Hello everyone, new poster here and new to hands-on engineering as well, so please be gentle!
I would really appreciate some help in selecting a suitable motor. I have a rapidly spinning cylinder that needs to change speeds very rapidly, meaning it must go from 4000 rpm to 2000 rpm in 0.01 s. It is critical that the cylinder does not oscillate during this process.
The cylinder would be driven by a rotary motor and the two are connected by a belt drive. The inertia of the rotating system is approximately 0.001 kg m2 (10,000 g cm2).
Does anyone know of any specific motors that could meet this task and if any of them are waterproof/splashproof or otherwise water resistant? The cylinder will be submerged and I would like to minimise the length of the belt drive.
Alternatively, could anyone share some advice on exactly what to look for in terms of motor type (e.g. AC vs DC, stepper vs servo) and specs (e.g. what terms to look for in the technical specifications) that will give me the information I need to work out how fast the motor can change speeds?
After my initial research, it seems that DC motors are better for speed control, servos in particular for speed changes (using PWM) and I will need one with an encoder so I can keep track of the rotational speed. I am not 100% sure about all these things though so I would appreciate confirmation/correction.
Also, is it ok to rapidly brake the motor (i.e. 2000 rpm to 0 rpm in 0.01 s) using an externally mounted brake or would this damage the motor in any way?
Thank you in advance for your help 
Anjali