Final answer:
The initial acceleration of a model rocket is primarily due to the thrust force created by the expulsion of fuel which overcomes gravitational pull.
Step-by-step explanation:
The initial acceleration of a model rocket fired vertically upward from rest is primarily due to the thrust force generated by the expulsion of fuel. When a rocket is launched, the gases expelled from the rocket's engines create a reaction force (thrust) in accordance with Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. The expelled gases exert a force on the rocket, propelling it upward. This thrust force is what overcomes the gravitational force pulling the rocket down and is responsible for its initial acceleration. The other factors listed, including gravitational pull, air resistance, and inertia, also play roles in the rocket's motion, but they do not primarily cause the initial acceleration. The gravitational pull acts to resist the rocket's upward motion, air resistance opposes the rocket's motion as it travels through the atmosphere, and inertia is the resistance of any physical object to change in its state of motion or rest. However, it is the thrust that gives the rocket the necessary acceleration to move upwards initially.