In physics si unit is also known as the international system of units , we call it because in French we call it Système international d'unités. The international si unit of length is meter. Let's look at the quantity length and units , we will write each of them one by one , 

1 Decimeter = 0.1 meter 

1 centimeter = 0.01 meter 

1 millimeter = 0.001 meter 

1 micro meter = 0.000001 meter

1 kilometer = 1000 meter 

This was about all the unit of length written in meter form 

Cubic meter (meter cube ) 

1 cubic centimeter = 0.000001 meter cube 

1 cubic decimeter = 0.001 meter cube 

Questions 

1. When you have an irregular shape it gets harder to measure its volume unless who have the measuring cylinder and water on it , when drop the irregular shape inside the cylinder , the irregular shape takes up the space of water which makes the water level high , for example if the volume of the shape is 20 cm square the water level increases 20 cm square , which is the volume of the figure , that's how we measure the volume of irregular figures like stone , piece of wood and etc 

2. The volume of the piece of wood is 25 cm square as the water level before the piece of wood was added was 50 cm square , now when the piece of wood is added to water level increases 25 cm square which is the volume of the piece of wood

   
   

Density

Density are the measurement of the small cubes of the whole of cube called volume , mass is the weight of all the small cubes inside the object , volume is the measurement of the length ,width and height of the cubes while density is the measurement of one such cube between all one them , the formula for density is , D = M/V , density = mass/volume. 

Units of Mass, volume , density 

Mass 

Kilogram - Kg 

Gram - G

Volume 

Cubic meter - m cube

Cubic centimeter = cm cube 

Cubic decimeter = dm cube 

Density

Kilograms per cubic meter

Kilograms per cubic centimeter

Kilograms per cubic centimeter