# Why is the stopped clock second hand always at the “9” position

Why is the stopped clock second hand always at the “9” position
There is an interesting phenomenon. I don’t know if you have noticed: When the clock hanging on the wall stops running when the battery power is exhausted, the second hand often stops at the “9” position on the dial. When the battery in the quartz clock runs out of power and the second hand stops on the dial, gravity produces the maximum reverse torque. We know that the direction of the second hand is clockwise, so gravity here will produce the largest reverse torque value: maximum counterclockwise. Weight moment = magnitude of gravity × gravity arm. When the second hand reaches 9 points on the dial, the distance from the fulcrum to the direction of gravity action is the largest, that is, the gravity arm is the largest, so the weight moment is the largest, so the second hand will stop at the 9 position on the dial. In the end, there was no power, it would be a little above and below 9, and the tolerance range was very small. The second hand of the quartz clock moves downwards from the number 12 to the number 6. The gravity it receives will help the second hand to rotate clockwise. When the second hand moves from the number 6 to the number 12, it is an upward movement. The gravity will become resistance to hinder the rotation of the second hand, and when the second hand turns to the number 9, the resistance arm is the longest, so that the resistance moment of gravity is the largest.

Why don’t mountains grow infinitely
Imagine: the mountains will grow indefinitely, penetrate the upper atmosphere, and create a rocky maze for the pilot. Maybe this happens somewhere in the universe, but it doesn’t happen on Earth. What prevented the mountains from growing? There are two limiting factors. The first limiting factor is gravity. Many mountains are formed by the movement of the Earth’s surface, known as plate tectonics. When two crustal plates collide, the impact forces the material to move upward from the edge it touches, forming a mountain range. As the mountains continue to grow, at some point, the mountains will become too heavy, and their own mass prevents the upward growth caused by the collision of the two plates. In other words, if the earth’s gravity decreases, its mountains will grow even higher. For example, on Mars, which has a lower gravity than Earth, Olympus is about 26,000 meters high. The second factor limiting the growth of mountains on Earth is the river. At first, the rivers made the mountains appear higher-they cut into the edges of the mountains, eroded material, and formed deep cracks near the bottom of the mountains. However, erosion will also cause landslides, taking away material from the mountains and limiting their height.

Why the trunk grows round
In order for all living things to survive, of course, they must develop towards the most adaptable aspect of the environment. Plants are no exception. The trunk is cylindrical to meet the needs of growth. First, the circle has the largest area with the same occupied material. The same amount of material is expected to make the largest volume, circular is the most suitable. Water pipes, gas pipes, etc., are imitations of this natural phenomenon. Secondly, the cylindrical shape has the largest supporting force. The weight of the tree is all supported by a trunk, which must be supported by a strong trunk to survive. Finally, the round shape prevents foreign injuries. The cortex of a tree is a channel for trees to transport nutrients. Once the cortex is interrupted, the tree will die. Trees are perennial plants, and they will inevitably suffer many external injuries throughout their lives. If the trunk is square, flat, or has other angular shapes, it is more vulnerable to external shocks. Therefore, the shape of the trunk is also the result of the adaptation of the tree to the natural environment and the result of long-term evolution.