Easy Physics Step-by-Step: With 95 Solved Problems

With the wide variety of review books on the market designed to supplement coursework, you may be asking, Why do we need another review book? Easy Physics Step-by-Step is unique because it is not designed as a test preparation book. This book will guide you, step-by-step, through the material covered in a typical high school physics course. It will take a unique approach to problem solving without teaching to the test. The emphasis is on presenting a review of fundamental concepts and encouraging you to think critically and creatively while solving problems.

In the spirit of this approach, Chapter 1 introduces a problem-solving ring as a guide to enhancing your skills as you progress through the material. You will notice that there are no multiple-choice questions. These types of assessments have their use, but the development of creative and critical thinking involves writing out the problems and their solutions in order to actively and constructively promote learning and understanding.

The material covered in this book is taught in most high schools. It reflects a beginning course in elementary physics and so tricky or difficult problems are kept to a minimum. Many sample problems are presented in a step-by-step manner to help ease you into solving more difficult problems. At the end of each chapter, there are several additional practice problems for you to solve on your own. An answer key is provided at the end of the book.

Topics and their level of difficulty have been selected based on 30 years’ experience in teaching high school physics and represent material that will give the reader a good introduction and review of elementary physics. It is assumed that the reader has a working knowledge of basic algebra and trigonometry at the secondary school level.

I would like to thank Christopher Brown (from McGraw-Hill Education) and Grace Freedson (from the Grace Freedson Publishing Network) for facilitating this project. My colleagues Vanessa Blood, Robert Draper, Patricia Jablonowski, and Joseph Vaughan, from Scarsdale High School, have always been very supportive and helpful. Finally, I would like to thank my wife, Karen, my daughter Marissa and her fiancé Eli Lieberman, and my daughter Ilana for all of their love and support.

In this chapter you will learn about the methods of problem solving in physics. These are techniques that you will use throughout this book as you learn about the mechanical universe.

The Nature of Science

What is physics? This is a difficult question to answer. If you are reading this book, then you are either taking a class in physics or are interested in learning more about physics. Science presents us with a worldview that relies on our sense experiences in conjunction with the rules of logic. We observe the universe in a state of motion and change all around us. How do we make sense of it all? What framework and structure can we build to understand and make predictions about the universe and also understand the practical applications of this knowledge?

Science involves identifying problems; all problems have goals and givens. The task of a physicist is to identify the problems that can be solved using the so-called scientific method. The method requires a scientist to identify the goals and givens in a particular problem.

As a physics student, you will be asked to solve problems that help you to understand some of the concepts covered in a typical high school physics class. To do so, you will use basic algebra and introductory trigonometry. To help you in this task, you will be introduced to step-by-step methods that will make the task of problem solving easier. These techniques will help you solve problems in other areas as well.

The Problem-Solving Ring and the Process Triangle

At the heart of any problem is the goal. Solving a problem involves first identifying the goal. The goal may be explicit: Calculate the velocity of an object dropped from a height of 30 meters after it has been falling for 2 seconds. The goal may also be implicit, for example: A person is driving a car at 25 m/s. When the car is 50 meters from an intersection, a traffic light turns from green to red. Does the car have enough time to stop?

After identifying the goal of the problem, you need to evaluate all of the given information. What information is implied and what information is given explicitly? What general area of physics is the problem dealing with (motion, energy, electricity, optics, etc.)?

The next step is to decide on the best solution path. What concepts do you need to know? What equations will allow you to solve the problem efficiently? Once you have selected the path, you need to implement it. This usually involves substituting the given information into the equations and using the correct units (see Chapter 2). Before the chosen solution path can be correctly implemented, however, several steps may be required, as more information may need to be deduced. You may find your chosen solution path is either not correct or not the most efficient method (for example, given time constraints on an exam).

Figure 1.1   Problem-solving ring

At the end of the mathematical process, you will obtain an answer. Now you must ask, Does this answer make sense? Do the units correctly match the desired goal? Have I in fact reached the goal? This is the evaluation stage. Once you evaluate your answer, you can decide if your goal or solution path needs to be modified. These steps are summarized in the problem-solving ring shown in Figure 1.1.

Embedded within this ring is a triangle representing three very important processes: analysis, synthesis, and modification. At all points along the problem-solving ring, you must continue to analyze (examine critically) your given information and problem-solving procedures. Synthesis involves linking together all of the different areas of physics and the problem-solving methods that help you to achieve the desired goal. Finally, you must be willing to modify what you are doing at every step. The triangle represents the idea that all three of these processes are linked and reinforce each other.

Whether you are solving a simple, one-step, plug-and-chug problem or a multistep problem, you can use this problem-solving cycle to help you. As you read through this book, you will see various aspects of this cycle employed. I have found it successful in the classes I teach, and I think you will find it successful as well.

In this chapter you will learn about how units and measurements are used in the study of physics. Then you will learn about how data is collected and analyzed using significant figures. This is a very important first step before we begin our exploration of the mechanical universe.

Fundamental (SI) Units

Physics is the study of the mechanical universe. Like all sciences, physics requires a set of rules and standards for making predictions about how the universe operates. You should be familiar with some of these rules, such as the scientific method, from your other science courses.

In physics, we use both observations and mathematical reasoning to establish relationships between physical quantities (like proportions). These proportions can also be visualized as patterns by using graphs (see Chapter 3).

We are also guided by the rules of reasoning outlined by Isaac Newton in his major work Philosophiae Naturalis Principia Mathematica (The Mathematical Principles of Natural Philosophy), published in 1687. They are summarized as follows.

1.  Keep the number of causes to a minimum to explain an observed effect.

2.  Assign the same cause to the same observed effect.

3.  Certain universal qualities exist in all bodies (like length and mass).

4.  Change our hypotheses about phenomena only when observed experimental evidence forces us to do so. If we do make those changes, then we must do more experiments to test the new hypotheses.

When making observations, we often use measurements as a way to establish a direct set of values for comparison. The idea of a unit of measurement allows us to make these comparisons fairly. The unit of measurement tells us not only what physical quantity we are measuring (length, area, volume, mass, energy, temperature, electric charge, frequency, etc.) but also what scale is being used for the measurement (we will discuss the concepts of accuracy and precision later in this chapter).

Problem    Which is larger, 500 millimeters (mm) or 5 meters (m)?

In physics, certain quantities are considered fundamental (recall Newton’s fourth rule of reasoning) and, therefore, have fundamental units, also referred to as Standard International (or SI) units. They are summarized in Table 2.1.

The remaining units used in physics, called derived units, are all based on these fundamental units. For example, we will learn in Chapter 3 that average speed is a measure of distance divided by time. The derived units for this concept would be m/s. Area is measured in units of m², which is derived from the units of length. As you learn a new concept, make sure you memorize the appropriate unit and use it in all calculations!

Conversion of Units

Sometimes quantities are not measured in fundamental units. In that case, we often have to convert them back into fundamental units. When recording measurements, certain prefixes are introduced to indicate the scale factor that is being used. Table 2.2 is a quick review of the powers of 10 and exponential notation. Table 2.3 shows some of the more common prefixes for units.

Problem    Convert the following units (note that a liter [L] is a unit of liquid volume).

a.  125 mm = _____________ m

b.  336 m = _____________ cm

c.  456 nm = _____________ m

d.  200 mL = _____________ L

e.  45 g = _____________ kg

f.  25 kg = _____________ g

a.  1 mm = 0.001 m

b.  1 m = 100 cm

c.  1 nm = 1 × 10–9 m

d.  1,000 mL = 1 L

e.  1 g = 0.001 kg

f.  1 kg = 1,000 g

a.  125 mm = 0.125 m

b.  336 m = 33,600 cm

c.  456 nm = 4.56 × 10–7 m

d.  200 mL = 0.200 L

e.  45 g = 0.045 kg

f.  25 kg = 25,000 g

Sometimes a problem will use common units for time that must be converted into seconds. Recall the following conversions.

1 microsecond (μs) = 10−6 seconds

1 millisecond (ms) = 0.001 seconds

60 seconds = 1 minute

60 minutes = 1 hour = 3,600 seconds

24 hours = 1 day (approximately) = 86,400 seconds

365.25 days = 1 year (approximately) = 31,557,600 seconds

Problem    Convert the following time measurements into seconds.

a.  300 ms = _____________ s

b.  468 μs = _____________ s

c.  18.5 minutes = _____________ s

d.  2.5 years = _____________ s

e.  5.77 days = _____________ s

f.  36.5 hours = _____________ s

a.  1 ms = 0.001 s

b.  1 μs = 10−6 s

c.  1 minute = 60 s

d.  1 year = 31,557,600 s

e.  1 day = 86,400 s

f.  1 hour = 3,600 s

a.  300 ms = 0.300 s

b.  468 μs = 4.68 × 10–4 s

c.  15.5 minutes = 1,110 s

d.  2.5 years = 78,894,000 s

e.  5.77 days = 498,528 s

f.  36.5 hours = 131,400 s

Accuracy and Precision

All measurements are subject to uncertainty. This does not mean that they are wrong! The choice of the correct instrument for measurement is related to the concepts of accuracy and precision. Accuracy refers to how close a measurement is to an accepted value for a physical quantity. Precision refers to the degree of agreement between successive measurements using a given instrument.

Let us suppose an experiment is performed to measure the acceleration due to gravity. As you will learn in Chapter 3, acceleration refers to a change in velocity (or speed) over time. It turns out that the accepted value for the acceleration due to gravity (near the surface of the earth) is the same for all dropped objects (objects that are falling from rest and with no air resistance acting on them to impede their motion). This is approximately equal to 9.81 m/s² (notice that the unit for acceleration is a derived unit; see Chapter 3 for more details). Suppose measurements were obtained by using a meterstick and a stopwatch to measure the time it takes objects to fall from various heights. The stopwatch can record hundredths of a second and the meterstick is marked with millimeters (0.001 m). Using formulas developed in Chapter 3, a student group obtains values of 8.36 m/s², 8.42 m/s², 8.38 m/s², and 8.39 m/s².

How would we analyze this data? In one sense, the students did not obtain the accepted value of 9.81 m/s². How consistent are their values? How reliable are their measurements? The values may appear to be consistent, but should the group have expected more accurate results given the measurement techniques? Should the students have obtained more data? If so, how much more?

These are difficult questions to answer. In reality, we must make a statement of what is an acceptable range of uncertainty for the purposes of accuracy. How much rounding did the students do? When they released the object, did the timer start immediately? Was it electronically controlled, or did someone say go and then someone else started the stopwatch by hand? What kind of data analysis should the students do? Should they use an average value? Should they make a graph of the data? How will they interpret their analysis? All of these issues should be discussed with the teacher before the experiment and in a laboratory report.

An average value (using a simple method of adding up the values and dividing by the number of values) would be equal to 8.3875 m/s² on a calculator. How is this number reported? Which numbers are significant? If the accepted value of the acceleration due to gravity is equal to 9.81 m/s², then maybe the students should report the average value as 8.39 m/s². A simple percent error, using the value of 8.39 m/s², could then be calculated from the formula

In this example, the percent error would be equal to approximately 14.5%. Is this an acceptable uncertainty? There are more sophisticated ways of analyzing data that we will not go into here. It is, however, important for you to understand that analysis of data is a crucial process in all areas of science.

Significant Figures and Data Analysis

When a measurement is made, it is very important to inform the reader about the level of accuracy and precision used. Even if you are using a calculator to compute the solution to a problem, the number of digits written down is important. It is for this reason that we need to consider the issue of significant figures.

The following sample problem shows you why the concept of significant figures is an important one.

Problem    What is the length of the box if the ruler (not drawn to actual scale) records centimeters?