Final answer:
The commonly used variable force causing a change in blade angle is related to aerodynamics in systems like wind turbines and to mechanical dynamics in applications such as doors or plates. Aerodynamic adjustments are made in response to factors like wind, while in mechanical systems, it is torque that often governs changes. Viscosity, angular velocity, and Newton's second law of motion are also relevant to understanding how these forces operate.
Step-by-step explanation:
The most commonly used variable force that causes a change in blade angle, working opposite to fixed force, is related to rotational dynamics and the principles of fluid mechanics. In the context of wind turbines and helicopter rotors, this force is primarily a matter of aerodynamics, whereas for systems like revolving doors or spinning plates, it is more about mechanical forces and torque.
In the case of aerodynamics, the blade angle or pitch can be adjusted (as mentioned in the concept related to long airplane wings) to change the interaction with airflow and therefore modify lift and efficiency. This adjustment is often made in response to wind speed or the need to control the rotational speed of the blades. Similarly, pushing a lawnmower involves dealing with forces and components of force as described, with friction and other factors influencing motion
Understanding the concept of viscosity and how it affects forces in fluid layers, as explained using the example of two plates moving relative to one another, is also essential in grasping how variable forces can impact mechanical systems. In helicopters, whether the force exerted by the blades is considered internal or external is pertinent to analyzing the system's dynamics.
The notion of angular velocity change, as when spinning a bike wheel, involves a different kind of variable force. Here, the force needed is dependent on the distance from the pivot and the mass, relates to rotational analogs of Newton's second law of motion, and is important for understanding how systems can be controlled and modified through their interaction with external forces.