# Does angular velocity of a body rotating on top of another rotating body add up or multiply?

I want to mount a small engine on a shaft of a large engine and rotate them both in the same direction, will the RPMs be a sum of the their RPMs or will the resulting number be a multiplication?

• Probably either addition, or failure. Can you include a diagram? Are you attaching the body of the small engine to the output shaft of the large engine? Does it mind being spun around? Feb 8 '18 at 17:42
• yeah thats the idea, to connect the output shaft of a large dc brushless engine to a custom mount which will hold tiny brushless engine and shaft of this small engine will rotate in the same direction.. Feb 8 '18 at 17:53
• In which case, addition. How are you planning to get a reliable electrical connection to the smaller motor? And, what is the design constraint that means you need this setup? Feb 8 '18 at 17:55
• I am trying to achieve extremely high rpms and I am hoping that the answer would be multiplication which would warrant further investigation as I believe this might be simpler than gears based solution for the angular velocity values that I am trying to achieve. The power would be transmitted either wirelesly or via brushed connection whichever would be easier at this stage. Feb 8 '18 at 18:00
• It's definitely not multiplication! Imagine you had a motor spinning in your hand at 1000rpm, and you decided to turn it over once every 30 seconds (2rpm). Would the output shaft be spinning at 2000rpm, or 1002rpm? Feb 8 '18 at 18:04