The Laws of Motion
The English scientist Sir Isaac Newton (1642–1727) formulated three laws of motion.
Newton's First Law The first law reads as follows: An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Basically, this means that an object will keep doing what it is doing unless something makes it change. The something that makes an object change its state is called a force. A force is a push or pull upon an object resulting from one object's interaction with another object. An example of this is a person pushing a swing or a person pulling a suitcase.
An object has inertia when it is moving. Inertia is merely the resistance to change. An example is the tendency of your body to keep moving forward when your car comes to a sudden stop. In a collision, a seatbelt can keep you in place when otherwise you might hurtle through the windshield because of inertia.
Friction is a force that results when the surface of one object touches the surface of another. Friction causes moving objects to slow down or stop. For example, if you shove a book across a table, it will soon slow to a stop because it is rubbing on the surface of the table. Friction between two objects causes heat.
Newton's Second Law According to Newton, an object will accelerate only if there is an unbalanced force acting upon it. The presence of an unbalanced force will accelerate an object, changing its speed, its direction, or both its speed and its direction. The second law states that the acceleration of an object is dependent upon two variables—the force acting upon the object and the mass of the object. The formula for acceleration is 
.
Newton's Third Law Newton's third law reads: For every action, there is an equal and opposite reaction. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction—a force on the chair and a force on your body. When a bird flaps its wings downward, the air pushes it upward, allowing it to fly. A gun recoils when it is fired.
Describing Motion In physics, there are various ways to describe motion. Speed is how fast an object is moving. A fast-moving object has a high speed, while a slow-moving object has a low speed. An object with no movement at all has a zero speed. The average speed during the course of a motion can be calculated using the following equation:
Velocity is defined as the rate and direction at which an object's position changes. Velocity includes both speed and direction, such as moving 55 miles/hour in a westerly direction or 6 meters/second upward.
Average velocity can be calculated using the equation:
Acceleration is defined as "the rate at which an object changes its velocity." An object is accelerating if it is changing its velocity. If an object is not changing its velocity, then the object is not accelerating. A falling object accelerates as it falls. If you could measure the motion of a falling object, you would notice that the object has an average velocity of 5 m/s in the first second, 15 m/s in the second second, 25 m/s in the third second, 35 m/s in the fourth second, and so on. By pushing down the gas pedal, you can accelerate your car.
Work, Energy, and Power
Some of the most important concepts in physics are work, energy, and power.
Work Work results from a force acting upon an object, causing it to change position or move from one place to another. There are three key aspects to work: force, movement, and cause. In order for a force to qualify as having done work on an object, there must be a change of position caused by the force. Here are some common examples of work: a person carrying a box of books upstairs, a horse pulling a plow through the fields, a weightlifter lifting barbells, a person pushing a grocery cart down the aisle of a grocery store, a shot putter launching the shot, and an ice skater lifting his partner overhead. In each case, there is a force exerted upon an object that causes the object to be displaced. Work is measured in joules.
Energy In physics, energy is defined as the ability to do work. Energy can be either potential or kinetic.
Potential Energy An object can store energy as the result of its position. A can of soup on a shelf has potential energy. When it falls from the shelf onto the floor, it releases its energy. A roller-coaster car has potential energy when it is at the top of the track. It releases the energy when it plunges downward. An arrow in a drawn bow has potential energy resulting from its position. If the bow is not drawn, or pulled back, the arrow has no potential energy. The can of soup and the roller-coaster car have potential energy that is caused by gravitation—being pulled toward Earth. The second form of potential energy is elastic potential energy. Elastic potential energy is the energy stored when an object is stretched or compressed. For example the arrow in a drawn bow has elastic potential energy. Other examples include stretched rubber bands, bungee cords, trampolines, and springs. The more stretch or compression that is exerted on the object, the more potential energy it holds.
Kinetic Energy Kinetic energy is the energy of motion. Any object that has motion has kinetic energy. The direction does not matter. A moving car has kinetic energy, as does a moving ice skater or a soup can falling off the shelf. When the soup can falls off the shelf, its potential energy is changed to kinetic energy.
Forms of Energy Energy cannot be created or destroyed; it merely changes from one form of energy to another. Forms of energy include:
Power Power is how much work is done over a given period of time. Work can be done very quickly or very slowly. For example, a rock climber scaling a sheer cliff may take a long time to raise his or her body just a few meters. But a hiker who selects an easier, less vertical path might raise his or her body the same few meters in a much shorter period of time. Even though the amount of work is the same, the hiker does the work in considerably less time than the rock climber. The hiker has a greater power than the rock climber.
The formula for power is
The standard metric unit of power is the watt. Watts are used to measure the amount of energy consumed by an electric device.
Waves
Sound waves, light waves, radio waves, microwaves, water waves, earthquake waves, and waves on a piece of string are just a few of the kinds of waves we are likely to encounter every day. Waves transport energy. Waves have different parts. This section will focus on sound and light waves.
The crest of a wave is the point where there is the maximum amount of upward displacement from the rest or equilibrium position. The trough of a wave is the point where there is the maximum amount of downward displacement from the rest or equilibrium position. The amplitude of a wave refers to the height of the wave from its equilibrium point.
The wavelength of a wave is simply the length of one complete wave cycle. The wavelength can be measured as the distance from crest to crest or from trough to trough or at any two similar points on the wave. The period of a wave is the amount of time it takes to get from one point on the wave to the same point on the next wave.
The amount of energy carried by a wave is related to its amplitude (the distance from the wave's equilibrium position to the maximum height or depth). A high-energy wave is characterized by a high amplitude; a low-energy wave is characterized by a low amplitude.
The frequency of a wave is the number of complete cycles per second that the wave makes, from equilibrium to its crest, then back through equilibrium to its trough, and back again to equilibrium.
The diagram of the electromagnetic spectrum shows that waves have a large range of frequencies. The diagram on page 150 depicts the electromagnetic spectrum and its various regions. The longer-wavelength, lower-frequency regions are located on the far left of the spectrum; shorter-wavelength, higher-frequency regions are on the far right. The different wavelengths of waves determine what we see and how we communicate.
Sound Waves Sound is a wave that is created by vibrating objects and that moves through a medium from one location to another. Sound cannot travel through a vacuum. The medium is usually air, but it could be any material, like a liquid or solid. The vibrating object could be the vocal chords of a person, the string of a guitar, or a tuning fork, or the vibrating diaphragm (see respiratory system earlier in this overview) of a television announcer.
The loudness of sound depends on the amplitude of the wave. The higher the amplitude, the louder the sound. Loudness is measured in decibels. The pitch of the sound depends on the frequency of the sound waves. A high-pitched sound is created by a high-frequency wave, and a low-pitched sound is created by a low-frequency wave.
Sound travels faster in solids than in gases. Sound also travels faster in substances that have a higher temperature.
Sound waves are subject to a phenomenon called the Doppler effect. You may have heard the results of the Doppler effect without knowing it. Have you ever listened to an ambulance siren moving toward you and then detected a change in pitch when the siren started to move away from you? This is an example of the Doppler effect—a shift in the frequency of the waves coming from a moving object. As the object moves toward you, the pitch is higher; when it is moving away, the pitch is lower.
Light Waves Visible light occurs within a very tiny range of the electromagnetic spectrum. Each color in visible light has a distinct wavelength. This can be seen most clearly when you pass white light through a prism that separates the light into its "rainbow" of colors. Dispersion of visible light produces the colors red (R), orange (O), yellow (Y), green (G), blue (B), indigo (I), and violet (V). The red wavelengths of light are the longer wavelengths, and the violet wavelengths of light are the shorter wavelengths. The visible light spectrum is shown in the diagram below. To remember the order from long wavelengths to the shorter wavelengths, think of the letters ROY G BIV.
White is really not a color; rather, it is the combination of all the colors of the visible light spectrum. Black is not really a color, either. Black is the absence of the wavelengths of the visible light spectrum or the absence of light or color. If you are in a room that is totally black/dark, it means that there are no wavelengths of visible light striking your eye.
Refraction Light refracts, or bends, when it passes through different media. Different substances bend light in different amounts. Every substance that can pass light has what is called a refractive index, the amount that the substance will bend light.
Light is bent when it goes through lenses. Lenses can be convex or concave, and light behaves differently when it passes through each type of lens. Convex lenses are thicker in the middle than at the edges. Concave lenses are thicker at the edges than in the middle.
When light rays travel through a convex lens, the lens converges the light to a focal point. The more curved the lens, the closer the focal point is to the lens.
A concave lens, by contrast, diverges the light so that the light rays will never meet.
Reflection Light also has the property of reflection. A ray of light, perhaps coming from a flashlight or other light source, approaching a mirror is known as an incident ray (I). The ray of light that leaves the mirror is known as the reflected ray (R). At the point where the ray strikes the mirror, a perpendicular line can be drawn; this line is known as a normal line (N). The normal line divides the angle into two equal angles. The angle between the incident ray and the normal is known as the angle of incidence. The angle between the reflected ray and the normal is known as the angle of reflection.
Like lenses, mirrors can be concave (converging) and convex (diverging).
A converging mirror takes light rays and converges them to a focal point. The focal point is the point in space at which light will meet after reflection. Flashlights and headlights on a car use concave mirrors to produce direct light beams in a narrow area.
A diverging mirror spreads the light after it is reflected, so that the light beams will never intersect. For this reason, convex mirrors produce virtual images that appear to be located somewhere behind the mirror. Examples of these are the security mirrors you seen in stores so that clerks can see people in a large part of the store. They can also be found in the outside rearview mirrors in cars, which help you see a larger viewing area than a flat mirror. Convex mirrors make objects look farther away and smaller. That's why you have the warning in your outside rearview mirror that states that objects appear more distant than they really are. Telescopes use both types of lenses to refract light and mirrors to reflect light.
Lasers Light from a laser is all of one wavelength, and all the waves are in phase (all crests and troughs are in tandem) and are traveling in the same direction. As a result, a laser beam of light does not spread out very much and directs its energy in a very small area. This characteristic allows lasers to be used as cutting tools in situations like eye surgery, for transferring digital music to CDs, and for reading bar codes in such places as grocery and department stores. Because laser light doesn't spread, lasers are used to carry information across long distances in space or through fiber-optic cable on land.
Blueshift and Redshift The Doppler effect is also a phenomenon of light. Scientists use this phenomenon to determine whether objects like stars and galaxies are moving toward Earth or away from it. If an object is moving toward Earth, the light from the object appears bluer (a blueshift). If an object is moving away from Earth, the light from the object appears redder (a redshift). Scientists have used this information to determine that the universe is expanding, creating a shift in wavelengths toward the red end of the spectrum.
Heat
Heat is created by moving molecules. The faster the molecules move, the more heat is generated. Heat is basically kinetic energy (review the section on energy earlier in this chapter). Heat is measured by a thermometer.
Heat transfers or flows from warmer objects to colder objects when they are in contact. Transfer takes place in three ways: radiation, conduction, and convection.
Radiation The first method of heat transfer is radiation, which takes place via invisible waves through the air or even through a vacuum. The energy from the sun warms us through radiation, just as a fire in a fireplace warms our hands. A microwave oven heats food via radiation.
Conduction The second method of heat transfer is conduction, which refers to heat transferred between atoms bumping into each other in a substance. What happens if you put a spoon in boiling water? Eventually the handle of the spoon gets too hot to hold. The heat from the water causes the molecules in the spoon to move and collide, creating heat that eventually is conducted up the spoon handle to your hand.
Some materials are good conductors of heat, and some are not. Metals are good conductors of heat, with gold, silver, and copper being the best conductors. Copper is used more because it is cheaper and more plentiful. Some materials are called insulators because they are not very good at conducting heat. Materials made of cotton or wool, for example, are not good conductors of heat. If you wrap yourself in a wool blanket, rather than conducting the heat away from your body, it traps the heat and you get warmer. Air is a good insulator, as are plastic, wood, rubber, and tile (think of the tiles on the space shuttle that protect the shuttle from the heat of friction created when the shuttle strikes the atmosphere during its return to Earth).
Think about heat conduction when you get into a car on a hot sunny day when the windows are closed. Vinyl or leather seats will feel very hot, as they are better conductors of heat then fabric seats.
Convection Heat also transfers via a process called convection. Convection generally occurs in gases or liquids. For example, as the temperature of a pan of water on the stove increases, the molecules start to move around more quickly. As they do this, the distance between the molecules increases and the liquid expands. The denser, cooler water then sinks to the bottom of the pan and forces the warmer liquid upward. A circulation of the colder and warmer liquid begins. On a much larger scale, this is the process that helps to create currents in the oceans.
The same process creates currents in the atmosphere. When land is heated by the sun, it heats the air that is in contact with it. The warmer air expands and rises, and the cooler, heavier air sinks down. The rising air creates upward motion, which is used by birds and hang gliders to stay aloft. Sometimes these upward currents are called thermals. Thermals can reach many thousands of feet up and will give people in airplanes a bumpy ride.
Convection also creates land and sea breezes at a shoreline. Land heats faster than water. As the land heats up, it warms the air above it. The air expands and rises. It is then replaced by cooler air flowing in from over the water. This process creates a wind current that goes from the water to the land. We call this current a sea breeze. In the evening, the reverse happens. The land cools faster than the water, so the air current flows in the opposite direction. This creates what is called a land breeze—a breeze from the land to the sea.
Magnetism
Magnets can be made by placing a certain material, such as iron or steel, in a strong magnetic field. Pieces of the steel or iron are generally arranged in a random order. When these pieces, or domains, are placed in a strong magnetic field, they line up in the direction of the field. When most of the domains are aligned, the material becomes a magnet.
A bar magnet has three-dimensional lines of force around it on all sides. If the lines were visible, they would look like the following diagram.
When opposite poles of two magnets are brought together, the lines of force join up and the magnets pull together.
When like poles of two magnets are brought together, the magnets repel each other and the lines of force push away.
Earth as a Magnet Because of its iron-nickel core, Earth behaves like a giant magnet. It happens that Earth's magnetic poles are fairly close to the geographic poles. A compass has a needle that is attracted to the magnetic north pole and is repelled by the magnetic south pole.
Earth's magnetic field, called a magnetosphere, is produced by electric currents. Deep in Earth's core, there is convection as hot molten magma rises, cools, and sinks. The process repeats itself time and time again. Within these rising and falling masses of magma, the rotation of Earth creates organized patterns of circular electric currents, called eddies. The interior of Earth seems to act like a giant dynamo. Earth's magnetic field deflects most of the charged particles streaming from the sun.
Magnets are used to make electric motors and generators. Generators produce electricity, which supports our lifestyle in many ways.
Electricity and Magnetism The movement of electrons around the nucleus of an atom produces a magnetic field. The wire that contains an electric current is surrounded by a magnetic field. A wire with a current running through it that is wrapped around an iron core is called an electromagnet. Electromagnets are used in a variety of ways in our lives in everything from doorbells to magnetic high-speed trains.
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