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Read the text and fulfill the tasks given in Comprehension check.

 

MECHANICS

 

Classical mechanics deals with the motion of bodies under the influence of forces or with the equilibrium of bodies when all forces are balanced. The subject may be thought of as the elaboration and application of basic postulates first enunciated by Isaac Newton in his Philosophiae Naturalis Principia Mathematica (1687). These postulates are called Newton's laws of motion. They may be used to predict with great precision a wide variety of phenomena ranging from the motion of individual particles to the interactions of highly complex systems.

In classical mechanics the laws are initially formulated for point particles in which the dimensions, shapes, and other intrinsic properties of bodies are ignored. Thus, in the first approximation even objects as large as the Earth and the Sun are treated as pointlike--e.g., in calculating planetary orbital motion.

In the framework of modern physics, classical mechanics can be understood as an approximation arising out of the more profound laws of quantum mechanics and the theory of relativity. However, that view of the subject's place greatly undervalues its importance. Our present-day view of the world and man's place in it is firmly rooted in classical mechanics. Moreover, many ideas and results of classical mechanics survive and play an important part in the new physics.

Classical mechanics consists of kinematics and dynamics. Kinematics is concerned with the geometrically possible motion of a body or system of bodies without consideration of the forces involved (i.e., causes and effects of the motions). Dynamics is the study of the action of forces in producing either motion or static equilibrium (the latter constituting the science of statics). In statics, interest centres on the topic of equilibrium, in which any number of forces balance each other and thereby cancel. A rigid framework, such as a bridge, a vehicle chassis, or the timber frame of a house or roof, will experience various stresses according to the loading characteristics. The study of statics is important for determining the loading limits of such structures.

If the forces acting on a system do not cancel, motion will result; the analysis of this situation falls within the province of dynamics. Before the work of Galileo and Newton, ideas about the motion of material bodies were vague and inaccurate. There was a common belief that, in the absence of forces, all bodies eventually achieve a state of rest. Forces were thus required to act continuously to produce any sort of motion. This led to difficulties in explaining how, for example, an arrow can continue to fly through the air when the only evident source of propulsion is the bow. Problems also arose in accounting for the Earth's continuing motion around the Sun.

Newton produced the first systematic set of mathematical laws to describe the motion of bodies. Newton maintained that mere motion does not require force: only accelerated motion needs a mechanism. Uniform motion in a straight line is "natural" and will continue indefinitely unless some agency interferes. This is the essential content of Newton's first law of motion. Thus, a vehicle slows to rest only because frictional forces sap its energy. In space, there is no friction to restrain the Earth's motion within the solar system.



Newton's second law relates the acceleration of a body to the forces acting on it. Newton's third law expresses the equality of action and reaction. These laws of classical mechanics embody the laws of conservation of energy, momentum, and angular momentum, which play a central part in understanding all isolated mechanical systems.

It is a remarkable fact that, although Newton's laws are no longer considered to be fundamental, nor even exactly correct, the three conservation laws derived from Newton's laws--the conservation of energy, momentum, and angular momentum--remain exactly true even in quantum mechanics and relativity. In fact, in modern physics, force is no longer a central concept, and mass is only one of a number of attributes of matter. (Though force and mass are still central concepts in classical mechanics.) Energy, momentum, and angular momentum, however, still firmly hold centre stage. The continuing importance of these ideas inherited from classical mechanics may help to explain why this subject retains such great importance in science today.

 

Comprehension check

Exercise 1. Choose the correct ending to the following sentences.

1. Classical mechanics

a) is concerned with the motion of the bodies under the influence of forces.

b) is connected with the motion of photons.

c) deals only with the bodies that are not affected by forces.

2. Newton’s laws

a) can be used to describe the motion of atoms.

b) are formulated for pointlike objects.

c) describe the motion of particles having a spherical shape.

3. Classical mechanics consists of

a) kinematics and statics.

b) dynamics and kinematics.

c) dynamics and statics.

4. Dynamics

a) studies the motion of bodies under the influence of forces.

b) studies the motion of bodies under the action of forces or if the force of equilibrium is equal to zero.

c) is concerned with the motion of bodies not taking into account forces.

5. Newton’s first law states that

a) a body moves uniformly when some force acts on it.

b) uniform motion will continue indefinitely unless some force starts acting.

c) a body does not move if no force affects it.

6. Newton’s laws of motion

a) are still thought to be fundamental.

b) are not true in quantum mechanics and relativity.

c) are true both in classical and quantum mechanics.

7. Energy, momentum and angular momentum

a) are still central concepts.

b) are no longer main concepts.

c) are concepts that are employed only in mechanics.

 

Exercise 2. Find the answers to the following questions in the text.

1. What does classical mechanics deal with?

2. How is classical mechanics connected with Newton’s laws?

3. What can Newton’s laws predict?

4. For what objects are Newton’s laws formulated?

5. What is the role of classical mechanics in modern physics?

6. What does classical mechanics consist of?

7. What did people think about the motion of bodies before the work of Galileo and Newton?

8. What difficulties or problems arose in explaining motion of bodies?

9. What did Newton state about motion?

10. What laws can be derived from Newton’s laws of motion?

11. What concepts are still central in modern physics and what aren’t? Why?

 

Exercise 3. The sentences given below are jumbled. Arrange them in the logical order to sum up the contents of the text “Mechanics”.

1. It consists of kinematics and dynamics.

2. Before Newton’s time people knew little about the motion of bodies.

3. Classical mechanics is connected with the motion of the bodies under the influence of forces and with bodies at rest when all forces are balanced.

4. Newton produced a number of laws that describe the motion of bodies.

5. These laws are formulated for pointlike bodies.

6. Newton’s laws of motion laid the foundation of classical mechanics.

7. Kinematics deals with the motion of bodies not taking into account the forces involved.

8. In modern physics Newton’s laws no longer hold centre stage.

9. Classical mechanics still plays an important part in modern physics.

10. Dynamics studies the action of forces in producing motion or static equilibrium.

11. The three conservation laws are derived from Newton’s laws of motion.

 


Date: 2015-01-12; view: 1099


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