forces like friction only do negative or zero work and not positive? Can ...
299 views

### Is friction a conservative force? [duplicate]

Suppose a block of mass $m$ is being pulled on a hill by a force $F_{app}$, the block is being pulled slowly such that $\Delta KE = 0$. Our teacher showed that the expression of work done by friction ...
212 views

### Inclusion/Exclusion of linear kinetic energy in a rope-pulley system

I find it difficult why in the first problem the linear kinetic energy is not included in the calculation, but in the second it is included, yet the questions are virtually the same.
229 views

### Why Energy is not conserved?

Two masses $\mathrm{A}$ and $\mathrm{B}$ each of $5~\mathrm{kg}$ are suspended by a light inextensible string passing over a smooth, massless pulley such that mass $\mathrm{A}$ rests on a smooth table ...
64 views

### Generalized forces in Richard Fitzpatrick's Newtonian Dynamics

In Richard Fitzpatrick's Newtonian Dynamics, page 128, generalized forces are defined as \begin{equation} \tag{9.6} Q_i = \sum_{j=1}^{\mathcal{F}} f_j \cdot \frac{\partial x_j}{\partial q_i}. \end{...
238 views

### Getting energy from decelerating a space-ship

This question comes from another answer. The proposed system is one where the moving space-ship decelerates and some object which is free to move within the ship feels the g forces of this ...
594 views

### Bounce of a ping pong ball vs bounce of other things (How does the material affect ?)

Yesterday I saw that a ping pong ball bounced nicely on a hard surface but poorly on a mattress.On the contrary ,I saw that other things (a pen for example) bounced poorly on a hard surface but nicely ...
57 views

### Ring motion in an hemispherical bottle

Suppose we add a ring of radius $r$ and mass $m$ into an hemispherical bottle of radius $R$, how can we find the differential equation for it in terms of the angle $\theta$ made from vertical ...
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### Collisions in space/vacuum

Say an object in space collides with another object at 100mph. Say the same thing happens on Earth. Would the impact in space exert more force on the receiving object than on Earth because some energy ...
101 views

### Mechanical Energy Conservation with a spring against a wall

Here is the sketched situation: We have a bloc of mass M, and a spring attached to one end of that bloc. The bloc is moving and moves into a wall. The spring is compressed up until a certain maximum ...
221 views

### Can the kinetic energy of a photoelectron be greater than what's calculated by the photoelectric equation?

The photoelectric equation, which relates the energy of the incident photons to the maximum kinetic energy of the photoelectrons, is given as $E=h\upsilon -\phi$, where $h\upsilon$ represents the ...
180 views

### Confusion about Conservation of energy

I getting a problem regarding law of conservation of energy. Suppose there is a hypothetical gravitational field which you switch on/off. you take an object to the opposite direction thus working and ...
673 views

### What is the difference between applying force and spending energy? [duplicate]

I have a follow-up to this question on how objects can be held up, against gravity, without work being performed. This comment is what I get tripped up on: Unlike your arm, the table does not need ...
2k views

### What causes more damage, a head-on car crash at 30km/h or one car at 60km/h crashing into a stationary one?

Which will cause a greater impact? Two identical cars colliding head on (in opposite directions), each going at $30\;\mathrm{km/h}$ OR One car travelling at $60\;\mathrm{km/h}$ colliding into a ...
8k views

### Why is Kinetic Energy = (-) Total Energy and Potential Energy = 2 $\times$ Total Energy?

I came across this relation while reading on the Bohr atomic model. Are there any other forces for which these relations hold good?
848 views

### Work-energy theorem for a system

While studying energy conservation on Morin I found this explanation about the work-energy theorem for a system. The work–energy theorem stated before is relevant to one particle. What if we are ...
1k views

### Condition for looping the loop

Consider a ball tied to a string and it is imparted a velocity we have studied that condition for looping the loop is that tension at the uppermost point must be zero, but why is this condition ...
229 views

### Where Potential increase gets its energy from?

Imagine there are two particles: A,B; of which, we observe A to be stationary, while B is moving towards A at velocity v. A,B has same mass; and they are charged both positive. B has a potential wrt ...
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### Must one include friction and gravitational force in the work term?

Example problem: "A sled has an initial velocity up a ramp of 5 m/s, the ramp has an angle of 20 degrees to the horizontal, and the coefficient of dynamic friction between the sled and the plane ...