Following the high school physics textbook to learn physics, today we will start to study the third chapter of force
In the very beginning, books tell us that objects in nature do not exist in isolation, that there are many kinds of interactions between objects, and that in mechanics, the interaction between objects is abstracted into a single concept - force.
This chapter examines the common forces of gravity, elasticity, and friction.
In junior high school, we learned the relevant content, and we know that there are three elements of force, size, direction, and action point. There are force-applied objects and force-bearing objects. You can also draw a diagram of the force or a diagram of the force to represent the force. These can be reviewed on their own.
The first section of the book talks about gravity and elasticity, we start with gravity today, here we pay attention to the definition of gravity, "the force received by an object due to the attraction of the earth", gravity is not the attraction of the earth to an object, and we will learn that the attraction of the earth to an object is called gravitational force.
The magnitude of gravity G=mg, I learned it in junior high school, and g is the acceleration of free fall that I just learned in free fall. The unit of g can be m/s² or N/kg in junior high school, and we will know later that these two units are equivalent. The magnitude of gravity can be measured with a spring dynamometer.
The direction of gravity is vertically downward, and junior high school must also emphasize that it is not vertically downward.
The object of gravity is the Earth.
All parts of an object are affected by gravity, and from the perspective of effect, it can be considered that the gravitational force of each part is concentrated in one point, which is called the center of gravity. The center of gravity can be seen as the point of action of gravity. If the mass of the object is evenly distributed, the center of gravity is only related to the shape, and the center of gravity of a regularly shaped object is at the center of the geometry. If the shape is irregular, we can use the knowledge of the balance of two forces to determine the position of the center of gravity. The book introduces the use of the hanging method to find the center of gravity of a thin sheet.
If the mass distribution of the object is not uniform, then the center of gravity is not only related to the shape, but also to the mass distribution, and if the mass distribution changes, then the position of the center of gravity will also change. The book uses trucks to carry different loads to illustrate, and we can also think about the fact that when a person's posture changes, the position of the center of gravity also changes. For example, if a person bends down and jumps high on his back, the position of the center of gravity will change, and it can also be found here that the center of gravity can be on the object or outside the object.
To sum up.