Final answer:
The force of friction increases when the contact surface area is larger, as in the case with the girl's leg being dragged. The coefficient of kinetic friction remains constant, and friction always opposes motion.
Step-by-step explanation:
When the girl's leg is being dragged while riding the skateboard, the force of friction increases because it increases the contact surface area between the leg and the ground. More surface area in contact means that there are more microscopic interactions between the surfaces, leading to a higher frictional force. It is important to note that the coefficient of kinetic friction itself does not change with the speed of motion, but the force of friction is the product of this coefficient and the normal force, which remains constant as it is equal to the girl's weight perpendicular to the contact surface.
The concept also ties in with the fact that friction always opposes motion, and if a body is moving down an incline with negligible friction, all objects would accelerate at the same rate due to gravity alone, as given by the equation a = g sin(θ).