## energy and force

This is all an unfortunate result of the popular media and high school teachers using the word energy where it does not belong, people get the idea that it's some galactic currency that gets things going.

Energy is just a convenient quantity that is time independent in closed systems.

 Quote by Bandersnatch Another example for constant energy despite acceleration is any body in free fall in a gravitational field - it merely exchanges potential energy for kinetic energy, but the total remains the same.
There is a problem .
For free falling in gravitational field we have a system not a object .
When you moving up a box from earth you must expended energy and because of this total mass of system increase . but total mass of earth and box remains constant .

This expended energy stored in gravitational field and after that the gravitational potential energy applies force to object and give it kinetic energy .
So please give a example that we have accelerator but object don't get energy .
Your example was a system involves two objects and gravitational filed between them .

 Quote by DaleSpam No, not necessarily. The object in motion could have been created already moving at high speed. It will then continue to move at high speed without ever having work done on it.

Yes . when we give energy to photon it never changing it's speed because C is limit speed in universe and when a black hole apply force to photon it never get accelerator because it's moving at C . but it's not true for object's that they move slower than light .
It never violate What I said .

 Quote by DaleSpam Look, your idea is simply wrong on multiple levels: 1) energy does not need to be expended to create a force 2) force is not required for velocity 3) energy is not required for acceleration
Let me give another example :
Consider a man wants to pull a box along a concrete floor . the box has a mass of 500 kg and the coefficient of friction between the box and the floor is 0.2 and g = 10 m/s2 .
Now if he applies 700 N on box , box produce 700 N in opposite direction
if he applies 800 N on box , box produce 800 N in opposite direction
if he applies 900 N on box , box produce 900 N in opposite direction
if he applies 1000 N on box , box produce 1000 N in opposite direction
in this case "net force" is zero and man doesn't do work .

According to what you said If we have a engine that applying 600 N on box in a day , the engine doesn't need energy for do this during this time .
Is it correct ?

But when he apply 1001 N on box , box starts to moving and gets accelerator and energy and now we expend energy .

So i say when we have non-zero "net force" that "act" on a object the force always do work and for produce a non-zero "net force" you must expend energy .
I know when theta be 90 degree work doesn't done but in reality there is example that "net force" be vertical and object moves in horizontal ?

Thanks

Mentor
 Quote by big_bounce Yes . when we give energy to photon it never changing it's speed because C is limit speed in universe and when a black hole apply force to photon it never get accelerator because it's moving at C . but it's not true for object's that they move slower than light . It never violate What I said .
The example I gave was for muons which are massive particles that travel slower than c and do, in fact, violate what you said.

And you returned to your refuted personal theories as I warned you not to do. Thread closed.