# Tagged Questions

2answers
142 views

### Perpendicular Elastic Collision (different masses, different velocities)

I'm stuck on a mechanics problem and I can't make any headway past momentum and kinetic energy being conserved. Here is the problem: Two hover cars are approaching an intersection from ...
5answers
172 views

### Why is momentum conserved (or rather what makes an object carry on moving infinitely)?

I know this is an incredibly simple question, but I am trying to find a very simple explanation to this other than the simple logic that energy is conserved when two items impact and bounce off each ...
2answers
78 views

### Ice skater increase of energy

This may be a very basic question but I am not seeing how it works. Consider the standard example of an ice skate rotating about his/her center of mass and pulling in his/her arms. The torque is zero ...
2answers
199 views

### Can a neutron be created from pure energy

Is it possible to create a neutron out of pure energy, i.e. not by bringing a bunch of already-existing quarks together? (A quick calculation using E = mc2 shows the energy required would be about 1.5 ...
1answer
134 views

### Mathematical question on Collisions [closed]

A 2.5kg ball travelling with a speed of 7.5m/s makes an elastic collision with another ball of ...
1answer
181 views

### Tricky Conservation of Momentum problem: find the ratio of the carts by mass percentage lost [closed]

A wagon is coasting at a speed $v_A$ along a straight and level road. When 42.5% of the wagon's mass is thrown off the wagon, parallel to the ground and in the forward direction, the wagon is brought ...
3answers
746 views

### On what basis do we trust Conservation of Energy?

I'm happy to accept and use conservation of energy when I'm solving problems at Uni, but I'm curious about it to. For all of my adult life, and most of my childhood I've been told this law must hold ...
2answers
4k views

### Why can't energy be created or destroyed?

My physics instructor told the class, when lecturing about energy, and that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
3answers
236 views

### Energy non-conservation for time-dependent potentials

Written in a book I read that the "total energy is not preserved when the potential depends explicitly on time", i.e. $U=U(x,t)$. Is there any proof or explanation for this?
3answers
278 views

### Is the continuous transformation of energy from one form to another one free? Or it consumes some quantity of who knows what?

Sorry for the unclarity. I probably created some bias in your mind having tagged my question with energy-conservation and conservation-laws. I simply consider that, relatively independently of ...
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