In the following statements about leverage, the correct one is () A. The fulcrum is always between the power action point and the resistance action point. B. the longer the power arm is, the less labor is always saved. C. the direction of power action is always opposite to that of resistance action. D. the larger the ratio of power arm to resistance arm, the less labor is saved

In the following statements about leverage, the correct one is () A. The fulcrum is always between the power action point and the resistance action point. B. the longer the power arm is, the less labor is always saved. C. the direction of power action is always opposite to that of resistance action. D. the larger the ratio of power arm to resistance arm, the less labor is saved

A. Fulcrum refers to the fixed point of the lever in the process of rotation, which is not necessarily between the levers. This option is incorrect. B. the labor saving of the lever is determined by the length relationship between the power arm and the resistance arm. The longer the power arm is, the less the labor is. This option is incorrect. C. when the fulcrum is on one side, the direction of power and resistance is opposite. When the fulcrum is on the other side, the direction of power and resistance is opposite In the middle, the direction of power and resistance is the same, which is not correct; D, the larger the ratio of power arm and resistance arm, the less effort, which is correct

Leveraged
There is a 90cm long wooden stick with uneven thickness. The weight of the rod is 19.6n. When the fulcrum is 30cm away from the end of the thick stick, the stick can keep horizontal balance. If the fulcrum is moved to 50cm away from the thick section, which end should be hung with multiple objects to restore the horizontal balance of the stick.
To solve the problem!

It can be understood that the center of gravity of the stick is 30cm away from the thick end. When the fulcrum is 50cm away from the thick end, it is equivalent to the moment generated by the center of gravity force at the distance of 50-30cm = 20cm from the fulcrum. At this time, 19.6 * 20 = (90-50) * weight
Weight = 19.6 * 20 / 40 = 9.8N

Ask a question about leverage in junior physics,
A wood with uneven thickness has a total length of 4 meters. When the fulcrum is 1.4 meters away from the thick end, the wood is just balanced. If an 80 n weight is hung on the thin end, the fulcrum must move 0.4 meters to the thin end to restore the wood balance. What is the weight of the wood?
Please give me the idea and process. It's urgent

When the fulcrum is 1.4 meters away from the thick end, the wood is just balanced
According to the following equation: "if the thin end is hung with 80N weight, the fulcrum must move 0.4m to the thin end"
G * L1 = g * L2
80N*(4-1.4-0.4)=G*0.4
G=440N
The weight of this wood is 440n