Lesson 2 mechanics

Key terms: motion, mechanics, kinematics, dynamics, acceleration, free fall, distance, displacement, speed, constant speed, average speed, velocity, constant velocity

1. The motion of bodies - sailboats, automobiles, rockets, the Sun or the Moon - is an inescapable part of everyday life. Mo­tion is defined as the movement of an object, or any of its parts, from one place to another. The study of the motion of objects and of the forces that affect their motion is called mechanics. Mechanics is divided into two distinct categories: kinematics and dynamics. Kinematics is the description of the motion of objects without considering the cause of the motion. Dynamics deals with examination of the causes of the motion of objects.

2. The most common example of motion with nearly constant acceleration is that of a body falling towards the earth. In the absence of air resistance it is found that all bodies, regardless of their size, weight or composition, fall with the same acceleration at the same point of the earth's surface, and if the distance covered is not too great, the acceleration remains constant throughout the fall. This ideal motion, in which air resistance and the small change in acceleration with altitude are neglect­ed, is called free fall.

3. The acceleration of a freely falling body is called the accel­eration due to gravity and is denoted by symbol g.

4. The length of the path travelled by an object as it moves from one location to another is called distance. The change in position of an object is called its displacement.

5. Anything that moves takes a certain amount of time to trav­el a given ^distance. Speed is defined as the distance travelled per unit time. An object that travels the same distance every second has a constant speed. The symbol for constant speed is v. The speed of light in a vacuum is constant. At this speed it takes light from the sun about 8 minutes to reach the Earth.

6. It is difficult to drive a car or to walk at a constant speed all the time. Suppose that you are walking 2.0 km to your home. You walk the first kilometre in 10 minutes and the next kilome­tre in 20 minutes. You walked at a slower speed during the second kilometre than the first. Average speed is a useful quan­tity for describing the motion of objects that change speed. Average speed is calculated by dividing the total distance trav­elled by the time taken to travel that distance.

7. Many people use the terms speed and velocity interchange­ably in everyday language. However, they have very different meanings in physics. Velocity describes how fast an object is moving and its direction. It is defined as the displacement (change in position) of an object per unit time. An object that has a constant speed and a constant direction has a constant velocity.

Exercise 1. Give the English equivalents to the following words and word-combinations:

1. рух 6. вага та склад

2. сили, що впливають 7. поверхня Землі

3. прискорення 8. відстань та переміщення

4. опір повітря 9. висота

5. відсутність 10. пройдена путь.

Exercise 2. Find the synonyms to the following words in the text:

1. reason (1) 6. unchangeable (2)

2. place (4) 7. pay no attention to (2)

3. imagine (6) 8. not likely to be achieved (2)

4. count (6) 9. material thing (1).

5. quickly (7)

Exercise 3. Say if the statements are true or false:

1. An object in motion is always affected by some forces.

2. Mechanics deals with the motion of objects.

3. Velocity describes the change in position of an object for a certain period of time.

4. Acceleration due to the gravity is constant.

5. The symbol for constant speed is g.

Exercise 4. Choose the definition to the words:

1. speed 1 the acceleration of freely falling body

2. mechanics 2 movement of an object from one place to another.

3. motion 3 ideal motion

4. free fall 4 the description of motion without considering its cause.

5. kinematics is defined as… 5 the change in position of an object

6. distance 6 the distance traveled per unit time

7. dynamics 7 the study of motion of objects and of the forces that affect their

motion

8. acceleration due gravity 8 the length of the path traveled by an object

9. displacement 9 examination of the causes of the motion of objects

10. velocity 10 the displacement of an object per unit time

Exercise 5. Answer the following questions:

1. What is the speed of light?

2. What is the subject-matter of mechanics?

3. It is difficult for objects to travel with the constant speed, is not it?

4. What is more important for the division into categories of mechanics: the motion of objects itself or its causes?

5. What are the causes of any motion?

Exercise 6. Examine the difference between the terms ,,speed” and ,, velocity”.

Exercise 7. Explain why free fall is considered to be unreal.

Exercise 8. Define two categories of mechanics.

Exercise 9. Describe the example of motion with constant acceleration.

Exercise 10. Discuss the text according to the points:

1. The subject-matter of mechanics. Its categories.

2. The difference between the terms “speed” and ,,velocity”.

3. The ideal motion .The acceleration due to gravity.

4. The main concepts in mechanics and their definitions.

Notes:

1. acceleration due to gravity – гравітаційне прискорення

2. per unit time – за одиницю часу

3. regardless of – не зважаючи на

4. throughout – на всьому протязі

5. travel – пройти

6. meaning - значення.