When moving an object, the energy conversion is different from that when the human body does work on the bucket, but the result of energy conversion is similar. When moving an object, you need to overcome some forces to do work, and when lifting water, you need to lift the water. At this time, part of the energy paid by people is converted into the gravitational potential energy of the bucket, and part of it is converted into internal energy. The internal energy is due to the friction between the hand and the bucket. Friction and hand contact will deform the muscle skin, and friction with the handle will be converted into internal energy.
Generally speaking, the chemical energy of sugar in human cells is converted into internal energy and mechanical energy of barrels, and some heat energy is lost due to body temperature regulation. Regardless of muscle jitter, people only do work on the bucket during the bucket lifting, and do not do work when walking.
It seems that people standing in the same place are not moving the bucket, but need to grasp the handle of the bucket to provide friction to resist the gravity of the bucket. The movement of muscles is complicated at the micro level, so the bucket is actually shaking slightly. As a result, people keep carrying the bucket up and down, but the amplitude is very small, and there is always friction between the hand and the handle, so people still do work on the bucket.
This is why people's arms and hands are sore after holding objects for a long time. In fact, muscles are constantly tense, and anaerobic glycolysis produces more lactic acid, but the arms are powerless.
The reason lies in the composition of human muscles. Muscle is made up of countless muscle fibers, each of which has the ability to contract or relax. At a more subtle level, muscle protein is a multi-level structure, which is equivalent to a sleeve structure. They will slide relatively, because muscle fiber is composed of actin and troponin, and the combination of troponin and calcium ions will deform, driving the microstructure of muscle fiber to slide relatively, resulting in the shortening of muscle fiber.
When protein dissociates calcium ions, it drives the fiber to slide to its original position, so it is the continuous sliding of muscles that provides muscle strength. protein's athletic ability comes from ATP, that is, high-energy phosphate, and the energy in ATP high-energy bond comes from aerobic decomposition or anaerobic glycolysis of sugar.
In addition to producing metabolic wastes, muscles also have a certain refractory period. The refractory period can be understood as muscle fatigue. Protein's rate of binding calcium ions is certain, which will decrease with time, and the oxidation of sugar by mitochondria is also such a change, which leads to the feeling of being more and more boring after long-term exertion. However, such a process is necessary for bodybuilding. Fitness is not eating protein powder, the most important thing is weight-bearing training, that is, lifting weights constantly will make your arms sour and soft. However, this long-term training will make the muscle compensatory thickening, and the flow rate of receptors and calcium ions on it will change, thus making the muscle more powerful.
Of course, in high school physics, we don't need to consider the phenomenon of muscle jitter, we just need to consider the ideal situation, that is, standing in the same place with a bucket or walking on the flat ground with a bucket, and no one has done any work on the bucket.