I ran into trouble again with a course about drag force on skiing. This is the question, and its explanation:
Which of the following factors would not affect the magnitude of the drag force on an object moving through the atmosphere?
Explanation
Correct answer: Weight of the object
Drag forces in the atmosphere result from air molecules colliding with a moving object. The magnitude of the drag force depends on the speed and shape of the object, and the density of air.
If the speed of the object is larger, the object collides with more molecules each second. More collisions increase the total drag force. For similar reasons, the force increases in denser air. Density measures the total mass, which is related to the number of air molecules, in a volume of air.
The shape of the object also affects the drag force in two ways. First, whether the surface of the object is rounded or has sharp edges, or is smooth or rough, factors into the drag force. Smooth objects with a rounded, gentler shape allow air to flow around them more easily, leading to smaller drag. Second, the forward-facing area of the object perpendicular to the direction of motion also contributes to the force.
The drag force for irregular objects moving at moderate speeds can be calculated with$$F_{drag}=\frac{1}{2} C A \rho v^2$$
where $C$ is a drag coefficient that is often determined with experiments, $A$ is the forward-facing area, $\rho$ is the mass density of the fluid, and $v$ is the speed of the object relative to the fluid. Specifically, this model of drag applies only when motion through the fluid causes turbulence — swirling, unpredictable air flow patterns. Both skiers and skydivers meet this criterion.
The question also provided me with four choices:
A) Density of air
B) Weight of the object
C) Shape of the object
D) Speed of the object
Now why wouldn't the weight of an object affect the magnitude of the drag force on the object? The heavier the weight, the faster the speed of the object (due to gravity), which will lead to the object colliding into more air molecules per second and therefore making the magnitude of the drag force on the object slightly bigger.
Best Answer
This has nothing to do with a falling object. The question does not say anything about falling objects. If they were falling, then you would have an issue because the velocities of the objects would be changing at different rates, and they would reach different terminal velocities. Therefore they would have different drag forces acting on them the entire time.
But this question is not concerned with this scenario. It is more of a general question whose answer it seems like you can already make sense of. The answer is the only one that does not effect the number of collisions with air molecules. The writer of the question should have done a better job specifying the specific movent of the object.