Physics 151: Lecture Notes on Units, Conversions, and Scientific Notation, Study notes of Physics

Download Physics 151: Lecture Notes on Units, Conversions, and Scientific Notation and more Study notes Physics in PDF only on Docsity! PHYSICS 151 – Notes for Online Lecture #1 Whenever we measure a quantity, that measurement is reported as a number and a unit of measurement. We say such a quantity has dimensions. Units are a necessity – they are part of any answer and the answer is wrong without them. We will preferentially use metric units, although some of the problems in the book use English units. Conversions are given in the inside front cover of your book. SI BASE UNITS (Metric System) Quantity Metric American Conversion Length meter (m) foot (ft) 1 ft = .305 m Mass kilogram (kg) slug 1 slug = 14.6 kg Time seconds seconds There are other base units that we will get to later in the course. We will also use many derived units that are combinations of base units such as the unit for energy called a joule which is a kg-m2/s2. How many meters are in a mile? Dimensional Analysis – Any valid physics formula must be dimensionally consistent meaning that each term must have the same units. Is the equation Dimensionally consistent? [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] 0 2 v v at L L L T T T T L L L T T T = + = + = + You Try It! You Try It! 0x x at= + Scientific Notation - Mathematical Shorthand for expressing very large and very small numbers. The number is written with one digit to the left of the decimal place and then multiplied by a power of ten. Examples: the speed of light is c = 300,000,000 m/s = 3.0 x 100,000,000 m/s = 3.0 x 108 m/s the size of a human hair is 0.000070 m = 7.0 x 10-5 m - The nearest star is around 41,000,000,000,000,000 m from the sun, 41,000,000,000,000,000 m = _______________________ m The exponent is 16 since the decimal place was moved 16 places to the left. - The wavelength of visible light is 0.0000005 m 0.0000005 = ______________________ m The exponent is -7 since the decimal place was moved 7 places to the right. What is 360,000 in Scientific Notation? Metric Prefixes - We often use prefixes to simplify the notation. You’re already used to using prefixes – we use then in talking about ‘millions’ ‘trillions’, etc. Peta P 1015 tera T 1012 giga G 109 mega M 106 kilo k 103 centi c 10-2 milli m 10-3 micro µ 10-6 nano n 10-9 pico p 10-12 femto f 10-15 Can you express 0.024 in terms of a convenient metric prefix? You Try It! You Try It! EXAMPLE: A field measures 20 km by 30 km. What is the area in m2? A = (20 km) x (30 km) = 600 km2 28 2 22 8 2 22 m10x6 km m km10x6 km1 m 1000km006km006 =⎥ ⎦ ⎤ ⎢ ⎣ ⎡ =⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = This example illustrates why it is so important for you to include units when doing your calculations. If you accidentally use the wrong conversion factor, you should be able to catch yourself at the end when the units don’t work out correctly. EXAMPLE : How old are you in seconds? Solution: age in years 365 days 1 yr 24 hr 1 day 60 min 1 hr 60 s 1 min ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝⎜ ⎞ ⎠⎟ ⎛ ⎝⎜ ⎞ ⎠⎟ age in years x10 s yr 7× ⎛ ⎝ ⎜ ⎞ ⎠ ⎟32. Age in years Age in seconds 18 5.8 x 108 19 6.1 x 108 20 6.4 x 108 21 6.7 x 108 22 7.1 x 108 You Try It! A tile store sells tile at the rate of $2.69 per square foot. How much does the tile cost per square meter? Measurement and Significant Figures No measurement is exact. Although we have atomic clocks that are highly accurate, the accuracy of the watch on your wrist is probably good enough for you to make it to class on time. The accuracy you need depends on what you’re going to use the measurements for. Let’s return to the example of measuring the lengths of the metal rod. The ruler in question has as its smallest markings a tenth of an inch. You measure the length and find that it falls about halfway 4.1 and 4.2 cm. You estimate that the length is 4.15 cm, but the 0.05 in is a guess, so you would report that the length of the rod as 4.15 ± 0.05 mm. The smallest marking on the measuring device represents the precision of your measurements. The accuracy of a measurement can be determined by repeating it more than once. The accuracy of a measurement is reflected in the way the number is written. When a number is reported, assume that the number of digits reported is the number known with any certainty. The uncertainty is generally assumed to be one or two units of the last digits. When counting the number of significant figures: • All digits 1 through 9 count as significant figures • Zeroes to the left of all of the other digits are not significant • Zeroes between digits are significant • Zeroes to the right of all other digits are significant if after the decimal point and may or may not be significant if before the decimal point. For example Number Number of Significant Figures Possible Range of the Real Measurement 1.2 2 1.1 - 1.3 3.61 3 3.60 –3.62 19.61 4 19.60 - 19.62 0.017 2 .0016 - .0018 10.25 0.1020 80 Sometimes, the number of significant digits can be unclear. For example, if we write 80 it is not clear whether the zero is significant or not. Is the measurement between 70 and 90 or between 79 and 81? If you write 80.0, there are 3 significant figures, because zeroes to the right of the decimal point are significant Exponential notation removes this ambiguity. We write all of the significant figures out front and then show what power of ten the significant figures should be multiplied by. For example (assuming that there is one unit of uncertainty in the last significant figure: 8 x 101 - means 8 (±1) x 101 = measurement is between 70 and 90 8.0 x 101 means 8.0 (± 0.1) x 101 = measurement is between 79 and 81 Rules for manipulating significant figures • Addition or subtraction: keep the place of the digit which is the same as the least significant place of the numbers you are adding/subtracting. • Multiplication or division: keep the same number of digits as the multiplicand with the least number of significant figures. EXAMPLE 2-6: A room is measured to be 5.5 feet wide and 6.75 feet long. What are the area and perimeter of the room? A = width × length = 5.5 feet × 6.75 feet = 37.125 feet2. Do we know this to 5 significant figures? Nope. 5.5 feet = 2 s.f. 6.75 feet = 3 s.f. We keep the same number of significant figures as the multiplicand with the smallest number of significant figures, so we can only use 2 s.f. A = width × length = 5.5 feet × 6.75 feet = 37.125 feet2 = 37 feet2. P = 2(l + w) = 2 (5.5 + 6.75) = 24.50 ft = 24.5 ft You Try It! Calculate the volume of a long cylinder that has a radius of r = 4.2 cm and a length of 26.52 cm. After making the calculation convert your volume to cubic inches.