Where do you get those 9000 mph from? as far as I can see the edge velocity will be 65.45 m/s or 236 km/h: Fill in the blanks, m/s: v (in m/s) = pi * D * r

Where do you get those 9000 mph from? as far as I can see the edge velocity will be 65.45 m/s or 236 km/h: Fill in the blanks, m/s: v (in m/s) = pi * D * rpm / 60 (D in m) 3.141593 * 0.01 * 125,000 / 60 = 65.45 (m/s) or, in km/h v (in km/h) = pi * D * rpm * 60 / 1000 (D in m) 3.141593 * 0.01 * 125,000 * 60 / 1000 = 235.62 (km/h) These values are valid for a disc with a 5 mm radius. If that '5 mm' indicates the diameter the values would be halved to 32.72 m/s or 118 km/h.