## 9th Class Physics Chapter 1 Notes

Physics is a branch of Science that deals with matter, energy and their relationship.

Some main branches of Physics are mechanics, heat, sound, light (optics), electricity and magnetism, nuclear physics and quantum physics.

Physics plays an important role in our daily life. For example, electricity is widely used everywhere, domestic appliances, office equipments, machines used in industry, means of transport and communication etc. work on the basic laws and principles of Physics.

A measurable quantity is called a physical quantity.

Base quantities are defined independently. Seven quantities are selected as base quantities. These are length, time, mass, electric current, temperature, intensity of light and the amount of a substance.

The quantities which are expressed in terms of base quantities are called derived quantities. For example, speed, area, density, force, pressure, energy, etc.

A world-wide system of measurements is known as international system of units (SI). In SI, the units of seven base quantities are metre, kilogramme, second, ampere, kelvin, candela and mole.

The words or letters added before a unit and stand for the multiples orsubmultiples of that unit are known as prefixes. For example, kilo, mega, milli, micro, etc.

A way to express a given number as a number between 1 and 10 multiplied by 10 having an appropriate power is called scientific notation or standard form.

An instrument used to measure small lengths such as internal or external diameter or length of a cylinder, etc is called as Vernier Callipers.

A Screw gauge is used to measure small lengths such as diameter of a wire, thickness of a metal sheet, etc.

Physical balance is a modified type of beam balance used to measure small masses by comparison with greater accuracy.

A stopwatch is used to measure the time interval of an event. Mechanical stopwatches have least count upto 0.1 seconds. Digital stopwatch of least count 0.01s are common.

A measuring cylinder is a graduated glass cylinder marked in millilitres. It is used to measure the volume of a liquid and also to find the volume of an irregular shaped solid object.

All the accurately known digits and the first doubtful digit in an expression are called significant figures. It reflects the precision of a measured value of a physical quantity.