Mechanical energy
Mechanical energy is the sum of the kinetic energy, or energy of motion, and the potential energy, or energy stored in a system by reason of the position of its parts.
Mechanical energy is constant in a system that has only gravitational forces or in an otherwise idealized system—that is, one lacking dissipative forces, such as friction and air resistance, or one in which such forces can be reasonably neglected. Thus, a swinging pendulum has its greatest kinetic energy and least potential energy in the vertical position, in which its speed is greatest and its height above the Earth least; it has its least kinetic energy and greatest potential energy at the extremities of its swing, in which its speed is zero and its height is greatest. As the pendulum moves, energy is continuously passing back and forth between the two forms. Neglecting friction at the pivot and air resistance, the sum of the kinetic and potential energies of the pendulum, or its mechanical energy, is constant. Actually the mechanical energy of the system is diminished at the end of each swing by the tiny amount of energy transferred out of the system by the work done by the pendulum in opposition to the forces of friction and air resistance. The mechanical energy of the Earth-Moon system is nearly constant as it is rhythmically interchanged between its kinetic and potential forms. When the Moon is farthest from the Earth in its nearly elliptical orbit, its speed is least. Its kinetic energy has become least, and its potential energy is greatest. When the Moon is closest to the Earth, it travels fastest; some potential energy has been converted to kinetic energy.
Task 2. Complete the following sentences. Your answers must be related to the ideas contained in the text.
Mechanical energy is the sum …
Mechanical energy is constant in …
A swinging pendulum has its greatest kinetic energy and least potential energy in …
It has its least kinetic energy and greatest potential energy in …
As the pendulum moves, energy …
The mechanical energy of the Earth-Moon system is nearly constant because …
Task 3. Formulate the main idea of the text.
Task 4. Speak about mechanical energy. Make use of the scheme suggested in exercise 4 (unit 2) for the text “Newtonian Mechanics”.
Read the text “Heat and Nuclear Energy” and fulfill the tasks given below.
Heat and nuclear energy
Heat energy is transferred from one body to another as the result of a difference in temperature. If two bodies at different temperatures are brought together, energy is transferred--i.e., heat flows--from the hotter body to the colder. The effect of this transfer of energy usually, but not always, is an increase in the temperature of the colder body and a decrease in the temperature of the hotter body. A substance may absorb heat without an increase in temperature by changing from one physical state (or phase) to another, as from a solid to a liquid (melting), from a solid to a vapour (sublimation), from a liquid to a vapour (boiling), or from one solid form to another (usually called a crystalline transition). The important distinction between heat and temperature (heat being a form of energy and temperature a measure of the amount of that energy present in a body) was clarified during the 18th and 19th centuries.
Nuclear energy, also called atomic energy, is released in significant amounts in processes that affect atomic nuclei, the dense cores of atoms. It is distinct from the energy of other atomic phenomena such as ordinary chemical reactions, which involve only the orbital electrons of atoms. One method of releasing nuclear energy is by controlled nuclear fission in devices called reactors, which now operate in many parts of the world. Another method for obtaining nuclear energy, controlled nuclear fusion, had not been perfected by the late 1980s. Nuclear energy has been released explosively by both nuclear fusion and nuclear fission.
Task 1. Use the following key words and word combinations to give the main ideas of the text.
Heat energy, to transfer, temperature difference.
The effect, energy transfer, an increase, temperature, a decrease, the colder/hotter body.
A substance, to absorb, heat, to change, a physical state.
Nuclear energy, to release, processes, to affect, atomic nuclei.
Nuclear energy, to releaser, reactors.
Nuclear energy, to obtain, controlled nuclear fusion.
Task 2. Complete the following sentences in accordance with the contents of the text.
Heat energy is transferred …
The effect of energy transfer is …
A substance that has absorbed heat …
Heat and temperature are different notions because …
Nuclear energy is released …
One method of releasing nuclear energy is …
Another method of obtaining nuclear energy is …
Task 3. Describe the forms of energy mentioned in the text.