Conservation Definition and 999 Threads

Hi PF, I've got a very trivial conceptual question regarding the conservation of energy with respect to thermodynamics and heat transfer that I can't seem to figure out. Suppose I have an electric heating element with a 240 volt, 30 amp supply, in contact with a solid volume of an arbitrary...

Hi everybody! I'm preparing myself for upcoming exams, and I struggle a little with conservation of angular momentum. Can anybody help me understand how to solve such problems? 1. Homework Statement (for a better comprehension, see the attached image) We have a wooden cylinder of mass mZ =...

1. Homework Statement For part (iii) , I used the principle of conservation of energy, K.E of the 2 kg particle after collision + E.P.E = K.E of the 2 kg particle at the furthest distance away from A + E.PE, But the solution for this question did not include the E.P.E of the string...

By squeezing a spring until it coils up, it then has more potential energy. So it will then have more mass, because mass is energy. So doesn't that violate the law of conservation of energy. The spring has more energy and mass after its squeezed than when it wasn't squeezed?

Dear all, So I have a question concerning atomic conservation in an ionized hydrogen gas. So imagine we have ## H_2 ## initially. Later the gas is taken to an appreciable temperature such that at equilibrium the following species are present, ## e^-, \ H, \ H^+, \ H_2, \ H^-, \ \text{and} \...

Hello Forum, Conservation of mechanical energy ME= KE+PE happens when the net work done by the non conservative forces is zero. Conservation of total momentum, instead, happens when the external net force is zero (or close to zero). In the case of mechanical energy, the non-conservative forces...

Hello, I have a simple question. Has the discovery that some neutrinos change their flavor posed any issues with conservation of energy?! How has this been solved?! Thank you.

Homework Statement Actually, my confusion originated from solving two different problems... 1) A point object of mass 'm' moving horizontally hits the lower end of the uniform thin rod of length 'l' and mass 'm' and sticks to it. The rod is resting on a horizontal, frictionless surface and...

When a particle (or any objects) accelerates when acted upon by fundamental forces, what is happening to the amount of energy in that particle? A large example would be an asteroid in space caught by Earth's gravity and accelerates toward Earth as a result. The fundamental forces here can be...

Homework Statement a 5.0 x 10^5kg railroad car moving at 8.0m/s. collides with a stationary railroad car of equal mass. after the collision the two cars lock together and slide forward. What is the final velocity of the wrecked cars? using conservation of energy M1 = 5.0 x 10^5 kg M2 = 5.0 x...

I'm a little confused about the hamiltonian. Once you have the hamiltonian how can you find conserved quantities. I understand that if it has no explicit dependence on time then the hamiltonian itself is conserved, but how would you get specific conservation laws from this? Many thanks

Homework Statement A spherical object is dropped from an elevation great enough such that it will achieve terminal velocity for some period of time before hitting the ground. Once terminal velocity is achieved what is gravitational potential energy converted to. Homework Equations Ug = mgh Ke...

Homework Statement An 8.00-g bullet is fired horizontally into a 9.00 kg block of wood on an air table and is embedded in it. After the collision, the block and the bulet slide along a frictionless surface together with a speed of 10cm/s. What is the initial speed of the bullet? M1 = 0.008kg...

Not a scientist, please be nice :) Let's assume I have a singe axis gyro (flywheel) spinning in space. I apply a force to a point which results in a change in pitch. I apply this force until the gyro is at 5* pitch and then stop. Will the gyro continue to change pitch after the force stops...

Homework Statement A 60.0 kg woman stands at the western rim of a horizontal turntable having a moment of inertia of 500 and radius 0f 2.00 m. Turntable is initially at rest and is free to rotate around frictionless vertical axle through its center. Woman then starts walking around the rim at...

Hello, I have a question about using the properties of conservation of angular momentum to provide mechanical resistance. Basically, I'd like to create a device that spins a disk similar to a gyroscope. The device has an external input that, depending on the configured orientation of the disk...

Imagine I have a rocket with a certain amount of energy stored as chemical energy, let's say its 10Js, that exhausts itself after 5 seconds. If I attach this rocket to a (relative to an observers frame) stationary cart in such a way that it pushes the cart, it will add 10J of kinetic energy to...

I am having trouble wrapping my head around a physics concept. If we roll solid sphere down an inclined plane it will reach the bottom at a different time than if we were to say, roll a hoop down the same inclined plane. This is because they have different rotational inertias, and so more of...

Electrons have a theoretical rest mass. They can move at varying speeds through space, unlike photons. They ehxhibit quantum-characteristics in their behavior. If an electron collides with, say, an atom, does conservation of momentum apply in the classical sense or does measurable mass (an atom...

Homework Statement We have an ordinary LRC circuit with inductance L, capacitance C and resistance R with an oscillating voltage with low frequency (U^e). Using the energy conservation law and Poynting's theorem, find the differential equation: $$L \frac{\partial ^2}{\partial t^2}I + R...

This is it; most likely the last time I bother the people of this website with my questions on traffic flow. I'm trying to figure out some concrete examples to demonstrate utilization of the conservation equation in traffic flow: \frac{\partial \rho }{\partial t} + \frac{\partial q(\rho...

Homework Statement Consider a gun of mass M (when unloaded) that fires a shell of mass m with muzzle speed v. (That is, the shell's speed relative to the gun is v.) Assuming that the gun is completely free to recoil (no external forces on gun or shell), use conservation of momentum to show that...

Homework Statement I get that less percentage energy is conserved from potential to kinetic energy by measuring h and v with two balls for the heavier ball. I am trying to sort of why actually is like that! Two balls where dropped down from a ramp with different masses and volume. The smaller...

Homework Statement A mass ##m_1 = 5.0 kg## is hanging from the end of a thread, of negligible mass, that slides on a pulley, of negligible mass too and without friction. At the other end of the thread, at the same height of ##m_1##, there is another hanging mass ##m_2 = 3.5 kg##. Using the Law...

Two things I'd like to discuss: 1. The conservation of angular momentum. If you have two discs rotating on the same fixed rigid axis, will these nullify each other? I.e. Create no net angular momentum? 2. How / is it possible to convert angular momentum to linear momentum in the sense to be...

I would like to preface this by saying that I do not in any way resemble a physicist - and I'm sure the crudeness of my work will confirm that, but I thought that this was so cool, I wanted to share it. I'm just a follower of physics. I understand that some of the concepts I'm going to be...

Homework Statement A block of mass m slides down a frictionless ramp from height h above the floor. At the base of the ramp it collides and sticks to the lower end of a uniform rod, length L, mass 2m, that is suspended about a pivot at point O, about which it is free to rotate. Express...

Homework Statement This lab I have the mass and change in distance measurements for inelastic collisions; and then we did an elastic collision one where we determined the mass and measured the times three of them simultaneously where we measured it as it bumped the car. So I am supposed to...

Homework Statement Two objects collide and bounce off one another. After the collision, object m1 = 2.74 kg moves off at 12.8 m/s at a heading of 295 degrees. Object m2 = 2.28 kg moves off at 12.8 m/s at a heading of 241 degrees. Initially, m2 was traveling at 11.1 m/s at a heading of 334...

Homework Statement Question - Models of global warming predict that large sections of the polar ice caps will melt. Explain what effect this will have on the rotation of the Earth, however slight. Homework Equations L = Iw The Attempt at a Solution Assuming polar ice caps protrude the earth...

1. A ball hanging on a pendulum hits an object standing on the table. The interaction is elastic and linear. After that, the object falls on the floor. Homework Equations From state 1 to 2, we have the conservation of the potential energy of the pendulum to its kinetic energy, right before...

Hi, I'm looking for the real solutions to the system \begin{array}{rcl} 1 & = & v_1+v_2+v_3+v_4+v_5 \\ 1 & = & v_1^2+v_2^2+v_3^2+v_4^2+v_5^2 \end{array} Background: I'm looking at a Newton's cradle with 5 balls, each of mass ##m=1##. The first ball is pulled away and let go such that it hits...

Homework Statement A stationary nucleus undergoes radioactive decay. A beta particle and a neutrino are detected leaving the nucleus. What is the recoil velocity of the remaining nucleus? If the recoil velocity measured is significantly different from the calculated velocity, what conclusion...

Hi. In an ideal pendulum, energy is conserved. Potential energy gets transformed to kinetic energy and vice versa. However, momentum is not conserved. The latter means that the pendulum is not an isolated system, which is plausible, since gravity is an external force. But why is energy...

Homework Statement Why for a given constant force, in a study reference system, which point of application moves from A to B, the work of the force is conservative? Homework EquationsThe Attempt at a Solution The only thing I know is that if the angle ##(\vec{F},\vec{AB})## is acute...

A person of 60 kg is holding on a rope of 3m while standing on a the ledge of a building of height 7m. The rope is fixed to a point at roughly eye level 3 m from ledge. The person walks off the building and is swung in a vertical circle. If the person let's go at approximately the same height he...

Hi PF! Can someone help me understand why, when writing the continuity equation we write: $$\frac{\partial}{\partial t} \iiint_v \rho \, dv$$ instead of $$ \iiint_v \frac{\partial}{\partial t} \rho \, dv$$ I understand the two are not necessarily the same, but why derive it the first way...

If there is no net torque acting on a system total angular momentum of the system will be conserved as well as angular momentum of each body present in the system will be conserved. And if there are two bodies /two charges present as a system and one of them (let's say body 1 )produces torque...

i got in an argument with my physics teacher about a test question recently, and am still reluctant to abandon my logic. The question is a standard explosion problem, akin to this: A firework is placed in the midst of some motionless billiard balls. The firework goes off and the billiard balls...

Having problems with part (c) here, question is attached below in full. Homework Statement Using the equation 1/2 * Load * Displacement = Sum of (P^2 * L/2AE) From the past questions I have the following info, these are also included in the solutions so are accurate: A = 1x10^3 m^2 FAB =...

Homework Statement A 20.0-kg block is connected to a 30.0-kg block by a string that passes over a light frictionless pulley. The 30.0-kg block is connected to a spring that has negligible mass and a force constant of 250 N/m. The spring is unstretched when the system is as shown in the figure...

Homework Statement There are two problems: (A) Consider two identical billiard balls (spheres), each of mass M and radius R. One is stationary (ball 2) and the other rolls on a horizontal surface without slipping, with a horizontal speed v (ball 1). Assume that all the frictional forces are...

Homework Statement A solid sphere is set into motion on a rough horizontal surface with a linear speed v in the forward direction and angular speed v/r in the anticlockwise direction. Find the linear speed of the sphere when: a) When it stops rotating b) when slipping ceases Homework...

I was reading through some main stream scientific literature, and I came across Sean Caroll's "Energy Is Not Conserved" post. Essentially, he contends that through general relativity energy is not conserved, at least not in conventional manner of thinking about energy. Anyways, some portions of...

This had me thinking for a while. Imagine a photon emitted by a very distant object at a redshift of z = 2.0 for example. As the photon travels through space, due to space expansion the photon's wavelength will shift towards red. With an increase in the wavelength there must come a decrease in...

Definition 1: The expectation value of the observable related to the parity operator ##\hat{P}## is constant over time. That is, \frac{d}{dt}\langle P\rangle=0 \int\Psi^*(r)\ \hat{P}\ \Psi(r)\ dr=constant \begin{align}\int\Psi^*(r)\ \Psi(-r)\ dr=constant\end{align} Definition 2: If the...

I have a question. According to Noether's theorem, "For each symmetry of the Lagrangian, there is a conserved quantity." But soon I thought that I can also prove "For each conserved quantiry, there is a symmetry of the Lagrangian." Actually I can prove the second statement if I start prove...

Well this question has been in the mind since a long time. I believe that the law of conservation of energy is not true. If it is/was true then why would the universe expand and into what is it expanding ? obviously energy is created when the universe expands into "NOTHING". I will be waiting...

Consider a free particle moving in a general time-dependent scalar potential. Energy & momentum are not conserved. However, there is a symmetry in the lagrangian: the velocity appears only as its square, so we can rotate it without affecting the value of L. What conservation law results from...

Homework Statement [/B] A block of mass m is attached to a spring with a force constant k, as in the above diagram. Initially, the spring is compressed a distancex from the equilibrium and the block is held at rest. Another block, of mass 2m, is placed a distance x/2 from the equilibrium as...