1. The problem statement, all variables and given/known data Daniel's weight is 800 newtons. According to Newton's Third Law, if the Earth pulls on Daniel with a force of 800 newtons, then Daniel pulls on Earth with a force of 800 newtons in the opposite direction. Why then is Daniel able to trip and fall down to the ground? 2. Relevant equations None 3. The attempt at a solution Well, I've been stuck on this for a while now. I remember learning something about this with a leaf falling, but I can't remember/find it anymore. I do have one theory, however. Even though I said there are no relevant equations, I believe I can use Newton's Second Law (F = ma) to figure out this problem. I need to rearrange this equation to solve for acceleration, as so: a = F / m. If force F is the same for both the earth and for Daniel, then the only changing variable is m. The earth has an extremely higher mass than Daniel, so the acceleration for the small force of 800 newtons is so small, it's nearly nonexistent. However, Daniel's mass is much smaller, so the force of 800 newtons causes his acceleration to be considerably higher than the earth's, so much as to make him fall down.