Материал: Андрианова English Thesaurus of Student-Physicist 2013

velocity of an object depends on the velocity of the observer with respect to the object. On highways, for example, cars moving in the same direction are often moving at high speed relative to Earth, but relative each other they hardly move at all. To an observer at rest at the side of the road, a car might be traveling at 60 mi/h, but to an observer in a truck traveling in the same direction at 50 mi/h, the car would appear to be traveling only 10 mi/h.

So measurements of velocity depend on the reference frame of the observer. Reference frames are just coordinate systems. Most of the time, we use a stationary frame of reference relative to Earth, but occasionally we use a moving frame of reference associated with a bus, car, or plane moving with constant velocity relative to Earth.

1.A scalar has no direction. Is it right?

2.Give examples of vector quantities.

3.On what condition vectors are considered to be equal?

4.Give the definition of negative of a vector.

5.Consider motion in one direction. How do horizontal and vertical motion depend on each other?

6.What does measurement of velocity depend on?

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:

projectile, scalar quantity, vector lies, x-axis, rate of change, acceleration vector, relative velocity, y-component

1.The number of grapes in bunch is one example of a _____.

2.If the _____ in the second or third quadrant, the angle, as measured from the positive _____, will be the angle returned by your calculator plus 180º.

3.The _____ of the velocity is zero at the Highest Point of the projectile’s trajectory.

4.As a _____ moves in its parabolic path, the velocity vector and

_____ are perpendicular to each other at the peak of its path.

5.The _____ of the resultant vector with respect to time is the

_____ equation.

UNIT 4

THE LAWS OF MOTION

Read the following texts. Study the thesaurus on fig. 4, answer the questions after the texts

Classical mechanics describes the relationship between the motion of objects found in our everyday world and the forces acting on them. As long as the system under study doesn't involve objects comparable in size to an atom or traveling close to the speed of light, classical mechanics provides an excellent description of nature.

Newton's three laws of motion are simple and sensible. The first law states that a force must be applied to an object in order to change its velocity. Changing an object's velocity means accelerating it, which implies a relationship between force and acceleration. This relationship, the second law, states that the net force on an object equals the object's mass times its acceleration. Finally, the third law says that whenever we push on something, it pushes back with equal force in the opposite direction. These are the three laws in a nutshell.

Newton's three laws, together with his invention of calculus are used routinely today in virtually all areas of mathematics, science, engineering, and technology. Newton's theory of universal gravitation had a similar impact, starting a revolution in celestial mechanics and astronomy that continues to this day. With the advent of this theory, the orbits of all the plan-

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ets could be calculated to high precision and the tides understood. The theory even led to the prediction of "dark stars," now called black holes, over two centuries before any evidence for their existence was observed.

Forces

A force is commonly imagined as a push or a pull on some object, perhaps rapidly, as when we hit a tennis ball with a racket. We can hit the ball at different speeds and direct it into different parts of the opponent's court. This means that we can control the magnitude of the applied force and also its direction, so force is a vector quantity, just like velocity and acceleration.

Another class of forces doesn't involve any directphysical contact. Early scientists, including Newton, were uneasy with the concept of forces that act between two disconnected objects. Nonetheless, Newton used this 'action-at-a-distance' concept in his law of gravity, whereby a mass at one location, such as the Sun, affects the motion of a distant object such as Earth despite no evident physical connection between the two objects. To overcome the conceptual difficulty associated with action at a distance, Michael Faraday (1791-1867) introduced the concept of a field. The corresponding forces are called field forces.

The known fundamental forces in nature are all field forces: the strong nuclear force between subatomic particles; the electromagnetic forces between electric charges; the weak nuclear force, which arises in certain radioactive decay processes; and the gravitational force between objects.

Newton’s first law

Before about 1600, scientists felt that the natural state of matter was the state of rest. Galileo, however, devised thought experiments - such as an object moving on a frictionless surface, as just described - and concluded that it's not the nature of an object to stop, once set in motion, but rather to continue in its original state of motion. This approach was later formalized as Newton's first law of motion: An object moves with a velocity that is constant in magnitude and direction, unless acted on by a nonzero net force. The net force on an object is defined as the vector sum of all external forces exerted on the object. External forces come from the object's environment. If an object's velocity isn't changingin either magnitude or direction, then its acceleration and the net force acting on it must both be zero.

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Mass and Inertia

Imagine hitting a golf ball off a tee with a driver. If you're a good golfer, the ball will sail over two hundred yards down the fairway. Now imagine teeing up a bowling ball and striking it with the same club. Your club would probably break, you might sprain your wrist, and the bowling ball, at best, would fall off the tee, take half a roll and come to rest.

From this thought experiment, we conclude that while both balls resist changes in their state of motion, the bowling ball offers much more effective resistance. The tendency of an object to continue in its original state of motion is called inertia. While inertia is the tendency of an object to continue its motion in the absence of a force, mass is a measure of the object's resistance to changes in its motion due to a force.

Newton’s second law

Newton's second law answers the question of what happens to an object that does have a net force acting on it. Imagine pushing a block of ice across a frictionless horizontal surface. When you exert some horizontal force on the block, it moves with an acceleration of, 2 m/s2. If you apply a force twice as large, the acceleration doubles to 4 m/s2. Pushing three times as hard triples the acceleration, and so on. From such observations, we conclude that the acceleration of an object is directly proportional the net force acting on it. Mass also affects acceleration. The acceleration of an object is inversely proportional to its mass. This approach was later formalized as Newton's second law of motion: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

The Gravitational Force

The gravitational force is the mutual force of attraction between any two objects in the Universe. Although the gravitational force can be very strong between very large objects, it's the weakest of the fundamental forces. In addition to contributing to the understanding of motion, Newton studied gravity extensively. Newton's law of universal gravitation states that every particle in the Universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them. If the particles have masses m1 and m2 and are separated by a distance r, the magnitude of the gravitational force, F is G* m1* m2/ r 2, where G is the universal gravitation constant.

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