At the same time, the fingers feel that the keycap pushes the fingers upward, which is the role of the keycap opponent. This is also a kind of force, commonly called reaction. Note: Action and reaction appear at the same time and disappear at the same time, with the same size and opposite directions.
Gravity is the attraction of the earth under our feet to any object near the surface of the earth, which is called the gravity of objects. The direction is vertical downward. For example, computers, tables, ourselves, and furniture at home can all be placed firmly on the ground because of gravity. If it weren't for the attraction of the earth, they would all disappear.
A lever is a stick that cannot be deformed. There is a fulcrum below, or something is pulling on it. Generally, there are two forces acting on the stick on both sides or the same side of the fulcrum, and there may be more forces acting on it, depending on how the stick moves and what happens. Generally speaking, it is in a balanced situation, so it is easy to solve the problem.
Pulleys are more fun. According to the given situation, some pulleys can't move in the center, but just rotate in the same place. This is called a crown block. The center of some pulleys can move with the weight or goods connected to them. This kind of pulley is called moving pulley.
For the crane, because the distance from the center to the side is the same, it can be imagined as a simple lever, and the forces on both sides must be the same, so the crane does not change the magnitude of the force, but only changes the direction of the force.
For the moving pulley, because its position is always moving, it requires that the ropes on both sides of it must be tightened. If you don't tighten it, the pulley will fall off, and as a result, the ropes on both sides will be tightened. Because the ropes on both sides are the same in tightness, the forces on the ropes on both sides must be the same. Otherwise, the pulley will fall down until both sides are elastic. In essence, the tension of the rope on both sides must be the same. So if there is a heavy object under the moving pulley, or if it is pulled down, the gravity of the heavy object will be balanced by the two same pulling forces above, so the two same pulling forces above only need to be half of the gravity of the heavy object below, so it can be said that using the moving pulley can save labor: note that the resultant force cannot be saved at all. If the total force is saved, won't the things hanging below fall off? So what's the point of saving effort? Think about it: there is only one rope on this moving pulley with two sliders on it. At work, most people can only pull one head, but you can't pull two heads with your hands. What's the point? So people can solve the problem by pulling a head. In the case of pulling only one end, people actually only pay the force of one rope, and the rope at the other end is hung on it, so people don't need to pay the other half. Therefore, moving the pulley can save power. The simplest thing is to put a heavy object under the moving pulley, and people pull a rope to save half the effort.
Do more simple questions and find the feeling. Not difficult. It will be easy to understand if you learn to turn the language in the book into your own vernacular.