Final answer:
The direction of vertical ground reaction force can be upwards or downwards in the coordinate system, representing the normal force that counteracts gravity for an object at rest or during vertical motion.
Step-by-step explanation:
The directions for vertical ground reaction force are upward and downward in the coordinate system. When an object is at rest on a surface, the Earth exerts a gravitational force downward, and the surface provides an equal and opposite reaction force upward, according to Newton's Third Law. This upward force is known as the normal force and is denoted as N=mg in the upward direction.
In the context of motion, such as a basketball being thrown up or down, the vertical ground reaction force will primarily be directed either up or down, depending on the interaction between the object and the ground. For objects like a sled on the ground with rockets, there might be significant horizontal forces, but the vertical forces, such as the weight (W) and the normal force (N), tend to cancel out when there is no vertical acceleration. Hence, for an object resting on the ground, the direction of the vertical ground reaction force is upward, counteracting the downward force of gravity.