A kid's knees are initially on the ground (like in a kneeling position). Upon hopping straight from this state, the kid can reach a height of about 20 cm. While making the hop, his body rises a distance of 16 cm from the knees up. Assume that this is true for his entire body as well. Solve for the force F that the ground exerts on the kid during the hop. Express this in terms of the kid's weight, w.