You have just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these are forces acting on the rock? Select all that apply.

(A) the force of the kick, acting in the direction of motion normal force

(B) directed upward friction

(C) acting opposite the direction of motion weight force

(D) directed downward

B), C) and D.

Explanation:

Once kicked, the rock, assuming no other forces present, would continue moving along a straight line, at the same speed that when it was just kicked, forever.

This means that the force of the kick is no longer present, so A) can't be true.

However, we have another three forces acting on the rock:

1) gravity, always directed downward. (D)

2) Normal force, directed upward, as it is always perpendicular to the surface on which the object is moving, and we assume that the ground is level. (B)

3) Kinetic friction force, which will be ultimately the responsible for the rock coming to an stop, opposing to the relative movement between the rock and the ground, so it will be directed backwards. (C)

So, the answers B, C and D identify forces acting on the rock.