Three Cheers! for “shut up and Calculate”: sns.ias.edu/sites/default/files/files/ThreeCheersShutUp.pdf

by N. Arkani-Hamed

@ZeroTheHero that anharmonic oscillator helium thing is very cool, need to go through the history of this properly

Not much to do

it's not against the rules to be a bad reviewer

I've looked everywhere and I still can't explain to myself a few things regarding flywheels. So in an engine, the flywheel is connected to the crankshaft and one part of the flywheel can spin freely, right? Meaning, if the engine suddenly stopped, a part of the flywheel will continue to spin?

If that's correct, then how and what connects the two parts of the flywheel (the freely spinning one and the one that holds tight to the crankshaft)?

Also in a bycicle, the same thing there, right? We pedal which rotates some gears and that rotates a flywheel which connects to the big back wheel?

@JingleBells I'm pretty sure the flywheel is usually directly connected to the crankshaft. If the engine suddenly stops firing, the crankshaft should still spin because the flywheel has fairly high mass and radius, so it has good rotational momentum which keeps it spinning. This also smooths out the transmission in general, since pistons firing isn't actually that smooth for rotation (especially if you have less cylinders).

@JingleBells I don't think bicycles typically have flywheels, though you could make the case the tires kinda act as flywheels, especially with freehub/freewheel designs where you can stop pedaling and the bike keeps moving. You could probably put large radius weights on your tires if you wanted to make it harder to get going, but also take longer to lose speed.

@JMac Yes, so the flywheel has two parts, one that is connected to the crankshaft and one that is free to spin? My question is, how do the two parts connect so the rotational energy can get transferred to the rest of the stuff?

@JingleBells On a manual transmission that's typically the clutch. It presses against a disc on the flywheel to gain it's own rotation through friction typically, then you can release the pressure and the flywheel would stop being coupled with the shafts that lead to the wheels. Automatic transmission is apparently pretty cool, and they don't even use flywheels. It's often a rotating fluid coupling the two shafts with a torque converter, and there's a flexible plate to connect to the starter.

@JMac I'm confused... I'm struggling to understand what the flywheel is and how it works.

Is a flywheel just a big circle weight that's connected to the crankshaft and that's it? If that's so, then if the engine stops, the flywheel will keep it rotating for a while even with no gas?

Hmm I think I start to get it now

I thought the flywheel had two parts one that's connected to the shaft and other that is connected to the rest of the stuff and they can slide together but get locked when the engine spins the crankshaft faster than what the freely moving part of the flywheel is... but I guess that's wrong

I think flywheels are just plain old dumb weights to with good momentum and inertia to smooth things out

If that's so, I have a question about bikes and about how the spinning of the back wheel works. What is that clicking sound that happens when your stop rotating the pedals but the wheel keeps going

Crocodile Dundee voice: That's a flywheel.

@FadedGiant No, that's complicationism.

ok looks cool but still no idea about what it does and how it does it

I will just guess that the flywheel is a circulish weight with good momentum to keep things smooth and so on. What I'm really struggling to understand is the thing with bikes

When we spin the pedals forwards and when the rotating thing at the back reaches and/or exceeds the speed of the back wheel, the two parts connected and we can go faster, but otherwise, they are disconnected... how?

The picture shows a 9000 kW motor on top of a reactor coolant pump. In the event of loss of power, coolant flow is maintained by pump coastdown due to the inertia provided by the flywheel.

@JingleBells On bikes it's pretty different. What you're thinking of has two types as far as I can find, Freehub and Freewheels. They have ratcheting mechanisms in them to keep the wheel spinning if it goes faster than the driving axle. The clicking you hear is the ratcheting mechanism when it's sliding over, then when the driving axle goes faster the ratchet locks and provides torque to the wheels.

Regarding the flywheel - youtube.com/watch?v=reh79gYsaQE this solved all my confusion

@JMac Got it, thanks :-)

So here, 1 is connected to the wheel side, where 2 is connected to the chain/driving torque side. You can see that if 1 goes faster, it's going to make a lot of clicking as 2 pops into the slots, but if 2 is going faster, it locks in and makes 1 go as fast as 2.

just thought this drawing made it really intuitive how they do it, so I felt like sharing.

Cool thanks

I find it cool because it's really pretty simple, but also works really well to keep the wheels going without having to always pedal.

Yup, a simple genius mechanism

How do we distinguish local inertial frames in GR? In SR we can distinguish two frames by their relative velocity, but in GR tidal forces make this more complicated since for instance the gravitational field around the Earth admits an infinite number of LIFs