Question

how many pulley systems can you identify in the gym?
pulleys arent just used to provide mechanical advantage, they also allow forces to be redirected into a more convenient (or safe) direction.

could you sail a boat without pulleys?
it would be very difficult! the forces applied to the sails are strong enough to accelerate the whole boat. under good sailing conditions, these forces can be too strong for a human to counter without a pulley system.

could you design a pulley-based elevator for your tree fort?
when you use a pulley system, the pulley itself does some of the lifting - allowing you to pull yourself up in a way you cant by just, say, pulling up on your hair!

how do your shoes stay on?
shoes & laces act like a pulley system, leveraging your pulling force so that the shoe binds tightly to your feet

Explanation:

1. For gym pulley systems: Common examples include cable machines (lat pulldowns, cable rows), pull-up assist pulleys, and weight stack pulleys, as these use pulley mechanisms to redirect force or provide mechanical advantage.
2. For sailing without pulleys: Pulleys (blocks) are critical for managing sail tension; without them, humans cannot safely/effectively counter the large forces from wind on sails, making controlled sailing extremely difficult or impossible.
3. For tree fort elevator: A simple pulley system can be designed by anchoring a fixed pulley at the top of the tree fort, running a rope through it with one end attached to a small platform (elevator) and the other held by the user; pulling the rope lifts the platform, using mechanical advantage to reduce the force needed to lift the user/items.
4. For shoe laces as pulleys: When you pull the ends of laces, the crisscrossed laces act like pulley segments, redirecting pulling force to tighten the shoe evenly around the foot, leveraging pulley-like force redirection.

Answer:

1. Common gym pulley systems: Cable machines (lat pulldowns, cable rows), pull-up assist pulleys, weight stack pulleys.
2. Sailing without pulleys would be extremely difficult, as humans cannot safely counter the strong wind forces on sails without pulley systems to manage tension.
3. Yes: Anchor a fixed pulley at the tree fort's top, run a rope through it, attach one end to a small platform, and pull the free end to lift the platform (and yourself/items) with reduced force.
4. Shoe laces act like a pulley system: pulling the lace ends redirects force through the crisscrossed laces to tighten the shoe evenly around the foot.