tennis ball with elastic string and computer string it can use in computer test area,making tension test Tension force:custom ... . Item nameTennis ball with elastic string and computer stringMaterialPolyster/LatexDiameter0.45mm-0.5mm ColorBlack (we can ... Cotton 20/1, 20/2, 20/3, 30/1, 30/2, 30/3, 40/1, 40/2, 40/3, 50/1 ... , 50/2, 50/3 etc Nylon ...