This issue will keep asking objections from readers, which surprised the soldiers. At first, I thought it was just a single person, but I didn't expect that people kept mentioning it for so many days...

My son is in the first year of high school. Just after school, I asked him to draw a picture on this issue for stress analysis. He drew it in less than a minute and said that this is the knowledge from junior high school.

This is indeed junior high school physics knowledge...

Simply put, the carriage is subjected to a gravity component downward along the inclined surface and a tension force upward along the inclined surface. If the gravity component is greater than the tension force, that is, when the carriage is about to fall down, the friction is in the same direction as the tension force.

To put it vividly, to increase the friction, you can stop and rest when you can't drag it. Relying on the friction, you can only keep the carriage from sliding down with a very small amount of tension... If the friction is too small, that is, the road surface is too slippery, you can't stop in the middle and you must pull up the steep slope to the flat ground in one breath.

This is also the reason why the tires of the trolley and the trolley need to be added with patterns to increase friction.
Chapter completed!