# PROBLEM SOLVING EXERCISES IN PHYSICS ANSWER KEY

Describe the procedure for arising a problem-fixing strategy.Explain just how to discover the numerical solution to a trouble.Summarize the process for assessing the meaning of the numerical solution to a difficulty.

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Figure 1.13 Problem-resolving skills are important to your success in physics. (credit: “scui3asteveo”/Flickr)

Problem-addressing skills are clearly crucial to success in a quantitative course in physics. More necessary, the capability to apply wide physical principles—generally represented by equations—to specific instances is a really effective develop of knowledge. It is a lot more powerful than memorizing a list of facts. Analytical abilities and problem-solving abilities can be used to brand-new cases whereas a list of facts cannot be made lengthy enough to contain eextremely feasible circumstance. Such analytical skills are useful both for fixing troubles in this message and for using physics in day-to-day life.

As you are most likely well aware, a specific amount of creativity and understanding is compelled to deal with problems. No rigid procedure works eincredibly time. Creativity and also understanding grow with suffer. With practice, the basics of difficulty resolving end up being nearly automatic. One means to gain exercise is to work-related out the text’s examples for yourself as you read. Another is to work-related as many kind of end-of-area difficulties as possible, starting via the simplest to construct confidence and then advancing to the even more tough. After you come to be connected in physics, you will certainly watch it all roughly you, and also you have the right to begin to apply it to situations you encounter external the classroom, simply as is done in many type of of the applications in this message.

Although tright here is no straightforward step-by-step technique that functions for eexceptionally difficulty, the following three-phase procedure facilitates trouble resolving and provides it more systematic. The three stages are strategy, solution, and meaning. This procedure is provided in examples throughout the book. Here, we look at each stage of the process subsequently.

### Strategy

Strategy is the beginning stage of solving a difficulty. The principle is to figure out exactly what the trouble is and also then develop a strategy for addressing it. Some basic advice for this phase is as follows:

Examine the situation to recognize which physical ethics are involved. It regularly helps to draw a straightforward sketch at the outset. You frequently have to decide which direction is positive and note that on your sketch. When you have determined the physical ethics, it is much less complicated to discover and apply the equations representing those ethics. Although finding the correct equation is vital, store in mind that equations recurrent physical ethics, regulations of nature, and relationships among physical quantities. Without a conceptual understanding of a problem, a numerical solution is meaningless.Make a list of what is offered or have the right to be inferred from the difficulty as proclaimed (recognize the “knowns”). Many kind of troubles are proclaimed extremely succinctly and also need some inspection to identify what is known. Drawing a sketch deserve to be extremely advantageous at this suggest as well. Formally identifying the knowns is of particular prestige in using physics to real-people situations. For instance, the word stopped suggests the velocity is zero at that immediate. Also, we deserve to often take initial time and place as zero by the proper option of coordinate system.Identify specifically what demands to be established in the difficulty (identify the unknowns). In complex troubles, particularly, it is not always evident what demands to be found or in what sequence. Making a list deserve to aid identify the unknowns.Determine which physical values deserve to help you solve the problem. Since physical ethics tfinish to be expressed in the create of mathematical equations, a list of knowns and also unknowns deserve to aid right here. It is simplest if you have the right to find equations that contain just one unknown—that is, all the various other variables are known—so you deserve to deal with for the unwell-known conveniently. If the equation includes even more than one unknown, then extra equations are needed to resolve the difficulty. In some difficulties, numerous unknowns have to be determined to gain at the one essential a lot of. In such troubles it is especially crucial to keep physical principles in mind to protect against going astray in a sea of equations. You may have to use two (or more) various equations to obtain the final answer.

### Solution

The solution stage is once you carry out the math. Substitute the knowns (together with their units) right into the appropriate equation and also attain numerical remedies finish through units. That is, carry out the algebra, calculus, geomeattempt, or arithmetic necessary to find the unknown from the knowns, being sure to carry the systems via the calculations. This action is clearly vital because it produces the numerical answer, along with its devices. Notice, but, that this phase is just one-3rd of the all at once problem-addressing process.

### Significance

After having actually done the math in the solution phase of problem fixing, it is tempting to think you are done. But, always remember that physics is not math. Rather, in doing physics, we usage math as a tool to help us understand also nature. So, after you acquire a numerical answer, you have to constantly assess its significance:

### Summary

The 3 steras of the process for addressing physics troubles offered in this book are as follows:

Strategy: Determine which physical ethics are affiliated and develop a strategy for making use of them to resolve the problem.Solution: Do the math vital to achieve a numerical solution complete via systems.Significance: Check the solution to make certain it provides feeling (correct devices, reasonable magnitude and sign) and assess its meaning.

### Conceptual Questions

What indevelopment carry out you must select which equation or equations to usage to resolve a problem?

What need to you perform after obtaining a numerical answer as soon as resolving a problem?

Show Solution

Check to make sure it renders feeling and assess its significance.

Consider the equation y = mt +b, wright here the measurement of y is size and also the measurement of t is time, and m and b are constants. What are the dimensions and also SI units of (a) m and (b) b?

Consider the equation where s is a size and also t is a time. What are the dimensions and SI units of (a) (b) (c) (d) (e) and also (f) c?

Sexactly how Solution

a. and also units are meters (m); b. and also devices are meters per second (m/s); c. and units are meters per second squared (m/s2); d. and also devices are meters per second cubed (m/s3); e. and also units are m/s4; f. and also devices are m/s5.

(a) A car speedometer has actually a 5% uncertainty. What is the array of feasible speeds when it reads 90 km/h? (b) Convert this range to miles per hour. Note 1 km = 0.6214 mi.

A marathon runner completes a 42.188-kilometres course in 2 h, 30 min, and also 12 s. Tbelow is an uncertainty of 25 m in the distance traveled and also 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?

Sjust how Solution

a. 0.059%; b. 0.01%; c. 4.681 m/s; d. 0.07%, 0.003 m/s

The sides of a small rectangular box are measured to be 1.80 ± 0.1 cm, 2.05 ± 0.02 cm, and also 3.1 ± 0.1 cm lengthy. Calculate its volume and also uncertainty in cubic centimeters.

When nonmetric units were supplied in the United Kingdom, a unit of mass dubbed 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?

The size and also width of a rectangular room are measured to be 3.955 ± 0.005 m and also 3.050 ± 0.005 m. Calculate the area of the room and its uncertainty in square meters.

A auto 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 lessened in volume in cubic centimeters? (b) Find the uncertainty in this volume.

Sjust how Solution

a. 143.6 cm3; b. 0.2 cm3 or 0.14%

The initially atomic bomb was detonated on July 16, 1945, at the Trinity test site around 200 mi southern of Los Alamos. In 1947, the UNITED STATE government declassified a film reel of the explosion. From this film reel, British physicist G. I. Taylor was able to identify the rate at which the radius of the firesphere from the blast flourished. Using dimensional evaluation, he was then able to deduce the amount of power released in the explosion, which was a carefully guarded secret at the moment. Therefore, Taylor did not publish his results until 1950. This difficulty difficulties you to redevelop this famous calculation. (a) Using keen physical insight developed from years of suffer, Taylor determined the radius r of the fireround must depfinish only on time because the explosion, t, the thickness of the air, and also the energy of the initial explosion, E. Therefore, he made the educated guess that for some dimensionless continuous k and some unrecognized exponents a, b, and c. Given that = ML2T–2, identify the values of the exponents important to make this equation dimensionally continual. (Hint: Notice the equation implies that and also that ) (b) By analyzing data from high-power standard explosives, Taylor discovered the formula he acquired seemed to be valid as long as the consistent k had the value 1.03. From the film reel, he was able to identify many type of worths of r and the corresponding worths of t. For instance, he uncovered that after 25.0 ms, the fireball had a radius of 130.0 m. Use these worths, together 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 acquire energy in joules, you have to make sure all the numbers you substitute in are expressed in terms of SI base devices.) (c) The energy released in large explosions is frequently cited in systems of “lots 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 soon 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 appropriate before the remnants of the shock wave hit him and looked to watch exactly how far they were carried by it.)

The purpose of this problem is to present the entire idea of dimensional consistency deserve to be summarized by the old saying “You can’t add apples and oarrays.” If you have actually studied power series expansions in a calculus course, you recognize the traditional mathematical functions such as trigonometric attributes, logarithms, and also exponential features can be expressed as boundless sums of the develop wright here the are dimensionmuch less constants for all and x is the debate of the feature. (If you have not studied power series in calculus yet, just trust us.) Use this truth to define why the requirement that all terms in an equation have the same dimensions is adequate as a definition of dimensional consistency. That is, it actually implies the debates of typical mathematical functions should be dimensionmuch less, so it is not really crucial to make this latter problem a sepaprice necessity of the meaning of dimensional consistency as we have done in this area.

Sexactly how Solution

Due to the fact that each term in the power series requires the argument elevated to a various power, the only means that eincredibly term in the power series deserve to have the same dimension is if the dispute is dimensionmuch less. To watch this explicitly, mean = LaMbTc. Then, = n = LanMbnTcn. If we want = , then an = a, bn = b, and cn = c for all n. The only means this deserve to happen is if a = b = c = 0.