Why is the force to double acceleration not double the applied force?

Which of the following is true about an object in free fall? its acceleration depends on its mass. it has no inertia. it pulls on earth, and earth pulls on it. its momentum is constant.

Two steel balls, each of mass m, are welded to a light rod of length l and negligible mass and are initially at rest on a smooth horizontal surface. a horizontal force of magnitude f is suddenly applied to the rod as shown. determine (a) the instantaneous acceleration a of the mass center g and (b) the corresponding rate at which the angular velocity of the assembly about g is changing with time.

Scientific models are based on current knowledge, which can limit their effectiveness when new discoveries are made.