The Realm of Physics

Orders of Magnitude When quantities are written to the nearest power of 10, the quantity is being expressed as an “order of magnitude”. Often, part of learning physics is to appreciate the order of magnitude of things – whether they are masses, times, distances, forces, or just pure numbers such as the number of times your heart will beat in your lifetime.

Range of Length Length/m Distance to the edge of the observable universe10 26 Diameter of the Milky Way galaxy10 21 Distance to the nearest star outside our solar system10 16 Diameter of solar system10 13 Distance to sun10 11 Diameter of Earth10 7 Radius of Earth10 7 Size of a cell10 -5 Size of a hydrogen atom Size of a nucleus Size of a proton10 -17

The length of a page of the examination paper is approximately 30 cm. Which of the following gives the order of magnitude for the time taken for light to travel the length of the page? A.10 –7 s B.10 –8 s C.10 –9 s D.10 –10 s

The ratio nucleushydrogen ofdiameter atomhydrogen ofdiameter to the nearest order of magnitude is A B C. 10. D

Range of mass Mass/Kg The universe10 53 The Milky Way galaxy10 41 The sun10 30 The earth10 24 An apple10 -1 A raindrop10 -6 A bacterium Smallest virus A hydrogen atom An electron10 -31

A.α-particleβ-particleproton B.protonα-particleβ-particle C.protonβ-particleα-particle D.β-particleprotonα-particle Which list gives the masses of the particles in ascending order of magnitude? least → greatest

The mass of an atom of the isotope strontium-92 (92Sr) is of the order of A.10–23 kg. B.10–25 kg. C.10–27 kg. D.10–29 kg.

The mass of an electron is 9.1 ´ 10 –31 kg and that of a proton is 1.7 ´ 10 –27 kg. Which one of the following is the difference in the order of magnitude of the masses of the electron and the proton? A.10.8 B.7.4 C.5.4 D.3

Range of time Time/s Age of the universe10 17 Age of the earth10 17 Time of travel by light to nearest star10 8 One year10 7 One day10 5 Period of one heartbeat10 0 Period of red light Time of passage of light across a nucleus10 -24

Making approximations By using orders of magnitude, we can arrive at approximate solutions to questions like “how many times does your heart beat in your lifetime. Ie. We live approximately 10 2 years, each year contains approximately 10 7 seconds, and our heart beats about 1 time per second. So, your heart beats about 10 9 times in your lifetime.

A similar calculation Assuming your heart beats 1 time per second, how many times would it beat in 75 years?

Comparing quantities Quantities can be compared using order of magnitude. Ex: ratio of the size of a hydrogen atom to the size of its nucleus is What is the mass of our galaxy in terms of solar mass? How many hydrogen atoms does it take to make up the mass of the earth?

The volume of the Earth is approximately km 3 and the volume of a grain of sand is approximately 1 mm 3. The order of magnitude of the number of grains of sand that can fit in the volume of the Earth is A B C D

An elephant has a life expectancy of 60 years. Which of the following gives the order of magnitude of this lifetime? A s B.10 9 s C.10 7 s D.10 5 s

rounding the order of magnitude

The SI system (metric system) Physicists have an agreed system of units, called ‘Le Système International d’Unités’ (S.I. Units) There are seven base units, all other units are derived from these

Derived units Physical quantities other than those listed above have units that are combinations of the seven fundamental units. Such units are called derived units. Ex. The unit of force is the Newton (N) 1N = 1 kgms -2.

In mechanics, physicists use three basic quantities to derive additional quantities. Mass is one of the three quantities. What are the other two? a)length and force b)power and force c)length and time d)force and time

A.amperesecond B.coulombkilogram C.coulombnewton D.metrekilogram Which one of the following lists a fundamental unit and a derived unit?

Which one of the following is a fundamental unit in the SI system? A.Ampere B.Volt C.Ohm D.Tesla

A.MetreKilogramCoulomb B.SecondAmpereNewton C.KilogramAmpereKelvin D.KelvinCoulombSecond Which one of the following contains three fundamental units?

the velocity of a wave on a string is related to the length L and mass m of the string, and the tension force F the string is subjected to. How exactly does the velocity depend on these three variables?

The resistive force F acting on a sphere of radius r moving at speed v through a liquid is given by F = cvr where c is a constant. Which of the following is a correct unit for c? A.N B.N s –1 C.N m 2 s –1 D.N m –2 s

Common metric prefixes PowerPrefixSymbol attoa femtof picop nanon microµ millim centiC decid

Common metric prefixes PowerPrefixSymbol 10 1 dekada 10 2 hectoh 10 3 kilok 10 6 megaM 10 9 gigaG teraT petaP exaE