If an alternator/generator is temporarily disconnected, will it still be difficult to turn the shaft?

If the connections to the alternator were temporarily cut then would it still produce electricity? Would the forced required to spin the shaft significantly decrease?

in other words, is there a way to turn a generator ON or OFF even while its shaft is still spinning? (like an aircon compressor in a car where turning it off would lessen the load on the car engine)

If the alternator is open circuited, then no electrical power can leave the machine. If no electrical power can leave the machine, then the full mechanical power won't be taken in on the engine side. (This makes lots of simplifications for automotive applications only...)

There will still be mechanical friction and windage losses due to spinning the rotor, but these will be small compared to total machine power rating.
If you want to install a clutch, you can reduce off-time losses, but the easiest thing will be to switch open the alternator.
WARNING: Some alternators will produce excessive overvoltages and fail if they are open circuited. This is especially true of high-rpm motorcycle alternators. Be prepared to replace the alternator if you alter the charging circuit.
Just curious - What are you trying to do exactly?

EDIT - your additional details describe the main benefit of a hybrid. Energy conversion from Kinetic back to chemical potential in an efficient and reliable way. This is the trick that has all of the auto manufacturers frantically researching right now. You have a long way to go before you get a usable configuration. Keep trying though, just be sure not to hurt yourself in the process, i.e. get someone else to do the testing...
If you remove the load, the alternator or generator shaft will turn easily. ~
The voltage would still be at the output leads, but no current would be flowing.
the easiest way to disconnect the alternator is to loosen the belt

problem is most cars these days have a single belt that goes from engine to alternator, to air conditioner, no way to disconnect it while running.

An older (80s to 90s) VW has a single belt that goes from the engine to the alternator

The belt is what drives the alternator. By disconnecting you are not reducing the load on the engine any significant way.
It would appear that those who have answered did not completely read your question. Heinz M did an okay job, but missed your total point.
If the load is disconnected, the load on the machine spinning the "generator" will significantly decrease. Since the horse power from the driving source has a bearing on the watts, or power produced, a no load condition on the "generator" means that the horse power load also drops, provided there is a feed back system to tell the driving source that the electrical load is gone. If not, then there will be full voltage output of the "generator" at it's output terminals.
To answer the rest of your question, the exciter voltage for the magnetic field from which the "generator" makes it's power anyway, by cutting those lines of force, can be turned off. Most modern "generators" use the rotor as the magnet, and the stator coils are where the electricity is produced. There are a couple of points that need to be mentioned, the first is residual magnetism, magnetic force that is always present, would still cause electricity to be produced, even if the exciter voltage is turned off. The other point is that some "generators" are self magnetizing, this means that they are self contained for producing the magnetic field needed to produce electricity. It is common practice to shunt the output of a "generator" to a load bank to absorb any residual power that it might still be producing after it is taken off line. I hope this information helps out.

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