Physics

This article focuses on the question of whether the water flow in toilets in the Northern Hemisphere always rotates counterclockwise. It introduces the definition and principle of the Coriolis force (Earth's rotational force), points out that the Coriolis force effect is weak on a small scale, and the direction of the toilet vortex is mainly determined by its own structure. At the same time, it elaborates on the significant role of the Coriolis force in large-scale and long-term processes such as river erosion, railroad wear, and typhoon structure

This article focuses on the question of "why running the same distance is more exhausting than cycling". Through semi-quantitative physical estimates and exercise physiological measurements, the article analyzes aspects such as overcoming external dissipative forces, the energy consumption of the vertical movement of the center of gravity during running, and overcoming internal friction forces. It explains why cycling is much more energy-efficient than running over the same distance.

This article focuses on the issue of why a unicycle with a high center of gravity can stand upright without falling over. It explains the principle that relies on the driver controlling the pedals to generate ground friction torque to balance the overturning torque, and introduces the structure and application of the Segway (a powered scooter) based on the same balancing principle and using an automatic control system.

This article focuses on answering the question of "why is it harder to ride a bicycle with underinflated tires?" It points out that the reason why cycling is more difficult is not because of increased friction (friction is unrelated to the contact area, and the friction force of the bicycle's rear wheel is forward, which should make cycling easier). The real reason is that after the tire is deflated, the point of support on the ground and the point of gravity are separated. The torque generated by gravity will "resist" the torque generated by the chain, resulting in harder cycling, and the more deflated the tire is, the harder it is to ride the bicycle.

This article focuses on the problem that bicycles are prone to tip over when braking the front brake abruptly, and explains its principle: When the bicycle is traveling at a high speed, braking the front brake abruptly will cause the front wheel to lock, and the static friction between the front wheel and the ground will turn into sliding friction. The rolling friction of the rear wheel is much smaller than the sliding friction and can be ignored. Due to the inertia of the bicycle, when taking the bicycle as the reference frame, if the torque generated by the inertial force is equal to or greater than the torque of the gravity acting on the bicycle and the rider as a whole, the rear wheel will tilt forward, and the faster the braking, the more likely the bicycle will tip over.

This article focuses on the phenomenon that the hoop can automatically return after being thrown in the artistic gymnastics hoop routine, introduces the equipment parameters and performance action types of hoop routine, analyzes the translational and rotational forms of the hoop through physical ideal models such as point and rigid body, and combines the action of sliding friction to explain the three possible situations of the hoop's movement and the physical principle of its automatic return.

This article introduces the origin and development of the high jump. By explaining the mechanical principles of the high jump, the movement methods and technical characteristics of the back-over style high jump, it explains why world-class high jump athletes adopt the back-over style. The reason is that when they cross the bar, the center of gravity of the human body can pass under the bar, which has more advantages than other high jump postures

This article focuses on the question of "why cats can freely turn over in mid-air", introducing the phenomenon that cats can quickly turn over and land on all fours during a high-altitude fall, and describing the process of proposing and refuting the "tail-turning" theory, as well as the content, verification basis, and related principles of the currently accepted "curved spine" theory.

This article takes the phenomenon of children holding dumbbells and sitting on a swivel chair with accelerated arm rotation speed, and the principle of door rotation force as examples to explain the law of conservation of angular momentum (the law of conservation of momentum), and to explain the reason why figure skaters contract their bodies when spinning quickly.

This article explains the question of "why martial artists can make flying needles penetrate glass", pointing out that the speed of about 30 meters per second required for a steel needle to penetrate glass can generally be achieved by adults. The key to flying needles penetrating glass lies in the steel needle hitting the glass vertically. Sharpening the needle tip can also help it penetrate. At the same time, the examples of bullets bouncing off helmets and Li Guang shooting stones demonstrate the importance of angle. It is explained that martial artists, through long-term training, can simultaneously ensure the speed of the steel needle and control the angle of impact, thus achieving the effect of flying needles penetrating glass.

This article introduces the concept of pressure in physics through a comparative experiment between carrying a schoolbag and holding a thin string, and explains the principle that the pressure exerted on the wooden board is much greater than that exerted on the hand because the contact area between the pinhead and the wooden board is much smaller than that between the pinhead and the hand. Therefore, it's easy to push the pin into the wooden board with your hand.

This article, based on the scenario of water skiing, uses Newton's Third Law to explain why water skiers don't sink on the board. It points out that skiers rely on their skills to control the tilt angle of the board, so that the vertical component of the water's reaction force can balance their own weight, allowing them to slide quickly on the water surface.