buckstar
I have 3 x Flywheels of equal dimensions but different weight eg a.400lbs b.800lbs c.2000lbs,which one will keep turning the longest? Or is there no difference?

Nessdude14
If they start at the same speed, and you assume that they all have experience the same friction, the heaviest will spin the longest. However, the heavier wheel is likely to experience more friction than the lighter wheels.

Techian
Assuming zero friction, all three flywheels will continue to rotate forever, in accordance with the law of conservation of angular momentum.
But when equal amount of friction is assumed to act on all three flywheels, then the flywheel with maximum rotational kinetic energy will rotate longer.
Rotational kinetic energy=(mass moment of inertia * (angular velocity)^2)/2
Notice that the formula for rotational kinetic energy is similar to kinetic energy formula in linear motion.
Mass moment of inertia is the rotational counterpart of mass(inertia) in linear motion,
Mass moment of inertia = mass * (radius of gyration)^2
Radius of gyration is a measure of how far the mass is distributed away from the axis of rotation.

pa5tabear
Assuming the surfaces experiencing friction were the same (same frictional coeffiecient?), how much would the actual rotational friction differ due to different masses?

Nessdude14
Assuming the surfaces experiencing friction were the same (same frictional coeffiecient?), how much would the actual rotational friction differ due to different masses?

Friction is directly proportional the normal force. This means it's also directly proportional to mass, so doubling the mass will double the friction.