**Chapter 1 Measurement**

Physics is based on measurement.

We measure each physical quantity in its own units, by comparison with a standard. The unit is a unique name we assign to measures of that quantity – for example meter (or M) for the quantity length.

Physicists specified seven physical quantities as base quantities.

We define all other physical quantities in terms of these base quantities and their standards (called base standards).

**Changing Units: by chain-link method**

In this method, we multiply the original measurement by a conversion factor which is equal to unity.

Example: 2 hour * (60 minutes/ 1 hour) = 120 minutes

In the units hour and hour get cancelled leaving 120 minutes as the answer. The value of 60 minute/1 hour is equal to one in common units.

**Standard for length**

Current one defined by 17th General Conference on Weights and Measures: The meter is the length of the path traveled by light in a vacuum during a time interval of (1/299 792 458) of a second.

The time interval was chosen so that the speed of light c is exactly

C = 299 792 458 m/s

**Standard for Time**

The 13th General Conference on Weights and Measures in 1967 adopted a standard second based on the cesium clock:

‘One second is the time taken by 9 192 631 770 oscillations of the light (of a specified wave length) emitted by a cesium-133 atom)

**Standard for Mass**

The SI standard of mass is a platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris and assigned, by international agreement, a mass of 1 kilogram.

A second mass standard

It is the carbon-12 atom, which, by international agreement, has been assigned a mass of 12 atomic mass units (u).

The relation between amu and kg is

1 u = 1.6605402*10^-27 kg

A difficult problem: I tell my daughter to identify the difficult problems and make a note of it beside the problem. Many problems can be solved by direct application of formulas. But some problems cannot be solved by direct application of the formulas we know at that point to time. That is because, a new point or issue or interpretation is required in that problem. We have to revise this problem along with the formulas and concepts to capture the new idea introduced in this problem.

Sample problem 1-4 in the 6th edition is one such problem. The problem is to find out the radius of earth with a wrist watch and meter stick.

A special edition of sixth edition of Halliday, Resnick and Walker published by Wiley is available for less than Rs.500 for Indian Students.

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