Читать книгу Janice VanCleave's Physics for Every Kid - Janice VanCleave - Страница 10

Energy is the capacity to do work. In physics, work is done when a force is applied to an object causing it to move. A force is a pushing or pulling action on an object. Forces are measured in pounds (lb) or newtons (N), where 1 lb = 4.5 N. Work occurs only when a force causes something to move. If you push on a tree and it doesn't move, then no work has been done even though the effort may have exhausted you. Study how work is calculated in this important equation: W = F × d; where W is the work done; F is a specific force and d is the distance moved. Comparatively, when the equation is W = F_net × d, the work being done considers all forces (F_net) that are acting on the object that is being moved, including frictional forces. Weight is a measure of Earth's gravitational force pulling an object down toward the center of Earth. Gravity is the force of attraction between two objects with mass. Yes, your body has a force of attraction on other objects, but it is such a minute force that it is basically ineffective. Whereas, the mass of Earth is great enough to produce a force that pulls things on or near Earth's surface down. Down means toward the center of Earth; thus, when you drop something, it falls perpendicular to Earth's surface. Forces do not always cause linear motion, which is motion in a straight line. Torque is a turning force that causes motion around a center point, such as the turning of a lid or the spinning of a merry-go-round.

Movement is the change of an object's physical position. Linear movement is measured in feet (ft) or centimeters (cm), where 1 ft = 30 cm. Not all movement is linear or rotational but rather some objects vibrate, meaning they move back and forth. Frequency is a measure of the number of times something happens in a specific amount of time. Frequency can be measured in hertz (Hz), where 1 Hz = 1 cycle per second or one back and forth vibration.

Potential energy is the energy an object has because of its position relative to some zero position. It is energy that has the potential to do ‘work.’ Two types of potential energy investigated in this book are gravitational potential energy and elastic potential energy.

 Gravitational potential energy is the stored energy an object has because of its position above a specific ground zero. This type of potential energy is due to the force of gravity acting on the object. To obtain this energy, work had to be done on an object to raise it to a higher level above ground zero, such as placing a book on a top shelf with the floor below being ground zero. Gravitational potential energy is directly related to the mass of the object as well as its height above ground zero. When the book is dropped from a specific height, its gravitational potential energy is converted to kinetic energy as the book falls.

 Elastic potential energy is the energy stored in an object that can be stretched or compressed. A force is needed to compress or stretch an elastic object. Consider a trampoline, which has the greatest elastic potential energy when it is stretched the most, as does a rubber band. A coiled spring stores elastic potential energy when a force compresses it as well as when a force stretches it. In both cases, when the spring is released, the spring's elastic potential energy results in the wound coils moving back to their normal position. Thus, the elastic potential energy is converted to kinetic energy.

Kinetic energy (KE) is the energy of objects that are moving. Remember, kinetic energy does not cause an object to move, instead objects have kinetic energy because they are moving. A ball at the top of a ramp has gravitational potential energy. As the ball rolls down the ramp, its gravitational potential energy is converted to kinetic energy. There are three types of kinetic energy: vibrational, rotational, and translational. Vibrational KE is the energy caused by a back and forth movement; rotational KE is the result of turning about an axis, and translational KE is the result of linear movement from one place to another.

Mechanical energy is the sum of an object's potential energy and kinetic energy. Objects have mechanical energy if they are moving or have positional potential energy. Remember that an object doesn't have to be a machine to have mechanical energy. For example, both rivers and wind have mechanical energy.

In addition to mechanical energy activities, other types of energy will be investigated: sound energy, electrical energy, and light energy.

Sound is the sensation perceived by an organism's sense of hearing produced by the stimulation of hearing organs by sound waves. Sound energy is a type of energy produced by vibrating objects, such as when a guitar string is plucked. The movement of the string moves the air around it, producing a pattern of disturbances in the air called sound waves. Sound energy is transferred through mediums, such as solids, liquids, and gases. This type of energy can be heard by humans and other animals.

Electricity is a type of energy that we often take for granted until it is not available. All electric appliances, including computers and TVs do not work if the electric power line bringing electrical energy to your home is broken during a storm. You will discover more about current electricity as well as how to perform magic tricks using static electricity in the activities included in this book. You will also learn how the chemical energy stored in batteries produces the current electricity necessary for cell phones and tablets. Other electrical terms such as free electrons, conductors, insulators, polarization, and closed and open circuits will be investigated in the activities related to electrical energy.

Light energy is radiation. Radiation is a type of wave energy that does not need a medium to move through, such as radiation from the Sun that moves through space to Earth. The term radiation can sound dangerous and frightening, but did you know that common visible light and heat waves, called infrared light, are all forms of radiation? We can't imagine homes without our useful microwave oven. There are seven types of radiation: gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radiowaves.