Two large masses joined by a string are suspended over a light, low-friction pulley. When an additional small amount of mass is added to one side the system clearly exhibits uniformly accelerated motion.

Prepared demos

Find a pulley, ideally with good bearings, and set it up using rods. Use a light string to hang masses from either side. I like to use the hook for hanging the massive disks.

PIRA DCS 1G10.40