MODULE 1: PHYSICAL QUANTITIES, UNITS AND VECTORS MODULE 1: PHYSICAL QUANTITIES, UNITS AND VECTORS 1.1 Physics and its importance The word physics comes from Greek, meaning “of nature” or “natural philosophy”. Physics is concerned with the description of nature—that is, the description and explanation of natural phenomena. In other words, physics is concerned with how and why things work or behave the way they do. Physics is an experimental science. Everything we know about the physical world and about the principles that govern its behavior has been learned through experiment, that is, through observations of the phenomena of nature. The ultimate test of any physical theory is its agreement with experimental observations. These observations usually involve measurements; thus physics is inherently a science of experiment and measurements. 1.2 Physical Quantities The study of Physics involves dealing with a lot of physical quantities. In mechanics, we have the basic or fundamental quantities like mass, length and time. All others are considered as derived quantities because they are obtained or defined by simple relations between the fundamental quantities. The fundamental quantities combined to form the derived quantity are sometimes called the dimensions of the derived quantity. Basic Quantities and Units Unit Symbol Description Definition Meter m Measures length Distance light travel in 1/299792458 second. Kilogram kg Measures mass Mass of a special platinum-iridium cylinder in Paris. Second s Measures time 9192631770 oscillations of a special light emitted by cesium-133 atoms. Combinations of these basic quantities and units give various derived quantities and units. Example: Force:1newton (N) = 1 kg-m/s2 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 1 In the proper expression of physical quantities, there should at least be a number (to indicate how large or how small the quantity is) and the unit (to indicate the nature and type of the quantity). An expression that does not have one of these two is meaningless. 1.3 Standards and Units A standard is that quantity (usually in physical form i.e. an object) to which other quantities being measured are compared. The measured quantity is then expressed in terms of the standard, which now becomes the unit of the quantity. Example: When we say the height of the building is 10 meters, it means that the measured quantity (height of the building) is expressed in terms of the length of an object (standard), which is considered to be one meter long. Thus the "meter" is the unit for the height of a building. 1.4 Systems of Units: There are two systems of units in common use: The English or British system and the Metric system. A refinement of the old metric system was introduced in 1960 and is officially known as the International System of units or SI units. It is now modern practice to use this system. The English system is also known as the foot-pound-second (fps) system. The SI system may be classified into the meter-kilogram-second (mks) system and the Gaussian or centimeter-gram-second (cgs) system. It is therefore necessary to look at the more salient aspects of the SI system: 1. In the SI system, the standard units for the different basic quantities are well defined, clear and precise. Example: For Length: 1 meter is defined as the distance traveled by light in 1/299792458 sec The student is advised to look at the SI definitions for the other basic units. Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 2 2. In the SI system of units, larger or smaller variations of these units are obtained by attaching the proper prefix. Prefixes in the Metric System or SI system Prefix Abbreviatio Power of 10 n Prefix Abbreviatio n Power of 10 exa E 1018 deci d 10-1 peta P 1015 centi c 10-2 tera T 1012 milli m 10-3 giga G 109 micro 10-6 mega M 106 nano n 10-9 kilo k 103 pico p 10-12 hecto h 102 femto f 10-15 deka da 101 atto a 10-18 1.5 Unit Consistency and Unit Conversion Unit consistency means that in a physical equation, each side of the expression should have the same units otherwise the equation is an error. Unit conversion is the process of changing the unit of a quantity to another one within the same system or into another system. In physical computations, this is usually done to attain unit consistency. The process of unit conversion may be relatively easy but it has to be done in an orderly manner to avoid errors. One should also have considerable knowledge of the needed conversion factors to be able to do it successfully. Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 3 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 4 Steps in Performing Unit Conversion: Example problems on Conversion of units: 1. Convert 120 km/hr to mi/hr. Solution: 120 𝑘𝑚 1𝑚𝑖 𝒎𝒊 × = 𝟕𝟒. 𝟓𝟕 1 ℎ𝑟 1.6093 𝑘𝑚 𝒉𝒓 Note that the unit km cancels out due to division. 2. As an astute observer walking around on continental crust (granite), you might decide to test the hypothesis that the Earth is made entirely of granite. You weigh a 1.00 ft3 piece of granite on your home scale and find that it weighs 171 lbs. Thus you determine that the granite has a density of 171 lb/ft 3. Convert your granite's density to g/cm 3. Solution: 171 𝑙𝑏 1000𝑔 1𝑓𝑡 1𝑖𝑛 𝒈 𝑥 𝑥 𝑥 = 𝟐. 𝟕𝟒 𝑓𝑡 2.205𝑙𝑏 12𝑖𝑛 2.54𝑐𝑚 𝒄𝒎𝟑 Note that the units lb, ft3 and in3 cancels out due to division Formative Problems: Practice conversion of units by solving the following problems. 1. The density of propane is 36.28 lb/ft3. Convert this to kg/m3. (Ans..581.67) 2. A box measures 3.12 ft in length, 0.0455 yd in width and 7.87 inches in height. What is its volume in cubic centimeters? (Ans.. 7.91 x 103 cm3) 3. A block occupies 0.2587 ft3 . What is its volume in mm3 ? (Ans.. 7.326x106 mm3) Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 5 1.6 Vectors and Vector Operations Many physical quantities have magnitudes only but no direction. These are called scalars. Examples are mass, time, density, temperature, etc. There are however, many physical quantities such as force, velocity, displacement, etc. which have directions as well as magnitude and these aspects always have to be indicated when expressing these quantities. They are called vectors. In physical computation and analyses, we have to be aware of the difference between vectors and scalars because the mathematical treatments are not the same. For example, we add scalars arithmetically but we cannot do the same to vectors. Special methods are used. 1.6.1 BASIC ASPECTS ABOUT VECTORS 1. Vector Representation a. Graphical representation - Vectors are represented by arrows. b. Vector notations – Vectors are usually denoted with capital letters written in boldface or with special markings. (A or A, B or B, etc.) 2. Indicating Directions of (coplanar) vectors: METHOD 1: Using the angle θ that the vector makes with the “zero reference line (usually the positive x-axis) ” measured going Counterclockwise.Illustration: i. Vector A = 3 units at 35o is a vector having a magnitude of 3 units, and whose direction θA is 35o from the positive x-axis measured going counter-clockwise. Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 6 ii. Vector A = 3 units at 1250 is a vector having a magnitude of 3 units, and whose direction θA is 125o from the positive x-axis measured going counter-clockwise iii. Vector A = 3 units at 2250 is a vector having a magnitude of 3 units, and whose direction θA is 225o from the positive x-axis measured going counter-clockwise. Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 7 METHOD 2: Using Geographic Directions Illustration: Let us assume that the figure below shows vector A = 3 units, θA = 25o and vector B = 3 units, θB = 30o The figure above means that: i. Vector A = 3 units 25o EN (East of North) ii. Vector A = 3 units 65o NE iii. Vector B = 3 units 30o SW iv. Vector B = 3 units 60o WS Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 8 1.6.2 VECTOR OPERATIONS 1. VECTOR ADDITION AND SUBTRACTION: Vector addition is the process of combining two or more vectors into one. The combination is called the RESULTANT (R) of the vectors. Vector subtraction is just like addition. In vector subtraction, the negative of one vector is added to the other. For example, if two vectors A and B are to be added, the operation is indicated as A + B. However, if vector B is to be subtracted from vector A, the operation is indicated as A – B which is the same as A + (-B). NOTE: The negative of a vector is another vector whose magnitude is the same as the original vector but in the opposite direction. METHODS OF VECTOR ADDITION i. The Algebraic method (for co-linear vectors only). Co-linear vectors are vectors which lie along the same line. Example: For the vectors shown in the diagram, determine a) their resultant; b) C A-D E = 60 m B = 20 m C = 30 m D = 25 m A = 50 m Solution: For convenience we assign all vectors directed towards the right as positive while all vectors directed towards the left are negative. Since vectors are co-linear simple arithmetic is applied a) Resultant: R = A+B+C+D+E = 50m+(–20m)+(-30m)+ 25m+(-60m) = - 35 m, this implies that the magnitude of the resultant vector has a magnitude of 35 m and directed towards the left (negative sign) b) C-A-D = C+(-A)+(-D) = -30m + (-50m ) + (-25m) = -101m, this implies that the magnitude of C-A-D is 101m and directed towards the left (negative sign) c) The Parallelogram Method The procedure of "the parallelogram of vectors addition method" is a. b. c. d. draw vector 1 using appropriate scale and in the direction of its action from the tail of vector 1 draw vector 2 using the same scale in the direction of its action complete the parallelogram by using vector 1 and 2 as sides of the parallelogram the resulting vector R is represented in both magnitude and direction by the diagonal of the parallelogram A A A R B B B Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 9 e. Solve the resultant using sine law and cosine law d) The Polygon method (Graphical method in determining the magnitude and direction of the Resultant R) Many vectors can be added together in this way by drawing the successive vectors in a tip-to-tail fashion, as shown on the example below. Scale: 1 cm = 1 unit e) The Triangle method is similar to the Parallelogram Method but with the two vectors connected from tip-to-tail. Procedure: a. b. c. Construct the vector triangle by drawing the two vectors tip-to-tail. The vector that closes the triangle is the resultant. The resultant vector R of the two coplanar vectors can be calculated by trigonometry using "the cosine law" for a non-right-angled triangle. The angle between the vector and the resultant vector can be calculated using "the sine law" for a non-right-angled triangle. f) The Component Method Ay Many vector operations and analyses are carried out using their components. These are two or more vectors which when combined or A added will give the original vector. For coplanar vectors (assumed to be on the xy-plane) it is usually convenient to use two components which are perpendicular to each other: one along the x-axis which is then called the xcomponent and the other one along the y-axis which is then called the ycomponent. These two components are collectively called the rectangular components of the vector. Determining the Components of a Vector 1. F1x is the magnitude of the x-component of vector F 1. 2. The sign of F1x is positive if it points in the positive x-direction, negative if it points in the negative x-direction. Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 10 3. F1y is the magnitude of the y-component of vector F1. 4. The sign of F1y is positive if it points in the positive y-direction, negative if it points in the negative y-direction. UNIT VECTORS (3 – dimensional vectors) Let Vectors having a magnitude of unity with no units. Its purpose is to describe a direction in space. 𝚤 = unit vector pointing in the x – axis 𝚥̂ = unit vector pointing in the y – axis 𝑘 = unit vector pointing in the z – axis y 𝚥̂ 𝚤̂ x 𝑘 z If A and B are in terms of their components: A = Ax𝚤 + Ay𝚥 + Az𝑘 Addition: and B = Bx𝚤̂ + By𝚥̂ + Bz𝑘 A + B = (Ax + Bx)𝚤̂ +(Ay + By)𝚥 + (Az + Bz)𝑘 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any 11 means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. PRODUCT OF VECTORS: A) Scalar Product (Dot Product) o Results to scalar quantity i.e. magnitude only, no direction. A ∙ B = AB cos θ B Where A is the magnitude of vector A, and B is the magnitude of vector B, and θ is the angle between them. θ A If θ = 90o, A∙B = AB cos 90o, cos 90o = 0, A∙B = 0; scalar product of 2 perpendicular vectors is always 0. Using the unit vector computation: A∙B = (Ax𝚤 + Ay𝚥 + Az𝑘 ) ∙ (Bx𝚤 + By𝚥 + Bz𝑘 ) A∙B = AxBx + AyBy + AzBz B) Vector Product (Cross Product) o Vector quantity with a direction perpendicular to the plane of the vector and a magnitude given by: A x B = AB sin θ If A and B are parallel, θ = 0 or 180o then A x B = 0 since, sin 0 & sin 180o = 0. There are always two directions perpendicular to a given plane. Use the right hand rule. USING VECTOR REPRESENTATION: A x B = (Ax𝚤 + Ay𝚥 + Az𝑘 ) x (Bx𝚤 + By𝚥 + Bz𝑘 ) 𝚤 Where: 𝚤 x𝚤=0 𝚤 x𝚥=𝑘 𝑘 x 𝚥 = -𝚤 𝚥x𝚥=0 𝚥 x𝚤 =-𝑘 𝚤 x 𝑘 = -𝚥 𝑘 x 𝑘 = 0𝚥 x 𝑘 = 𝚤 𝑘 𝑘 x𝚤 =𝚥 + 𝚥 vector product of two parallel vectors is always zero. A x B = (AyBz – AzBy) ̂ + (AzBx – AxBz) ̂ + (AxBy = AyBx)𝒌 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 12 SAMPLE PROBLEMS FOR VECTOR ADDITION: 1. Given are the following vector quantities: A = 80 m due N B = 40 m 300 NW C = 60 m 150 NE D = 50 m SE Determine: a. Magnitude and direction of the resultant of vectors A and B (using Parallelogram Method) b. Magnitude and direction of the resultant of vectors A and B (using unit vectors) c. Magnitude and direction of the Resultant of the four given vectors using component method. The given vectors drawn in the Cartesian-plane: N A=80m B=40m W C=60m 300 150 45 E 0 D=50m S Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 13 a) Using PARALLELLOGRAM METHOD: Let vector RAB represent the resultant of vectors A and B N RAB A=80m A=80m 1200 B=40m 300 W E S By isolating the lower half of the parallelogram, our analysis in determining the magnitude and direction of RAB can be determined by applying Sine and Cosine Laws. By cosine law: RAB = 𝟖𝟎𝟐 + 𝟒𝟎𝟐 − 𝟐(𝟖𝟎)𝟒𝟎 𝐜𝐨𝐬 𝟏𝟐𝟎 = 𝟏𝟎𝟓. 𝟖𝟑𝒎 By sine law: sin 𝛷 sin 120 = 105.83 80 ⬚ Angle Φ = 40.89o Direction of the resultant = 300 + 41.1436o = 71.1436o Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 14 THEREFORE: RAB = 105.83m, 7.89 NW (or North of West) Note that the Triangle method will have the same solution as the parallelogram method. Connect the two vectors from tip-to-tail. The Resultant is a vector drawn from the tail of the first vector to the tip of the second vector. B=40m RAB N A=80m W E S Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 15 b) USING COMPONENT MET N A=80m W E S X-COMPONENT: AX =0 Y-COMPONENT: AY=80m N By B=40m 300 W E Bx X-COMPONENT: BX = 40 cos 30 = 34.64m Y-COMPONENT: BY = 40 sin 30 = 20m S Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 16 By adding all x components of the vector: (vectors to the right “+”; vectors to the left “-“), we have: RABX = AX +BX = 0 +(-34.64) = - 34.64m The negative sign means that the x-component of R AB is towards the left. By adding all y-components of the vector: (vectors upward “+”; vectors downward“-“), we have: RABY = AY + BY = 80 + 20 = 100m Since the value of the y-component of the resultant is positive, it means that it is directed upwards. The x and y components of the resultant can now be drawn as: N RAB RABY = 100m AB W E RABX = 34.64m S Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 17 By Pythagorean Theorem: RAB = √𝟑𝟒. 𝟔𝟒𝟐 + 𝟏𝟎𝟎𝟐 = 𝟏𝟎𝟓. 𝟖𝟑𝒎 From trigonometric functions of Right Triangles we have: Tan θAB = 𝟏𝟎𝟎 𝟑𝟒.𝟔𝟒 θAB = 70.89O THEREFORE: RAB = 105.83m, 7.89 NW (or North of West) c). Magnitude and direction of the Resultant of the four given vectors From Previous solution: The Components of vector A are: X-COMPONENT: AX =0 Y-COMPONENT: AY=80m The Components of vector B are: X-COMPONENT: BX = 40 cos 30 = 34.64m Y-COMPONENT: BY = 40 sin 30 = 20m For Vector C, the components are: N C=60m Cy 150 W Cx E Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 18 S X-COMPONENT: CX = 60 cos 15 = 57.96m Y-COMPONENT: CY = 60 sin 15 = 15.53m For Vector D, the components are: N W E 45 0 D=50m Dy Dx S X-COMPONENT: DX = 60 cos 45o = 42.43m Y-COMPONENT: DY = 60 sin 15o = 42.43m x-component of the Resultant: Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 19 RX = AX + BX + CX + DX = 0 + (-34.64m) + 57.96m + 42.43m = 65.75m (to the right) x-component of the Resultant: RY = AY + BY + CY + DY = 80m + 20m + 15.53m + (-42.43m) = 73.1m (upward) N Ry R θ W Rx E S RX = √𝟔𝟓. 𝟕𝟓𝟐 + 𝟕𝟑. 𝟏𝟐 = 𝟗𝟖. 𝟑𝟐𝒎 Tan θ = 𝟕𝟑.𝟏 𝟔𝟓.𝟕𝟓 θAB = 48.03O THEREFORE: R = 98.32m, 48.03 NE (or North of EAST) Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 20 Formative Problems: 1. A = 1km due south, B = 2km due west. Determine the resultant. (ans..2.24km, 63.4o W of S) 2. A = 72.4 m, 32.0° east of north, B = 57.3 m, 36.0° south of west, C = 17.8 m due south. Determine the resultant. (ans..12.7m, 39o W of N) SAMPLE PROBLEMS FOR DOT PRODUCT: Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 21 SAMPLE PROBLEMS FOR CROSS PRODUCT: EXERCISES: 1. Do the following conversions: (a) 15 m to ft, (b) 12 in to cm, (c) 30 days to sec 2. A football field is 300 ft long and 160 ft wide. What are the field’s dimensions in meters and its area in square centimeters? 3. In the Bible, Noah is instructed to build an ark 300 cubits long, 50 cubits wide and 30 cubits high. (A cubit was a unit of length based on the length of the forearm and equal to half of a yard.) What would the dimensions of the ark be in meters? What would its volume be in cubic meters? (Assume that the ark was rectangular.) 4. Which is longer and by how many centimeters, a 100-m dash or a 100-yd dash? 5. Vector is A =2.80 cm long and is above the x-axis in the first quadrant. Vector is 1.90 cm long and is below the x-axis in the fourth quadrant. Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 22 Find using triangle method for all the items: a) A + B = C, b) A – C c) B + C 6. Three ropes pull on a large stone stuck in the ground, producing the vector forces as shown in. Find the magnitude and direction of a fourth force on the stone that will make the vector sum of the four forces zero. DULE 2: Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 23