I never understood that designation. The rolamite and it’s relative the scrollerwheel just seem like applications of the wheel to me.
I can see how a simple wheel is an application of the lever. But what about ball bearings? Each one can be seen either to have no axis or to have infinite possible axes. Does that have any bearing? 
A ball bearing isn’t a lever, but it also isn’t a machine (in the sense that it has no mechanical advantage).
And the key difference between a rock and a hand is not that the rock is harder, but that the rock is more massive: You can put energy (in the form of kinetic energy) into a rock over a long distance (thus using relatively low force), but then get that energy out over a short distance (thus exerting a much greater force). If you don’t like the rock-and-hand example, then consider one of those pieces of construction equipment that drives a pole into the ground by repeatedly lifting and then dropping a weight: Sure, the weight is probably lifted via some sort of pulley arrangement (thus, the whole thing is a complex machine), but it’s a lot more effective to have the cable pulling it up and dropping it than it would be to have the cable pulling down. That extra effectiveness is the mechanical advantage of the hammerhead.
I guess I would say that rocks, hammerheads, and fists are all alike, the differing size, weight, and hardness is irrelevant. These are like strings and rails and sticks, components used in machines, even simple machines. Whether they are considered machines themselves doesn’t matter, they just have a different nature.
I don’t know what to call a drop hammer that just uses gravity to do its work. Obviously a machine is used to lift the weight, but it seem like the principle is the same as a spring where energy is stored in the weight by lifting it.
And a spring must be some kind of simple machine to explain how a bow and arrow works.
A bow and arrow stores kinetic energy in the bow when it’s flexed, like a spring when it’s stretched or compacted.
Does a weight lifted and then dropped store any energy? Can you store gravity?
And a ball bearing reduces energy lost to friction. Isn’t that an advantage?
You create potential energy by raising the weight. You release that energy by dropping it.
I don’t think there’s a simple definition for simple machines. I think machines are made of a number of different kinds of parts that have specific functions. Some of those parts are rocks and sticks, and non-rigid sticks called ropes. Some of those rigid sticks combine with a fulcrum to create a lever. Some of the levers are circular. Inclined planes redirect motion, and if wrapped around a stick become a screw that translates rotation to linear motion with a mechanical advantage. A free wheel on an axle can redirect rotation into linear motion through a curved inclined plane. When a wheel is fixed to an axle it’s a fancy lever when attached to a fulcrum.
I just don’t see a simple common explanation. We have discrete components of machines and other components made of multiple sub-components. At least for me, it’s not clear when components become a machines.
This doesn’t make sense to me. The energy stored in a lifted rock would amplify the drop caused by gravity, because the gravity is a given, so any other energy would be in addition to that. A spring or bow mechanism would obviously do so because that’s where the energy would be stored. But dropping something with only gravity would show no acceleration beyond gravity itself, with zero energy added. When you lift a rock the kinetic energy is stored in your arm; simply opening your fingers and dropping the rock would not transfer any of that energy to the rock. It would be released as the equal and opposite upward movement of your arm. Gravity is an external force that provides its own energy.
I mean, right?
I don’t know how to explain potential energy properly, but if you raise a weight up you can get back the work you put into raising it when it falls by hitting something with it, or letting it push down one end of a lever or tying a rope to it and pulling on something. I don’t think a spring is significantly different in regard to this discussion.
It’s certainly a practical thing to do. But “mechanical advantage” is a technical term here. It doesn’t just mean “a practical thing done by a contraption”.
I understand that, which is why I asked the question. A block and tackle offers an advantage by reducing the amount of energy required to lift a weight. A ball bearing reduces the amount of energy required to move one surface against another.
What’s the actual difference?
You insist upon your point without addressing mine. How is gravity “stored energy”? How is any such stored energy part of gravitational pull if it adds literally zero inertia or momentum?
I didn’t say gravity was stored energy, that is some concept of yours. However, this is an example from a number of sources that define potential energy and speak of energy being stored in an object:
" To summarize, potential energy is the energy that is stored in an object due to its position relative to some zero position. An object possesses gravitational potential energy if it is positioned at a height above (or below) the zero height. "
A machine doesn’t change energies; it changes forces. Let’s say that your muscles are strong enough to exert a force of 100 N, but that you need to accomplish some task that requires 200 N (for instance, lifting a heavy object). No amount of ball bearings will let you accomplish that. But a lever, or a ramp, or a set of pulleys, or many other contraptions, might.
The biggest difference between a rock and a hammer is that a rock lets you use your arm to drive the nail or spike while a hammer allows you to add wrist motion, thus recruiting all you forearm muscles as well as you biceps. Not to be ignored in the equation is shock transmission to one’s arm from the hammering stroke, which is close to 100% with an effectively-used rock but much lower with the hammer.
Those numbers arbitrary, and limit the definition to mechanical means of at least doubling the energy. Ball bearings reduce the amount of energy spent. Surely that’s enough toret any definition.
I don’t accept that. By that definition all objects at all times are storing potential energy, be wise they would drop if the6 lost support.
When you drop a rock, 100% of the energy is provided by gravity. Zero energy is provided by stored kinetic energy. Any kinetic energy expended in lifting a rock would be stored in the mechanism that lifted it; zero kinetic energy is stored in the rock.
Physics 101: when you raise a mass in a gravitational field, you are exerting a force counter to the force of gravity and doing work. Simply by being higher, the mass has stored (potential) energy. When you let it fall, the force of gravity converts that potential to “kinetic” energy.
You don’t need to use gravity to hammer something, you can hammer upwards or sideways. I’m not sure there’s a simple machine function there, you are just exploiting the effect of angular momentum - by rotating a mass about a radius, vector product r x p where r is radius, p the momentum of the mass - angle and radius substitutes for distance travelled. However, it does give the mass velocity which when it strikes something, the actual momentum can be imparted. By increasing the radius (long handle) or the angle travelled (more wrist action) as well as increasing the mass, one creates more momentum to drive nails harder.
A ball bearing is simply a special design shape of a roller bearing, which is effectively an exploitation of the simple machine “wheel”. The purpose of a wheel is to replace surface-on-surface slide friction with rolling (or “wheeling”) which is significantly less wasted energy and wear. A bearing piece can be anything from a cylinder to an ovoid to a sphere - it makes no difference, they only roll on one surface track along the circumference; spheres are most common because they (or ovoids) can be placed in concave tracks (races) which keep them “on track” so to speak. Cylinder roller bearings risk wearing the flat ends against the edges of the race. Spheres are easier to manufacture and assemble into bearings than are ovoids since orientation during assembly is unimportant
I think this is the best descriptor of a simple machine. .
0% of the energy that drops a weight to the ground is stored kinetic energy. Stored kinetic energy, like in a spring or a bow, adds to gravitational pull and will accelerate it beyond simply falling. Simply being dropped has no added energy, and gravitational pull is not the same as kinetic energy. Any stored kinetic energy from lifting a weight is stored in the device which lifted it, and not in the weighted object itself. We know this because zero momentum or acceleration is added to an object that has been merely dropped. Otherwise, you’re saying that gravity had no effect on an object unless it’s in freefall, and is irrelevant to an object held to the ground by gravity. Not to mention that objects would fall at different rates depending on the mechanism that lifted them.
Say there’s a stone on a beach. You dig beneath it, scooping out sand with your hands. The stone would make its way downward as you dig. You’re exerting energy to move the sand, but none of that energy is stored in the rock. It is subject to nothing but gravity, plus a little friction.
What if the 200 N task is pushing a heavy object? Pushing it along a floor covered with ball bearings would certainly make it much easier.