# Understanding net torque from an actuator

This will probably sound like a silly question, but here goes....

Let's say I have an actuator, with a torque rating of 1 Nm. I then connect it to a wheel, which starts rotating at a constant speed. From Newton's second law, I know that Net Force = Mass * Acceleration, or in this case, Net Torque = Mass * Angular Acceleration. Given the input torque of 1 Nm, this would mean that the wheel would have a net torque of 1 Nm applied to it. Therefore, the wheel should experience an acceleration of 1 Nm/mass. However, the wheel doesn't keep on accelerating -- it just rotates at a constant speed. So why doesn't the wheel accelerate, when the actuator is applying a fixed torque?

• Are you trying to understand the theory behind an actual device?
– Wasabi
Jun 9, 2016 at 17:51
• Well I'm trying to relate Newton's second law to the real-world behaviour of an actuator. It doesn't make sense to me to say that an actuator applies a torque of 1 Nm, but then the device it is connected to just rotates at a constant speed... Jun 9, 2016 at 17:54
• The constant acceleration assumes no friction, while in the real world you do have friction, which often a function of velocity. Jun 9, 2016 at 18:53
• Also worth noting acceleration of rotation is dependent on moment of inertia not mass per see even a small mass gan have a huge inertia if far away from the center. Jun 10, 2016 at 9:50
• Net Torque = Mass * Angular Acceleration can't be true. It's not homogeneous. Torque = moment of inertia * Angular Acceleration. Jun 10, 2016 at 12:23