A force exerted on an object is always due to:

1. Forces are not properties an object carries around by itself. 2. A force exerted on an object is always due to the interaction of that object with another object. 3. Forces that systems exert on each other are due to interactions between objects in the systems. 4. That is why every force can be traced back to a specific partner object. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 4 'Dynamics: Force and Newton's Laws of Motion', sections 4.1-4.8_