Final answer:
The magnitude of air resistance (Fr) when a car moves at constant velocity can be assumed to be balanced by the car's engine's forward force. Without further details on separate friction forces, air resistance might be the same as the engine's force, but the specific number cannot be precisely identified without additional information.
Step-by-step explanation:
To find the magnitude of air resistance (Fr) on a midsize car when velocity is constant, and given a force of friction of 250 N, we can assume that the force needed to maintain this constant velocity equals the resistive forces acting on the car. Newton's second law of motion, which states that the net force on an object is equal to the mass (m) of the object multiplied by its acceleration (a), gives us the basis for our calculation.
Since the car is moving at a constant velocity, its acceleration is zero. Therefore, the net force acting on the car is also zero. This means that all the forces acting on the car are balanced. The engine of the car exerts a forward force to counteract the resistive forces, which include both the friction between the car's tires and the road and the air resistance.
Considering that the student's initial question doesn't provide enough details to directly calculate the air resistance, we proceed with our own scenario. If the net force is zero and the force exerted backward by the tires is 2100 N, and the combined force of friction and air resistance is 250 N, then the air resistance can be deduced if we know the friction part separately. Unfortunately, the question doesn't separate these forces for us.
In a scenario where only air resistance is the force opposing the motion of the car, if we take the backward force to be equal to the forward force exerted by the engine to maintain constant velocity, then the magnitude of force caused by air resistance would be the same as the force the engine exerts, which is 2100 N in our scenario.