What is the object in the acceleration of an object practical? And why is it proportional?

This practical investigates the effect of changing the force on the acceleration of an object while keeping the mass of the object constant. Although the object is not just the toy car. It is the toy car, the string, and the mass on the end of the string because they are all attached. And this means if the mass is removed from the end of the string, it needs to be transferred onto the toy car. This keeps the overall mass of the object the same. As the resultant force is the weight of mass on the end of the string, the acceleration of the toy car would be proportional to mass on the other end of the string. This is because Newton’s second law of motion states the acceleration of an object is proportional to the resultant force acting on the object.