UNITS

PHYSICS

BILINGUAL GROUP FACULTY OF EDUCATION, UCM

UNITS

CONTENTS

▸ Introduction

▸ The International System of units

▸ Other system of units

▸ Conversion of units

▸ Scientific notation

▸ Significant figures and order of magnitude

WHY DO WE NEED UNITS?INTRODUCTION

UNITS

INTRODUCTION

▸ To measure is a requisite of science, and in physics, more than in any other field of knowledge.

▸ Measurement of any quantity involves comparison with some precisely defined unit value of the quantity.

▸ For example, to measure the distance between two points, we need a standard unit, such as the meter

https://www.youtube.com/watch?v=DQPQ_q59xyw

MAKING MEASUREMENTSSYSTEM OF UNITS

UNITS

SYSTEM OF UNITS▸ The statement that a distance is 25 meters means that it is 25

times the length of the unit meter. For this reason, it is important to include the unit! To say that a distance is 25 is meaningless.

▸ The magnitude of any physical quantity must include both a number and a unit.

▸ The choice of standard units for the aforementioned fundamental quantities determines a system of units. The system used universally in the scientific community is called SI (for Système Internationale).

https://ed.ted.com/lessons/why-the-metric-system-matters-matt-anticole

UNITS

SI UNITS▸ A small number of fundamental units are sufficient to express all

physical quantities. In fact, we only need three fundamental measures: length, mass, and time.

http://physics.nist.gov/cuu/Units/meter.html

http://physics.nist.gov/cuu/Units/kilogram.html

http://physics.nist.gov/cuu/Units/second.html

▸ We need three more: one for temperature, the degree Kelvin (K); one for the amount of substance, the mole (mol);and one for electrical current, the Ampere (A). There is another fundamental unit, the candela (cd) for luminous intensity.

https://www.youtube.com/watch?v=WuaxXcgX6Rc

UNITS

PREFIXES, CONVERSION & SCIENTIFIC NOTATION

▸ Prefixes can be applied to any SI unit: 0.001s is 1 millisecond (ms), 1,000,000 w is 1 megawatt (MW). http://physics.nist.gov/cuu/Units/prefixes.html

▸ Conversion factors: https://www.youtube.com/watch?v=HZ9weUkSdoY

▸ Scientific notation: The number is written as a product of a number between 1 and 10 and a power of 10.

https://www.youtube.com/watch?v=vRjGarICal4

UNITS

SIGNIFICANT FIGURES AND ORDER OF MAGNITUDE▸ Many of the numbers in science are the result of measurement and are

therefore known only to within some degree of experimental uncertainty.

▸ A reliably known digit (other than a zero used to locate the decimal point) is called a significant figure.

▸ In doing rough calculations or comparisons, we sometimes round off a number to the nearest power of 10. Such a number is called an order of magnitude.

https://www.khanacademy.org/math/pre-algebra/exponents-radicals/orders-of-magnitude/v/orders-of-magnitude-exercise-example-2

UNITSPHYSICS

BILINGUAL GROUP

FACULTY OF EDUCATION, UCM

More info:

Tipler P.A., Physics, 4th Edition, pp. 1-17 Sang D., Cambridge IGCSE Physics (2nd Edition), pp. 2-10 Holzner S., Physics I for dummies (2nd Edition), pp. 15-25

Please send your comments and suggestions regarding this presentation to juan.pena@quim.ucm.es