I got a rather small Netwon's cradle and when I start it, the effect is not very good since all the balls start to swing where the effect we want is obvious. The balls are small and I wonder if larger balls would make the swing more ideal, like only the outer balls appear to move and the 3 inner balls should be still? I know product recommendations are off-topic, but I'm leaning towards buying a cradle with larger balls and perhaps you can advice which alternative to get that best replicates the original ideal cradle where only the outer balls move and the inner balls appear still?
Newton's cradle fights air resistance and restitution (efficiency of rebound). You then want the highest density, hardest balls with the highest restitution. Cobalt-sintered tungsten carbide is magnetic. Polished hardened tool steel ball bearings are a good start.
Platinum plus gallium and indium alloys heat treat to a very hard and springy state, density around 19 g/cm^3 versus less than 8 g/cm^3 for tool steel (Steve Kretchmer, Niessing Co., Eastern Smelting; platinum SK(tm) alloys). For more shallow wallets, tungsten steel, thoriated tungsten.
95.5% Pt, 3.0% Ga, 1.5% In; 95.2% Platinum, 4.8% Ga, In, Cu; 1550-1650 C melt. 700 C for 30 minutes and slow cool to harden (not reducing atmospheres). Vickers Hardness 318/Rockwell A 76/Rockwell C 32. 125,000 psi tensile, 104,000 psi yield.
Here are some possible reasons the cradle may not work as expected. 1) Excessive friction at the support points where the strings mount to the frame, causing torsion to be passed thru the frame to other strings&balls. Similarly, the frame may be not sufficiently rigid. 2) The balls are not of uniform mass. 3) The balls' masses are not sufficiently greater than the mass of the strings, so the strings' masses can't be ignored in the momentum conservation equations. 4) less likely-- the balls are inelastic, leading to loss of kinetic energy. 5) less likely -- the balls are not aligned, causing the momentum vectors to wander off in 2 or three dimensions.