1 Units and Measurement
1 Chapter Review
Key Terms
- accuracy
- the degree to which a measured value agrees with an accepted reference value for that measurement
- base quantity
- physical quantity chosen by convention and practical considerations such that all other physical quantities can be expressed as algebraic combinations of them
- base unit
- standard for expressing the measurement of a base quantity within a particular system of units; defined by a particular procedure used to measure the corresponding base quantity
- conversion factor
- a ratio that expresses how many of one unit are equal to another unit
- derived quantity
- physical quantity defined using algebraic combinations of base quantities
- derived units
- units that can be calculated using algebraic combinations of the fundamental units
- dimension
- expression of the dependence of a physical quantity on the base quantities as a product of powers of symbols representing the base quantities; in general, the dimension of a quantity has the form LaMbTcIdΘeNfJgLaMbTcIdΘeNfJg for some powers a, b, c, d, e, f, and g.
- dimensionally consistent
- equation in which every term has the same dimensions and the arguments of any mathematical functions appearing in the equation are dimensionless
- dimensionless
- quantity with a dimension of L0M0T0I0Θ0N0J0=1;L0M0T0I0Θ0N0J0=1; also called quantity of dimension 1 or a pure number
- discrepancy
- the difference between the measured value and a given standard or expected value
- English units
- system of measurement used in the United States; includes units of measure such as feet, gallons, and pounds
- estimation
- using prior experience and sound physical reasoning to arrive at a rough idea of a quantity’s value; sometimes called an “order-of-magnitude approximation,” a “guesstimate,” a “back-of-the-envelope calculation”, or a “Fermi calculation”
- kilogram
- SI unit for mass, abbreviated kg
- law
- description, using concise language or a mathematical formula, of a generalized pattern in nature supported by scientific evidence and repeated experiments
- meter
- SI unit for length, abbreviated m
- method of adding percents
- the percent uncertainty in a quantity calculated by multiplication or division is the sum of the percent uncertainties in the items used to make the calculation.
- metric system
- system in which values can be calculated in factors of 10
- model
- representation of something often too difficult (or impossible) to display directly
- order of magnitude
- the size of a quantity as it relates to a power of 10
- percent uncertainty
- the ratio of the uncertainty of a measurement to the measured value, expressed as a percentage
- physical quantity
- characteristic or property of an object that can be measured or calculated from other measurements
- physics
- science concerned with describing the interactions of energy, matter, space, and time; especially interested in what fundamental mechanisms underlie every phenomenon
- precision
- the degree to which repeated measurements agree with each other
- second
- the SI unit for time, abbreviated s
- SI units
- the international system of units that scientists in most countries have agreed to use; includes units such as meters, liters, and grams
- significant figures
- used to express the precision of a measuring tool used to measure a value
- theory
- testable explanation for patterns in nature supported by scientific evidence and verified multiple times by various groups of researchers
- uncertainty
- a quantitative measure of how much measured values deviate from one another
- units
- standards used for expressing and comparing measurements
Key Equations
| Percent uncertainty | [latex]\text{Percent uncertainty}=\frac{\delta A}{A}\times 100%[/latex] |
Summary
1.1 The Scope and Scale of Physics
- Physics is about trying to find the simple laws that describe all natural phenomena.
- Physics operates on a vast range of scales of length, mass, and time. Scientists use the concept of the order of magnitude of a number to track which phenomena occur on which scales. They also use orders of magnitude to compare the various scales.
- Scientists attempt to describe the world by formulating models, theories, and laws.
1.2 Units and Standards
- Systems of units are built up from a small number of base units, which are defined by accurate and precise measurements of conventionally chosen base quantities. Other units are then derived as algebraic combinations of the base units.
- Two commonly used systems of units are English units and SI units. All scientists and most of the other people in the world use SI, whereas nonscientists in the United States still tend to use English units.
- The SI base units of length, mass, and time are the meter (m), kilogram (kg), and second (s), respectively.
- SI units are a metric system of units, meaning values can be calculated by factors of 10. Metric prefixes may be used with metric units to scale the base units to sizes appropriate for almost any application.
1.3 Unit Conversion
- To convert a quantity from one unit to another, multiply by conversions factors in such a way that you cancel the units you want to get rid of and introduce the units you want to end up with.
- Be careful with areas and volumes. Units obey the rules of algebra so, for example, if a unit is squared we need two factors to cancel it.
1.4 Dimensional Analysis
- The dimension of a physical quantity is just an expression of the base quantities from which it is derived.
- All equations expressing physical laws or principles must be dimensionally consistent. This fact can be used as an aid in remembering physical laws, as a way to check whether claimed relationships between physical quantities are possible, and even to derive new physical laws.
1.5 Estimates and Fermi Calculations
- An estimate is a rough educated guess at the value of a physical quantity based on prior experience and sound physical reasoning. Some strategies that may help when making an estimate are as follows:
- Get big lengths from smaller lengths.
- Get areas and volumes from lengths.
- Get masses from volumes and densities.
- If all else fails, bound it.
- One “sig. fig.” is fine.
- Ask yourself: Does this make any sense?
1.6 Significant Figures
- Accuracy of a measured value refers to how close a measurement is to an accepted reference value. The discrepancy in a measurement is the amount by which the measurement result differs from this value.
- Precision of measured values refers to how close the agreement is between repeated measurements. The uncertainty of a measurement is a quantification of this.
- The precision of a measuring tool is related to the size of its measurement increments. The smaller the measurement increment, the more precise the tool.
- Significant figures express the precision of a measuring tool.
- When multiplying or dividing measured values, the final answer can contain only as many significant figures as the least-precise value.
- When adding or subtracting measured values, the final answer cannot contain more decimal places than the least-precise value.
1.7 Solving Problems in Physics
The three stages of the process for solving physics problems used in this book are as follows:
- Strategy: Determine which physical principles are involved and develop a strategy for using them to solve the problem.
- Solution: Do the math necessary to obtain a numerical solution complete with units.
- Significance: Check the solution to make sure it makes sense (correct units, reasonable magnitude and sign) and assess its significance.
Conceptual Questions
1.1 The Scope and Scale of Physics
1.
What is physics?
2.
Some have described physics as a “search for simplicity.” Explain why this might be an appropriate description.
3.
If two different theories describe experimental observations equally well, can one be said to be more valid than the other (assuming both use accepted rules of logic)?
4.
What determines the validity of a theory?
5.
Certain criteria must be satisfied if a measurement or observation is to be believed. Will the criteria necessarily be as strict for an expected result as for an unexpected result?
6.
Can the validity of a model be limited or must it be universally valid? How does this compare with the required validity of a theory or a law?
1.2 Units and Standards
7.
Identify some advantages of metric units.
8.
What are the SI base units of length, mass, and time?
9.
What is the difference between a base unit and a derived unit? (b) What is the difference between a base quantity and a derived quantity? (c) What is the difference between a base quantity and a base unit?
10.
For each of the following scenarios, refer to Figure 1.4 and Table 1.2 to determine which metric prefix on the meter is most appropriate for each of the following scenarios. (a) You want to tabulate the mean distance from the Sun for each planet in the solar system. (b) You want to compare the sizes of some common viruses to design a mechanical filter capable of blocking the pathogenic ones. (c) You want to list the diameters of all the elements on the periodic table. (d) You want to list the distances to all the stars that have now received any radio broadcasts sent from Earth 10 years ago.
1.6 Significant Figures
11.
(a) What is the relationship between the precision and the uncertainty of a measurement? (b) What is the relationship between the accuracy and the discrepancy of a measurement?
1.7 Solving Problems in Physics
12.
What information do you need to choose which equation or equations to use to solve a problem?
13.
What should you do after obtaining a numerical answer when solving a problem?
Problems
1.1 The Scope and Scale of Physics
14.
Find the order of magnitude of the following physical quantities. (a) The mass of Earth’s atmosphere: 5.1×1018kg;5.1×1018kg; (b) The mass of the Moon’s atmosphere: 25,000 kg; (c) The mass of Earth’s hydrosphere: 1.4×1021kg;1.4×1021kg; (d) The mass of Earth: 5.97×1024kg;5.97×1024kg; (e) The mass of the Moon: 7.34×1022kg;7.34×1022kg; (f) The Earth–Moon distance (semimajor axis): 3.84×108m;3.84×108m;(g) The mean Earth–Sun distance: 1.5×1011m;1.5×1011m; (h) The equatorial radius of Earth: 6.38×106m;6.38×106m; (i) The mass of an electron: 9.11×10−31kg;9.11×10−31kg; (j) The mass of a proton: 1.67×10−27kg;1.67×10−27kg; (k) The mass of the Sun: 1.99×1030kg.1.99×1030kg.
15.
Use the orders of magnitude you found in the previous problem to answer the following questions to within an order of magnitude. (a) How many electrons would it take to equal the mass of a proton? (b) How many Earths would it take to equal the mass of the Sun? (c) How many Earth–Moon distances would it take to cover the distance from Earth to the Sun? (d) How many Moon atmospheres would it take to equal the mass of Earth’s atmosphere? (e) How many moons would it take to equal the mass of Earth? (f) How many protons would it take to equal the mass of the Sun?
For the remaining questions, you need to use Figure 1.4 to obtain the necessary orders of magnitude of lengths, masses, and times.
16.
Roughly how many heartbeats are there in a lifetime?
17.
A generation is about one-third of a lifetime. Approximately how many generations have passed since the year 0 AD?
18.
Roughly how many times longer than the mean life of an extremely unstable atomic nucleus is the lifetime of a human?
19.
Calculate the approximate number of atoms in a bacterium. Assume the average mass of an atom in the bacterium is 10 times the mass of a proton.
20.
(a) Calculate the number of cells in a hummingbird assuming the mass of an average cell is 10 times the mass of a bacterium. (b) Making the same assumption, how many cells are there in a human?
21.
Assuming one nerve impulse must end before another can begin, what is the maximum firing rate of a nerve in impulses per second?
22.
About how many floating-point operations can a supercomputer perform each year?
23.
Roughly how many floating-point operations can a supercomputer perform in a human lifetime?
1.2 Units and Standards
24.
The following times are given using metric prefixes on the base SI unit of time: the second. Rewrite them in scientific notation without the prefix. For example, 47 Ts would be rewritten as 4.7×1013s.4.7×1013s. (a) 980 Ps; (b) 980 fs; (c) 17 ns; (d) 577μs.577μs.
25.
The following times are given in seconds. Use metric prefixes to rewrite them so the numerical value is greater than one but less than 1000. For example, 7.9×10−2s7.9×10−2s could be written as either 7.9 cs or 79 ms. (a) 9.57×105s;9.57×105s; (b) 0.045 s; (c) 5.5×10−7s;5.5×10−7s; (d) 3.16×107s.3.16×107s.
26.
The following lengths are given using metric prefixes on the base SI unit of length: the meter. Rewrite them in scientific notation without the prefix. For example, 4.2 Pm would be rewritten as 4.2×1015m.4.2×1015m. (a) 89 Tm; (b) 89 pm; (c) 711 mm; (d) 0.45μm.0.45μm.
27.
The following lengths are given in meters. Use metric prefixes to rewrite them so the numerical value is bigger than one but less than 1000. For example, 7.9×10−2m7.9×10−2m could be written either as 7.9 cm or 79 mm. (a) 7.59×107m;7.59×107m; (b) 0.0074 m; (c) 8.8×10−11m;8.8×10−11m; (d) 1.63×1013m.1.63×1013m.
28.
The following masses are written using metric prefixes on the gram. Rewrite them in scientific notation in terms of the SI base unit of mass: the kilogram. For example, 40 Mg would be written as 4×104kg.4×104kg. (a) 23 mg; (b) 320 Tg; (c) 42 ng; (d) 7 g; (e) 9 Pg.
29.
The following masses are given in kilograms. Use metric prefixes on the gram to rewrite them so the numerical value is bigger than one but less than 1000. For example, 7×10−4kg7×10−4kg could be written as 70 cg or 700 mg. (a) 3.8×10−5kg;3.8×10−5kg; (b) 2.3×1017kg;2.3×1017kg; (c) 2.4×10−11kg;2.4×10−11kg; (d) 8×1015kg;8×1015kg; (e) 4.2×10−3kg.4.2×10−3kg.
1.3 Unit Conversion
30.
The volume of Earth is on the order of 1021 m3. (a) What is this in cubic kilometers (km3)? (b) What is it in cubic miles (mi3)? (c) What is it in cubic centimeters (cm3)?
31.
The speed limit on some interstate highways is roughly 100 km/h. (a) What is this in meters per second? (b) How many miles per hour is this?
32.
A car is traveling at a speed of 33 m/s. (a) What is its speed in kilometers per hour? (b) Is it exceeding the 90 km/h speed limit?
33.
In SI units, speeds are measured in meters per second (m/s). But, depending on where you live, you’re probably more comfortable of thinking of speeds in terms of either kilometers per hour (km/h) or miles per hour (mi/h). In this problem, you will see that 1 m/s is roughly 4 km/h or 2 mi/h, which is handy to use when developing your physical intuition. More precisely, show that (a) 1.0m/s=3.6km/h1.0m/s=3.6km/h and (b) 1.0m/s=2.2mi/h.1.0m/s=2.2mi/h.
34.
American football is played on a 100-yd-long field, excluding the end zones. How long is the field in meters? (Assume that 1 m = 3.281 ft.)
35.
Soccer fields vary in size. A large soccer field is 115 m long and 85.0 m wide. What is its area in square feet? (Assume that 1 m = 3.281 ft.)
36.
What is the height in meters of a person who is 6 ft 1.0 in. tall?
37.
Mount Everest, at 29,028 ft, is the tallest mountain on Earth. What is its height in kilometers? (Assume that 1 m = 3.281 ft.)
38.
The speed of sound is measured to be 342 m/s on a certain day. What is this measurement in kilometers per hour?
39.
Tectonic plates are large segments of Earth’s crust that move slowly. Suppose one such plate has an average speed of 4.0 cm/yr. (a) What distance does it move in 1.0 s at this speed? (b) What is its speed in kilometers per million years?
40.
The average distance between Earth and the Sun is 1.5×1011m.1.5×1011m. (a) Calculate the average speed of Earth in its orbit (assumed to be circular) in meters per second. (b) What is this speed in miles per hour?
41.
The density of nuclear matter is about 1018 kg/m3. Given that 1 mL is equal in volume to cm3, what is the density of nuclear matter in megagrams per microliter (that is, Mg/μLMg/μL)?
42.
The density of aluminum is 2.7 g/cm3. What is the density in kilograms per cubic meter?
43.
A commonly used unit of mass in the English system is the pound-mass, abbreviated lbm, where 1 lbm = 0.454 kg. What is the density of water in pound-mass per cubic foot?
44.
A furlong is 220 yd. A fortnight is 2 weeks. Convert a speed of one furlong per fortnight to millimeters per second.
45.
It takes 2π2π radians (rad) to get around a circle, which is the same as 360°. How many radians are in 1°?
46.
Light travels a distance of about 3×108m/s.3×108m/s. A light-minute is the distance light travels in 1 min. If the Sun is 1.5×1011m1.5×1011m from Earth, how far away is it in light-minutes?
47.
A light-nanosecond is the distance light travels in 1 ns. Convert 1 ft to light-nanoseconds.
48.
An electron has a mass of 9.11×10−31kg.9.11×10−31kg. A proton has a mass of 1.67×10−27kg.1.67×10−27kg. What is the mass of a proton in electron-masses?
49.
A fluid ounce is about 30 mL. What is the volume of a 12 fl-oz can of soda pop in cubic meters?
1.4 Dimensional Analysis
50.
A student is trying to remember some formulas from geometry. In what follows, assume AA is area, VV is volume, and all other variables are lengths. Determine which formulas are dimensionally consistent. (a) V=πr2h;V=πr2h; (b) A=2πr2+2πrh;A=2πr2+2πrh; (c) V=0.5bh;V=0.5bh; (d) V=πd2;V=πd2; (e) V=πd3/6.V=πd3/6.
51.
Consider the physical quantities s, v, a, and t with dimensions [s]=L,[s]=L, [v]=LT−1,[v]=LT−1, [a]=LT−2,[a]=LT−2, and [t]=T.[t]=T. Determine whether each of the following equations is dimensionally consistent. (a) v2=2as;v2=2as; (b) s=vt2+0.5at2;s=vt2+0.5at2; (c) v=s/t;v=s/t; (d) a=v/t.a=v/t.
52.
Consider the physical quantities m,m, s,s, v,v, a,a, and tt with dimensions [m] = M, [s] = L, [v] = LT–1, [a] = LT–2, and [t] = T. Assuming each of the following equations is dimensionally consistent, find the dimension of the quantity on the left-hand side of the equation: (a) F = ma; (b) K = 0.5mv2; (c) p = mv; (d) W = mas; (e) L = mvr.
53.
Suppose quantity ss is a length and quantity tt is a time. Suppose the quantities vv and aa are defined by v = ds/dt and a =dv/dt. (a) What is the dimension of v? (b) What is the dimension of the quantity a? What are the dimensions of (c) ∫vdt,∫vdt, (d) ∫adt,∫adt, and (e) da/dt?
54.
Suppose [V] = L3, [ρ]=ML–3,[ρ]=ML–3, and [t] = T. (a) What is the dimension of ∫ρdV?∫ρdV? (b) What is the dimension of dV/dt? (c) What is the dimension of ρ(dV/dt)?ρ(dV/dt)?
55.
The arc length formula says the length ss of arc subtended by angle ƟƟ in a circle of radius rr is given by the equation s=rƟ.s=rƟ. What are the dimensions of (a) s, (b) r, and (c) Ɵ?Ɵ?
1.5 Estimates and Fermi Calculations
56.
Assuming the human body is made primarily of water, estimate the volume of a person.
57.
Assuming the human body is primarily made of water, estimate the number of molecules in it. (Note that water has a molecular mass of 18 g/mol and there are roughly 1024 atoms in a mole.)
58.
Estimate the mass of air in a classroom.
59.
Estimate the number of molecules that make up Earth, assuming an average molecular mass of 30 g/mol. (Note there are on the order of 1024 objects per mole.)
60.
Estimate the surface area of a person.
61.
Roughly how many solar systems would it take to tile the disk of the Milky Way?
62.
(a) Estimate the density of the Moon. (b) Estimate the diameter of the Moon. (c) Given that the Moon subtends at an angle of about half a degree in the sky, estimate its distance from Earth.
63.
The average density of the Sun is on the order 103 kg/m3. (a) Estimate the diameter of the Sun. (b) Given that the Sun subtends at an angle of about half a degree in the sky, estimate its distance from Earth.
64.
Estimate the mass of a virus.
65.
A floating-point operation is a single arithmetic operation such as addition, subtraction, multiplication, or division. (a) Estimate the maximum number of floating-point operations a human being could possibly perform in a lifetime. (b) How long would it take a supercomputer to perform that many floating-point operations?
1.6 Significant Figures
66.
Consider the equation 4000/400 = 10.0. Assuming the number of significant figures in the answer is correct, what can you say about the number of significant figures in 4000 and 400?
67.
Suppose your bathroom scale reads your mass as 65 kg with a 3% uncertainty. What is the uncertainty in your mass (in kilograms)?
68.
A good-quality measuring tape can be off by 0.50 cm over a distance of 20 m. What is its percent uncertainty?
69.
An infant’s pulse rate is measured to be 130 ± 5 beats/min. What is the percent uncertainty in this measurement?
70.
(a) Suppose that a person has an average heart rate of 72.0 beats/min. How many beats does he or she have in 2.0 years? (b) In 2.00 years? (c) In 2.000 years?
71.
A can contains 375 mL of soda. How much is left after 308 mL is removed?
72.
State how many significant figures are proper in the results of the following calculations: (a) (106.7)(98.2)/(46.210)(1.01);(106.7)(98.2)/(46.210)(1.01); (b) (18.7)2;(18.7)2; (c) (1.60×10−19)(3712)(1.60×10−19)(3712)
73.
(a) How many significant figures are in the numbers 99 and 100.? (b) If the uncertainty in each number is 1, what is the percent uncertainty in each? (c) Which is a more meaningful way to express the accuracy of these two numbers: significant figures or percent uncertainties?
74.
(a) If your speedometer has an uncertainty of 2.0 km/h at a speed of 90 km/h, what is the percent uncertainty? (b) If it has the same percent uncertainty when it reads 60 km/h, what is the range of speeds you could be going?
75.
(a) A person’s blood pressure is measured to be 120±2mm Hg.120±2mm Hg. What is its percent uncertainty? (b) Assuming the same percent uncertainty, what is the uncertainty in a blood pressure measurement of 80 mm Hg?
76.
A person measures his or her heart rate by counting the number of beats in 30 s. If 40 ± 1 beats are counted in 30.0 ± 0.5 s, what is the heart rate and its uncertainty in beats per minute?
77.
What is the area of a circle 3.102 cm in diameter?
78.
Determine the number of significant figures in the following measurements: (a) 0.0009, (b) 15,450.0, (c) 6×103, (d) 87.990, and (e) 30.42.
79.
Perform the following calculations and express your answer using the correct number of significant digits. (a) A woman has two bags weighing 13.5 lb and one bag with a weight of 10.2 lb. What is the total weight of the bags? (b) The force F on an object is equal to its mass m multiplied by its acceleration a. If a wagon with mass 55 kg accelerates at a rate of 0.0255 m/s2, what is the force on the wagon? (The unit of force is called the newton and it is expressed with the symbol N.)
Additional Problems
80.
Consider the equation y = mt +b, where the dimension of y is length and the dimension of t is time, and m and b are constants. What are the dimensions and SI units of (a) m and (b) b?
81.
Consider the equation s=s0+v0t+a0t2/2+j0t3/6+S0t4/24+ct5/120,s=s0+v0t+a0t2/2+j0t3/6+S0t4/24+ct5/120, where s is a length and t is a time. What are the dimensions and SI units of (a) s0,s0, (b) v0,v0, (c) a0,a0, (d) j0,j0, (e) S0,S0, and (f) c?
82.
(a) A car speedometer has a 5% uncertainty. What is the range of possible speeds when it reads 90 km/h? (b) Convert this range to miles per hour. Note 1 km = 0.6214 mi.
83.
A marathon runner completes a 42.188-km course in 2 h, 30 min, and 12 s. There is an uncertainty of 25 m in the distance traveled and an uncertainty of 1 s in the elapsed time. (a) Calculate the percent uncertainty in the distance. (b) Calculate the percent uncertainty in the elapsed time. (c) What is the average speed in meters per second? (d) What is the uncertainty in the average speed?
84.
The sides of a small rectangular box are measured to be 1.80 ± 0.1 cm, 2.05 ± 0.02 cm, and 3.1 ± 0.1 cm long. Calculate its volume and uncertainty in cubic centimeters.
85.
When nonmetric units were used in the United Kingdom, a unit of mass called the pound-mass (lbm) was used, where 1 lbm = 0.4539 kg. (a) If there is an uncertainty of 0.0001 kg in the pound-mass unit, what is its percent uncertainty? (b) Based on that percent uncertainty, what mass in pound-mass has an uncertainty of 1 kg when converted to kilograms?
86.
The length and width of a rectangular room are measured to be 3.955 ± 0.005 m and 3.050 ± 0.005 m. Calculate the area of the room and its uncertainty in square meters.
87.
A car engine moves a piston with a circular cross-section of 7.500 ± 0.002 cm in diameter a distance of 3.250 ± 0.001 cm to compress the gas in the cylinder. (a) By what amount is the gas decreased in volume in cubic centimeters? (b) Find the uncertainty in this volume.
Challenge Problems
88.
The first atomic bomb was detonated on July 16, 1945, at the Trinity test site about 200 mi south of Los Alamos. In 1947, the U.S. government declassified a film reel of the explosion. From this film reel, British physicist G. I. Taylor was able to determine the rate at which the radius of the fireball from the blast grew. Using dimensional analysis, he was then able to deduce the amount of energy released in the explosion, which was a closely guarded secret at the time. Because of this, Taylor did not publish his results until 1950. This problem challenges you to recreate this famous calculation. (a) Using keen physical insight developed from years of experience, Taylor decided the radius r of the fireball should depend only on time since the explosion, t, the density of the air, ρ,ρ, and the energy of the initial explosion, E. Thus, he made the educated guess that r=kEaρbtcr=kEaρbtc for some dimensionless constant k and some unknown exponents a, b, and c. Given that [E] = ML2T–2, determine the values of the exponents necessary to make this equation dimensionally consistent. (Hint: Notice the equation implies that k=rE−aρ−bt−ck=rE−aρ−bt−c and that [k]=1.[k]=1.) (b) By analyzing data from high-energy conventional explosives, Taylor found the formula he derived seemed to be valid as long as the constant k had the value 1.03. From the film reel, he was able to determine many values of r and the corresponding values of t. For example, he found that after 25.0 ms, the fireball had a radius of 130.0 m. Use these values, along with an average air density of 1.25 kg/m3, to calculate the initial energy release of the Trinity detonation in joules (J). (Hint: To get energy in joules, you need to make sure all the numbers you substitute in are expressed in terms of SI base units.) (c) The energy released in large explosions is often cited in units of “tons of TNT” (abbreviated “t TNT”), where 1 t TNT is about 4.2 GJ. Convert your answer to (b) into kilotons of TNT (that is, kt TNT). Compare your answer with the quick-and-dirty estimate of 10 kt TNT made by physicist Enrico Fermi shortly after witnessing the explosion from what was thought to be a safe distance. (Reportedly, Fermi made his estimate by dropping some shredded bits of paper right before the remnants of the shock wave hit him and looked to see how far they were carried by it.)
89.
The purpose of this problem is to show the entire concept of dimensional consistency can be summarized by the old saying “You can’t add apples and oranges.” If you have studied power series expansions in a calculus course, you know the standard mathematical functions such as trigonometric functions, logarithms, and exponential functions can be expressed as infinite sums of the form ∑n=0∞anxn=a0+a1x+a2x2+a3x3+⋯,∑n=0∞anxn=a0+a1x+a2x2+a3x3+⋯, where the anan are dimensionless constants for all n=0,1,2,⋯n=0,1,2,⋯ and x is the argument of the function. (If you have not studied power series in calculus yet, just trust us.) Use this fact to explain why the requirement that all terms in an equation have the same dimensions is sufficient as a definition of dimensional consistency. That is, it actually implies the arguments of standard mathematical functions must be dimensionless, so it is not really necessary to make this latter condition a separate requirement of the definition of dimensional consistency as we have done in this section.
