Place a piece of paraffin with a density of 0.9 g / cm3 at the bottom of the beaker, and then add salt with a density of 1.1 g / cm3 A piece of paraffin with a density of 0.9 g / cm3 is placed at the bottom of the beaker, and then salt water with a density of 1.1 g / cm3 is slowly injected into the beaker. The experimental results show that the paraffin does not float up. Why? If you change paraffin into wood? What's the result? Why?

This is the real reason for buoyancy
The pressure difference between the upper surface and the lower surface of a liquid to an object
Suppose a cube is immersed in a liquid. Because there is pressure inside the liquid, it will exert pressure on the six surfaces of the cube. Therefore, the pressure received by the left and right, front and rear surfaces is equal (see "liquid pressure" for details), and the resultant force is 0. However, the pressure received by the upper and lower surfaces is different due to their different depths (the liquid pressure increases with the depth of the object). Generally speaking, the, If the pressure on the upper surface is less than that on the lower surface, the resultant force is upward and buoyancy is generated
Let's get back to the point,
Because the buoyancy is caused by the pressure difference between the upper and lower surfaces, and the paraffin in the question is close to the bottom of the cup because it is put into the bottom of the cup before the water is released again. There is no water on the lower surface of the paraffin. If the water pressure on the lower surface is 0, there is no buoyancy. The paraffin is only affected by the gravity and the pressure on the upper surface, and the resultant force is downward,
If you change it to a wooden block, the wooden block will float up, because the wooden block and the bottom of the cup will not be closely combined, so according to the above, buoyancy will be generated

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