## Basic and Derived Quantities:

This is a basic knowledge of physics. Every one knows there are 7 types of basic quantities found in the world of physics. These are; Mass, Time, Length, Electric current, Temperature, Light and amount of substance.

The Derived quantities are derived from the basic quantities such as speed which is derived from length and time.

## SI units:

SI units are used to symbolize the basic quantities. These are as follows:

## Prefixes for SI units:

The Prefixes are used to define the zeros in a math equation. For example; 0.00000010 has 6 zeros in it, then its prefix will be “u”, It will be written as 0.10^-6 um or 1 x 10^-6 um.

Prefixes are as follows:

This is especially useful for finding the molecular distance between various objects and is useful for math problems.

## Vernier calipers:

Vernier Calipers are used to calculate the length of the object that is being measured. The Vernier Caliper has four jaws. the upper right and left jaw which are used to measure the internal length or diameter of an object and the lower jaw which are bigger than the upper jaw for the purpose of measuring external length of an object. The vernier caliper has two scales, the main scale and the vernier scale. It has an accuracy of 0.1 mm

### How it Works:

1. The way it works is easy. You just fit an object in its lower jaw for the purpose of measuring its external length. Tighten the jaw on to it and make sure it isn’t lose. Then check the vernier scale of the instrument. The measurement on main scale, on which the 0 reading of the vernier scale sets on is the length according to the main scale.
2. It is not over yet. On the vernier scale there might be a measurement or reading that is directly opposite to the main scale reading (Note: The main scale reading is not the same one as in the previous step). This reading (the vernier scale reading) is another length. This reading should be noted down according to accuracy of the vernier caliper which is 0.1 mm as written on the previous paragraph. The number “1” in 0.1 is not a strict number and can be changed according to vernier scale reading.
3.  Add both readings and you will get the total length. (Note: The reading would be written in “mm”)

## Micrometer screw gauge:

Micrometer screw gauge is almost like the vernier caliper. This instrument does not has a jaw rather it has a spindle which tightens the placed object onto the anvil which is a fixed object on the opposite side of the spindle. The Micrometer has two scales the main scale and the thimble scale. The ratchet prevents the user to exert more pressure on the already tightened object. It has an accuracy of 0.01 mm.

### How it Works:

1. It is simple. Just tighten the object in the spindle and anvil and roll the ratchet. Then read the measurement on the main scale. The reading would be calculated from where the main scale touches the thimble scale. That number is the main scale reading.
2.        The reading on the thimble scale which is directly opposite to the main scale will be the thimble scale reading. This reading will be written according to its accuracy.