①Identify substances: Find the density of the object to be measured based on the conditions set in the question, and then compare the calculated value with the density table to determine the type of substance.

②Proportion problem: Known mass, density, volume, the multiple relationship or proportional relationship between certain physical quantities, solving the ratio of unknown quantities, etc.

③Hollow problem: including two cases of judging whether an object is hollow or solid and calculating the volume of the hollow part; to judge whether it is hollow or solid, you can use any of the three methods of comparing density, comparing volume, and comparing mass; calculating the hollow part When it comes to partial volume, the volume of the material that makes up the object must be found. V space = V total - V material

④Average density: ρ=Mtotal/VTotal. The total mass of the mixture is equal to the sum of the masses of the various substances. Without considering the volume change after the substances are mixed, the volume of the mixture is equal to the sum of the volumes of the individual substances.

Note: F is pressure, not gravity; S is the force-bearing area, not the surface area or base area of ​​the object

①The liquid exerts pressure on the bottom and side walls of the container, which increases with depth.

② There is pressure in all directions inside the liquid, and at the same depth, the pressure in all directions is equal

③Liquid pressure is related to the density of the liquid. At the same depth, the greater the density of the liquid, the greater the pressure.

④ Liquid pressure formula: p = ρ liquid gh, where ρ liquid represents the density of the liquid, h represents the depth, that is, the vertical distance from the desired pressure point to the free liquid surface.

The pressure of the liquid on the bottom of the container: F=pS= ρ liquid hs=G liquid

The pressure of the liquid on the bottom of the container: p=ρ liquid gh or p=G liquid/S

Imagine a cube submerged in water, with water pressure acting on all six surfaces. The pressure on the left and right sides is the same and opposite in direction. It is a pair of balancing forces. In the same way, the pressure on the front and rear sides is also a pair of balancing forces. Due to the difference in depth, the pressure on the lower surface is greater than the upper surface, so a force is generated

Using a spring dynamometer

h surface refers to the distance from the surface to the free liquid surface

F refers to the reading of the spring force gauge when the object is immersed in water.

When the object is floating, G=F float = ρ liquid gV discharge. The density of the liquid is different due to the different immersion depth of the density meter.

Density meter does not have 0 scale mark

The upper reading of the density meter is small, and the lower reading is large.

The density meter scale is uneven, sparse at the top and dense at the bottom.

Making steel into a hollow ship can displace more water and gain more buoyancy to float on the water.

The "draft" allowed for a ship to be fully loaded in different waters and seasons

Quality of water discharged by ships

By changing its own gravity, that is, the drainage and water inflow of the water tank, it achieves floating up and down.