And that is correct. All objects within a gravitational field have gravitational potential energy. Gravity grants these objectenergy, but the object does not move as long as it is supported. So that is potential energy. When the object actually moves, that is kinetic energy.
Energy is never created nor destroyed, so the kinetic energy can’t come from nothing. Gravitational kinetic energy must come from gravitational potential energy that has been converted.
This all seems quite confused, but I am having trouble even parsing what you are saying to correct it.
For starters, what do you mean by stored kinetic energy if not potential energy?
If I place a 10 kg rock onto platform 1 meter high (no matter how I do it), we can say that the rock has about 100 Joules of potential energy (if we want to be careful maybe we’d say the rock-Earth system has 100 Joules of potential energy) and 0 Joules of kinetic energy. This will take about 100 Joules of work by either me or the machine. If the platform breaks, then the potential energy of the rock will be continually converted to more and more kinetic energy as it accelerates downward, and just before it hits the ground it will have 100 J of kinetic energy and 0 J of potential energy.
Until the earth is a perfectly smooth glassy sphere, there will be parts of it that have the potential to fall and so have potential energy. The work to raise that stone or water or human may have been plate tectonics or other geological activity, wave action, sunlight evaporation followed by condensation - but to get to that point where it can fall, work was done to raise that mass. It may have been infinitesimal and slow, but it did happen.
As others say, it may be more precise to say the earth-boulder system has potential energy, until that boulder falls and it becomes kinetic.
Similarly a spring takes work to get compressed - when compressed it has potential energy; when released, that becomes kinetic energy. Ditto for a bow - it takes work to draw it, and like a spring, it has potential energy as the force to make the bow (wood? Fiberglas? Steel?) being bent. Release it and that spring energy is translated to momentum of the arrow.
Ball bearings might reduce friction from 200 N to 100 N (or some other lower value), but they still won’t let you exert 200 N. And the force output by a machine could be used for any task, even one for which friction is already negligible: I can’t draw an English longbow by myself, but I could make a lever that would let me do so, or an inclined plan, or a hydraulic ram, or any other simple machine. But ball bearings, no matter how I used them, wouldn’t let me draw that bow.
In most presentations of “simple machines” I’ve seen, they’ve been in the “physics problem world” where no (dynamic) friction exists. In that world, ball bearings don’t provide any benefit, since pushing a heavy object across a frictionless floor is just as easy.
“Work” using the physics definition, is Force times the distance the force is exerted over (i.e. W = FD). A simple machine is just something that allows you to do the same Work using a lower force, but a longer distance.
For example, using an inclined plane to lift something means you have push the thing farther (the length of the inclined plane, rather than straight up) but you can use less force to do it, so the amount of work remains the same.
(This thread gave me the excuse to indulge in some nostalgia and track down some videos on the topic that used to play on TVOntario. The whole series is gold.)
I would say that a load on a floor of ball bearings, or log rollers, etc. is just a rudimentary form of the wheel. The “Aha” moment with the wheel is affixing an axle so you don’t need a huge ongoing supply of circular assistance under foot to keep your load moving.
But a wheel used in that way isn’t a machine.
But the wheel in that sense is merely a friction-removing device, which isn’t a “machine” in friction-free physics world.
The wheel in the “simple machine” sense is something like an olde-timey ship’s wheel where a small force applied to the outer part of the wheel is converted to a large force on the tiller ropes (applied over a shorter distance) because the tiller ropes are connected closer to the centre of the wheel.
EK=mv2
“v” is speed
For an object in tension or compression, v=0, which means that the kind of energy stored in a bow or spring is quite literally not kinetic energy (the energy in the motion of a thing).
The bonus of a wheel as a machine is the axle. So, unlike roller logs or a floor full of ball bearings, you do not have to keep adding something in front to keep moving an object. A room full of ball bearings to me is like a bunch of bricks. I can lift an object a short distance, shove a brick under it, rest, lift some more, add a second brick, etc. but I would not call a bunch of bricks a simple machine. It’s not self-contained, it does not amplify labour.
It’s not storing kinetic energy. It’s a bookkeeping thing. If you change potential energy by moving up or down through gravity that energy goes or comes from somewhere. It could be speed, heat, deformation of an object, etc.
If you can lift it at all, that is. For a sufficiently-heavy object, you won’t even be able to lift it a little, to get that first brick in.
On the other hand, you could have an object that you can’t lift completely, but can lift one end up while the other is staying on the floor. So you lift up one end, slide a brick in, then lift the other, and repeat. In this case you are benefiting from a machine, but it’s still not the bricks: You’re using the object itself as a lever.
@Chronos - can you please explain the ball bearing part:
Imagine I have a box on the floor pulled by a rope . And say I need to apply 100N on a rope for the box to overcome static friction and start moving. If I grease the floor (at the chemical level, grease is nothing but small ball bearings), I can now move the box with say 50N. Why is the mechanical advantage of 100N —> 50N not considered a machine ?
Because it’s not changing the force you can exert. Your 50 N is still 50 N. You’ve managed to decrease the force required, but only in that specific situation: If the ball bearings truly had a mechanical advantage, you could use it for any situation requiring a force.
Plus, we can use my example - it’s not self-contained. In order to keep moving the box, you need to continuously add more and more grease in front. With a cart and axle, it will continue to roll as you pull, all across the wind-swept plains. (and other planes).
