The correct free-body diagram is diagram A
Explanation:
There are three forces acting on Evan as he goes down the hill:
- The weight
- The air resistance
- The normal force exerted by the hill on him
We have to determine the direction of each force. Let's start with the weight: the weight of an object is the gravitational force between the object and the Earth, and therefore, it always acts vertically and downward. This means that we can rule out option C and D, since in those free-body diagrams the weight is not acting vertical.
Now let's analyze the air resistance. Air resistance is a force that opposes the motion of Evan, due to the presence of the air, and it always acts in the direction opposite to the motion. Since Evan is going down along the slope, the air resistance must act up along the slope: this means that both choices A and B can be still correct.
Finally, let's analyze the normal force. This force always acts perpendicular to the plane where the body is moving. Therefore, we see that option B cannot be correct: in fact, in that diagram the normal force acts vertically upward, but the slope is inclined, so the normal force must be inclined as well (since it is perpendicular to the slope). Therefore, the only correct choice is A.
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