This discussion board is a place to discuss our Daily Challenges and the math and science
related to those challenges. Explanations are more than just a solution — they should
explain the steps and thinking strategies that you used to obtain the solution. Comments
should further the discussion of math and science.

When posting on Brilliant:

Use the emojis to react to an explanation, whether you're congratulating a job well done , or just really confused .

Ask specific questions about the challenge or the steps in somebody's explanation. Well-posed questions can add a lot to the discussion, but posting "I don't understand!" doesn't help anyone.

Try to contribute something new to the discussion, whether it is an extension, generalization or other idea related to the challenge.

Stay on topic — we're all here to learn more about math and science, not to hear about your favorite get-rich-quick scheme or current world events.

Markdown

Appears as

*italics* or _italics_

italics

**bold** or __bold__

bold

- bulleted - list

bulleted

list

1. numbered 2. list

numbered

list

Note: you must add a full line of space before and after lists for them to show up correctly

Suppose a stationary (relative to some frame of reference ) box explodes into two pieces in the vacuum of space. The net momentum of the system of particles before and after is zero, but the kinetic energy, being scalar, goes from zero to a posotive quantity, hence kinetic energy isn't conserved.

Decay. In the rest frame, initially there's an unstable particle with 0 kinetic energy and 0 momentum. After its decay into multiple particles, there still must be 0 momentum, but the particles each carry some kinetic energy.

think this scenario violates the law of conservation of energy: A ball is rolling twards a fan, witch provides a constant force on the ball, with some amount of kinetic energy. As the ball gets closer to the fan, its kinetic energy is converted to "fan potential energy". Then, when the ball's kinetic energy is copletly converted to potential energy, the fan turns off. An instantanious force is then applied on the ball, and the ball rolls away from the fan. Now, the energy of the ball is smaller than the energy it had at first, because its kinetic energy is essentially zero and its fan potential energy is constantly decreasing. Where does the energy go? oh, also, the surface the ball is on has no friction

Sure, whenever we have a case where the Lagrangian of a system isn't independent of time, while being independent of translation, i.e., not invariant with respect to time but invariant with respect to space.

Usually, the Lagrangian in any classical mechanics system is invariant with respect to time, i.e., doing an experiment now will produce the same results as "yesterday" or "tomorrow", but as the universe evolves, not necessarily over great spans of time.

Note: This reply was given before the question was modified to "conservation of kinetic energy". In that case, wow, there's lots of places where that "before" kinetic energy could get squirreled away into many other forms of energy "afterwards". Meanwhile, if the entire system is isolated, then its center of mass remains in uniform motion and thus "linear momentum" is conserved.

If you're going to go around saying I'm the Wikipedia of Brilliant, then I am going to have to add a caveat to this matter of conservation of kinetic energy. Given any closed system, with a fixed total mass and uniform center of mass motion, then the kinetic energy of the total mass as defined by the motion of its center of mass is always conserved. We complicate the issue by introducing other forms of energy such as heat or electric fields, so that the total energy of the closed system could be greater than the kinetic energy defined as above---and then we involve external influences (like the planet Earth!) so that it's not truly a closed system.

When doing physics, we need to pay particular attention to whether a dynamic system is truly closed or open. A lot of laws of physics depend on such a distinction.

Nihar stating that energy conservation is violated is not the correct way of saying what you probably wanted to.You should rephrase it so that what you intend is clear and more importantly an incorrect idea is not conveyed.

Is the law of conservation energy really violated i mean the enegy is still converted into a non recoverable form it is still energy. Being one of the 3 most fundamental conservation primciples i dont expect it to get violated.

Easy Math Editor

This discussion board is a place to discuss our Daily Challenges and the math and science related to those challenges. Explanations are more than just a solution — they should explain the steps and thinking strategies that you used to obtain the solution. Comments should further the discussion of math and science.

When posting on Brilliant:

`*italics*`

or`_italics_`

italics`**bold**`

or`__bold__`

boldNote: you must add a full line of space before and after lists for them to show up correctlyparagraph 1

paragraph 2

`[example link](https://brilliant.org)`

`> This is a quote`

Remember to wrap math in`\(`

...`\)`

or`\[`

...`\]`

to ensure proper formatting.`2 \times 3`

`2^{34}`

`a_{i-1}`

`\frac{2}{3}`

`\sqrt{2}`

`\sum_{i=1}^3`

`\sin \theta`

`\boxed{123}`

## Comments

Sort by:

TopNewestA spring block system. ......

Log in to reply

Perfectly inelastic collision

Log in to reply

Inelastic collision

Log in to reply

Inelastic collisions...

Log in to reply

Inelastic collision

Log in to reply

Suppose a stationary (relative to some frame of reference ) box explodes into two pieces in the vacuum of space. The net momentum of the system of particles before and after is zero, but the kinetic energy, being scalar, goes from zero to a posotive quantity, hence kinetic energy isn't conserved.

Log in to reply

Fluid flow over a fix plate and other plate is movable..

Log in to reply

Catching a ball

Log in to reply

Inelastic collision,the loss in kinetic energy becomes a form of internal elastic of the system or lost as heat !

Log in to reply

In an inelastic collision the K.E is not conserved but momentum is . (some energy goes in deformation due to in elasticity)

Log in to reply

Inelastic collision

Log in to reply

An example is Inelastic collision.

Log in to reply

Decay. In the rest frame, initially there's an unstable particle with 0 kinetic energy and 0 momentum. After its decay into multiple particles, there still must be 0 momentum, but the particles each carry some kinetic energy.

Log in to reply

think this scenario violates the law of conservation of energy: A ball is rolling twards a fan, witch provides a constant force on the ball, with some amount of kinetic energy. As the ball gets closer to the fan, its kinetic energy is converted to "fan potential energy". Then, when the ball's kinetic energy is copletly converted to potential energy, the fan turns off. An instantanious force is then applied on the ball, and the ball rolls away from the fan. Now, the energy of the ball is smaller than the energy it had at first, because its kinetic energy is essentially zero and its fan potential energy is constantly decreasing. Where does the energy go? oh, also, the surface the ball is on has no friction

Reference https://www.physicsforums.com/threads/does-this-situation-violate-conservation-of-energy.346004/

Log in to reply

Inelastic collision such as a car crash

Log in to reply

A vertical elastic collision would also work. Kinetic energy is converted into potential energy.

Log in to reply

That would work only if you include Earth itself as part of the dynamic system with a center of gravity in uniform motion.

Log in to reply

I hope it may be inelastic collision

Log in to reply

I hope it may inelastic be collision

Log in to reply

Inelastic and perfectly inelastic collision

Log in to reply

An inelastic collision between two bodies? Energy is lost as heat/used to deform the body but linear momentum is still conserved.

Log in to reply

Was weds

Log in to reply

It is better to say that mechanical energy is changed. It is in case of Inelastic and some oblique collisions.

Log in to reply

Sure, whenever we have a case where the Lagrangian of a system isn't independent of time, while being independent of translation, i.e., not invariant with respect to time but invariant with respect to space.

Usually, the Lagrangian in any classical mechanics system is invariant with respect to time, i.e., doing an experiment now will produce the same results as "yesterday" or "tomorrow", but as the universe evolves, not necessarily over great spans of time.

Note: This reply was given before the question was modified to "conservation of kinetic energy". In that case, wow, there's lots of places where that "before" kinetic energy could get squirreled away into many other forms of energy "afterwards". Meanwhile, if the entire system is isolated, then its center of mass remains in uniform motion and thus "linear momentum" is conserved.

Log in to reply

How do you have so much loads of information? You are the "Wikipedia of Brilliant" :P

Log in to reply

If you're going to go around saying I'm the Wikipedia of Brilliant, then I am going to

haveto add a caveat to this matter of conservation of kinetic energy. Given any closed system, with a fixed total mass and uniform center of mass motion, then the kinetic energy of the total mass as defined by the motion of its center of mass is always conserved. We complicate the issue by introducing other forms of energy such as heat or electric fields, so that the total energy of the closed system could be greater than the kinetic energy defined as above---and then we involve external influences (like the planet Earth!) so that it's not truly a closed system.When doing physics, we need to pay particular attention to whether a dynamic system is truly closed or open. A lot of laws of physics depend on such a distinction.

Log in to reply

Nihar stating that energy conservation is violated is not the correct way of saying what you probably wanted to.You should rephrase it so that what you intend is clear and more importantly an incorrect idea is not conveyed.

Log in to reply

Thanks. Edited.

Log in to reply

Is the law of conservation energy really violated i mean the enegy is still converted into a non recoverable form it is still energy. Being one of the 3 most fundamental conservation primciples i dont expect it to get violated.

Log in to reply

I think he means Kinetic Energy. I answered according to that

Log in to reply

Thanks. Edited.

Log in to reply

Any example of Inelastic collision is true.

Log in to reply

Yes , correct. Can you think of more examples?

Log in to reply