What Do You Mean By "DENSITY" ?

 

DENSITY

 

When designing a large building or a bridge, an architect or an engineer will need to know the masses of iron girders he intends to use so that he may plan supports of sufficient strength for them. From his plans, he can calculate their volumes, and if he knows the mass of a cubic metre of iron, simple multiplication sums will give their masses. Similarly, in case of a tank lorry to carry petrol, the mass of the load should not be excessive for the strength of the axles, etc. and a knowledge of mass of a cubic metre of petrol would enable the size of the tank to carry the greatest permissible load to be calculated. These examples illustrate the usefulness of a knowledge of mass per unit volume of the substance. This quantity is called the density of the substance. It is evident that it will often be inconvenient to try to obtain exactly 1 m3 of a substance in order to find its density. But if both mass and volume of any amount of a substance are known, its density can be found by dividing its mass by its volume. Therefore,

 

Density of a substance is defined as its mass per unit volume.

 

d=M/V

 

Unit of density. Since mass (M) is measured in kilogram (kg) and the volume (V) is measured in metre³ (m³), the unit of density is kg/m³

 

 RELATIVE DENSITY

 

In many cases, instead of dealing with the density of a substance, it is preferrable to consider the number of times the substance is as dense as water. This is called the relative density.

 

Relative density of a substance is defined as the ratio of its density to that of water at 4°C.

 

Density of substance Thus, Relative density = Density of water at 4°

 

C Unit of Relative Density Since relative density is a ratio of two similar quantities, it has no unit. Further, relative density = density of substance

 

 

density of water at 4°C mass of substance/ volume of substance

 

mass of water / volume of water at 4°C If the volume of a given substance is equal to the volume of water at 4°C, relative density = mass of substance

 

mass of an equal volume of water at 4°C Relative density can also be defined as the ratio between the mass of the substance and the mass equal volume of water at 4°C.

 

 

Relative density is also sometimes called specific gravity. It tells us as to how many times a given substance is heavier or lighter than water at 4°C. If a given substance has more density than water, it is called a heavy substance or it has a higher relative density. On the other hand, if the given substance has less density than water, it is called a lighter substance or it has a lower relative density.

 

 

 

1. Mass per unit volume of a substance is called its density, i.e., density (d) =volume (V)/ mass (M)

 

2. The SI unit of density is kg/m3 and its cgs unit is g/cm³. 1 kg/m³ = 1000 g/cm³ 3. Density of a substance is one of its characteristic properties and it enables us to determine its purity.

 

4. Relative density of a substance is defined as the ratio of its density to that of water at 4°C. Being a ratio of two similar quantities, it has no units. Relative density of a substance is also defined as the ratio of the mass of the substance to the mass of an equal volume of water at 4°C.

 

5. If a given substance has more density than water, it is called a heavy substance or it has a higher relative density. Conversely, if the substance has less density than water, it is called a lighter substance or it has a lower relative density.

 

6. According to Newton's third law of motion, to every action, there is always an equal and opposite reaction.

 

7. The forces of action and reaction are always equal and opposite. They act on two different objects and never cancel each other. Each force produces its own effect.

 

8. Though action and reaction forces are always equal in magnitude, yet these forces may not produce accelerations of equal magnitude. This is because each force acts on a different object, which may have different mass.

 

9. Some examples of Newton's third law of motion are : walking ; swimming ; recoiling of gun; man and boat ; flying of rockets and jet planes ; the case of a hose pipe etc.

 

10. According to the law of conservation of linear momentum, when two or more bodies interact with one another, the vector sum of their linear momenta remains constant (ie., conserved), and is not affected due to their mutual action and reaction. The only condition is that no external unbalanced forces should be acting on the system of bodies. This law is deduced from Newton's third law of motion.

 

11. All applications/examples of Newton's third law of motion can be explained in terms of the law of conservation of linear momentum.

 

12. When a bullet is fired from a gun; the gun recoils, i.e., the gun moves backwards.

 

13. From his observations, Galileo established that an unbalanced external force is required to initiate the motion (from state of rest). But no unbalanced force is needed to sustain the uniform motion. Objects continue moving with a constant speed along a straight line, when no external force acts on them.   14. According to Newton's first law of motion, a body continues to be in a state of rest or in a state of uniform motion along a straight line, unless an external force is applied on the body to change the state.   15. Newton's first law of motion gives us qualitative definition of force. Further, this law means that a body on its own, cannot change its state of rest or state of uniform motion along a straight line. This property is called inertia. Therefore, Newton's first law of motion is also called the law of inertia.   16. Quantitatively, inertia of a body is measured by the magnitude of force required to change the state of the body. When body is heavy, force required to change its state is large. Therefore, a heavy body has large inertia. Hence mass of a body is a measure of inertia of the body in linear motion. Larger the mass, greater is the inertia.   17. Inertia of a body is of three types : (i) Inertia of rest (ii) Inertia of motion and (iii) Inertia of direction.   Inertia of rest means that a body at rest cannot start moving on its own. Inertia of motion means that a body in motion cannot stop on its own.   Inertia of direction means that a body moving along a particular direction cannot change its а direction of motion by itself.   In all the three cases, external forces are required for changing the state of the body.