Your body applies force to the ground as weight. if you weigh 450 newtons and carry a backpack that weighs 45 newtons, what is the net force applied downward to the ground?

Forces are vector quantities and we can add them as such. Because both the weight of the person is 450 N and the weight of the bag pack is 45 N, both of those forces act in the same direction and we simply add our weight and the weight of the bag pack:

450 N + 45 N = 495 N. 

So the net force that acts on the ground is 495 N.