calculating velocity of pneumatic cylinder with force as input (not acompressed air)

phneoix

Hi,
i want to use a pneumatic cylinder for slowing down the motion of an assembly. There will be no input supply of compressed air. The force will be acting directly on the piston. The air inlet end of cylinder will be open to allow air to escape. I would like to know how do i calculate the velocity of the piston relational to the input force applied (if possible in SI units).

Thanks