Pull up exercise will not shorten your height. There is no scientific evidence that pull ups will shorten or extend your height.

Gravitational Pull, push, or force.

Spermicide condoms unless you want a baby then go pull out method.

112lbs psi

Gravity is the force in nature that quite literally binds everything together. One atom attracts electrons through its pull from the nuclei mass. The Earth pulls me from its center mass. The amount of matter in one object is able to pull other objects near it as long as it is able to sustain that attracted mass. In the case of the the Earth, the Earth is so massive that it has a gravitational pull reaching very far into space that can pull objects as big the moon into it. Amazing stuff, I know!