Материал: Андрианова English Thesaurus of Student-Physicist 2013

Sometimes it's more convenient to locate a point in space by its plane polar coordinates (r, £). A point is then specified by the distance r from the origin to the point and by the angle £ between the reference line and a line drawn from the origin to the point. The standard reference line is usually selected to be the positive x-axis of a Cartesian coordinate system. The angle £ is considered positive when measured counterclockwise from the reference line and negative when measured clockwise.

Trigonometry

The basic trigonometric functions defined by such a triangle are the ratios of the lengths of the sides of the triangle. These relationships are called the sine, cosine, and tangent functions. Another important relationship, called the Pythagorean theorem, exists between the lengths of the sides of a right triangle.

1.What physical quantities do the basic laws of physics involve?

2.What are the three most fundamental quantities in mechanics?

3.What do you know about the standard system of units called SI?

4.What is the standard of length nowadays?

5.What three relationships in trigonometry could you name?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

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3.Fill in the gaps with the missing words from the list: dimension, slag, coordinate system, second, SI units,, estimate, foot

1.In physics, the world _____ denotes the physical nature of quanti-

ty.

2.In U.S. customary system the units of length, mass, and time are the _____, _____, and _____.

3._____ are almost universally accepted in science and industry.

4.Any _____ under ten thousand are small compared with Earth’s total population, but a million or more would be alarming.

5.Many aspects of physics deal with location in space, which require the definition of a _____.

UNIT 2

MOTION IN ONE DIMENSION

Read the following texts. Study the thesaurus on fig. 2, answer the questions after the texts

Life is motion. Our muscles coordinate motion microscopically to enable us to walk and jog. Our hearts pump tirelessly for decades, moving blood through our bodies. Cell wall mechanisms move select atoms and molecules in and out of cells. From the prehistoric chase of antelopes across the savanna to the pursuit of satellites in space, mastery of motion has been critical to our survival and success as a species.

The study of motion and of physical concepts such as force and mass is called dynamics. The part of dynamics that describes motion without regard to its causes is called kinematics. In this Unit, the focus is on kinematics in one dimension: motion along a straight line. This kind of motion - and, indeed, any motion - involves the concepts of displacement, velocity, and acceleration. Here, we use these concepts to study the motion of objects undergoing constant acceleration.

The first recorded evidence of the study of mechanics can be traced to the people of ancient Sumeria and Egypt, who were interested primarily in understanding the motions of heavenly bodies. The most systematic and detailed early studies of the heavens were conducted by the Greeks from about 300 b.c. to a.d. 300. Ancient scientists and laypeople regarded the Earth as the center of the Universe. This geocentric model was accepted by

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such notables as Aristotle (384-322 b.c.) and Claudius Ptolemy (about a.d. 140). Largely because of the authority of Aristotle, the geocentric model became the accepted theory of the Universe until the 17th century.

About 250 b.c, the Greek philosopher Aristarchus worked out the details of a model of the Solar System based on a spherical Earth that rotated on its axis and revolved around the Sun. He proposed that the sky appeared to turn westward because the Earth was turning eastward. This model was- n't given much consideration, because it was believed that if the Earth turned, it would set up a great wind as it moved through the air. We know now that the Earth carries the air and everything else with it as it rotates.

The Polish astronomer Nicolaus Copernicus (1473-1543) is credited with initiating the revolution that finally replaced the geocentric model. In his system, called the heliocentric model, Earth and the other planets revolve in circular orbits around the Sun.

This early knowledge formed the foundation for the work of Galileo Galilei (1564-1642), who stands out as the dominant facilitator of the entrance of physics into the modern era. In 1609, he became one of the first to make astronomical observations with a telescope. He observed mountains on the Moon, the larger satellites of Jupiter, spots on the Sun, and the phases of Venus. Galileo's observations convinced him of the correctness of the Copernican theory. His quantitative study of motion formed the foundation of Newton's revolutionary work in the next century.

Displacement

Motion involves the displacement of an object from one place in space and time to another. Describing the motion requires some convenient coordinate system and a specified origin. A frame of reference is a choice of coordinate axes that defines the starting point for measuring any quantity, an essential first step in solving virtually any problem in mechanics.

Because displacement has both a magnitude (size) and a direction, it's a vector quantity, as are velocity and acceleration. In general, a vector quantity is characterized by having both a magnitude and a direction. By contrast, a scalar quantity has magnitude, but no direction. Scalar quantities such as mass and temperature are completely specified by a numeric value with appropriate units; no direction is involved.

Velocity

In day-to-day usage, the terms speed and velocity are interchangeable. In physics, however, there's a clear distinction between them: Speed is a

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scalar quantity, having only magnitude, while velocity is a vector, having both magnitude and direction.

Why must velocity be a vector? If you want to get to a town 70 km away in an hour's time, it's not enough to drive ata speed of 70 km/h; you must travel in the correct direction as well. This is obvious, but shows that velocity gives considerably more information than speed, as will be made more precise in the formal definitions.

The average speed of an object over a given time interval is defined as the total distance traveled divided by the total time elapsed.

Acceleration

Going from place to place in your car, you rarely travel long distances at constant velocity. The velocity of the car increases when you step harder on the gas pedal and decreases when you apply the brakes. The velocity also changes when you round a curve, altering your direction of motion. The changing of an object's velocity with time is called acceleration. The average acceleration during the time interval is the change in velocity divided by time (meter per second per second, m/s2).

Acceleration is a vector quantity having dimensions of length divided by the time squared. For the case of motion in a straight line, the direction of the velocity of an object and the direction of its acceleration are related as follows: When the object's velocity and acceleration are in the same direction, the speed of the object increases with time. When the object's velocity and acceleration are in opposite directions, the speed of the object decreases with time. The minus signs indicate that the velocities of the car are in the negative direction; they do not mean that the car is slowing down! Positive and negative accelerations specify directions relative to chosen axes, not "speeding up" or "slowing down." The terms "speeding up" or "slowing down" refer to an increase and a decrease in speed, respectively.

The value of the average acceleration often differs in different time intervals, so it's useful to define the instantaneous acceleration. The instantaneous acceleration is the limit of the average acceleration as the time interval.

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