Материал: Андрианова English Thesaurus of Student-Physicist 2013

occurs, the gas presses against the chamber in all directions, but can't press against anything at the hole, where it simply escapes into space.

1.What phenomenon goes to one of the most powerful principle in physics?

2.What phenomenon the change of amount of motion goes to?

3.Describe the phenomenon known as recoil.

4.What is the result of physical contact between two objects?

5.What forces make rocket move?

6.In what case the total momentum of the system doesn't change with the passage of time?

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:

3. Fill in the gaps with the missing words from the list:

inelastic, tonometer, thrust, elastic, multistage rocket, momentum, collisions

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1.Billiard ball _____ and the collisions of air molecules with the walls of a container at ordinary temperatures are highly _____.

2.In an _____ collision, _____ is conserved but kinetic energy is

not.

3.A _____ to measure the pressure releases a puff of air against the outer surface of the eye and measures the speed of the air after reflection from the eye.

4.The _____ on the rocket is defined as the force exerted on the rocket by the ejected exhaust gases.

5.Space Shuttle and a number of other rockets, such as Titan4 or Russian Proton used concept of the _____.

UNIT 7

ROTATIONAL MOTION AND THE LAW OF GRAVITY

Read the following texts. Study the thesaurus on fig. 7, answer the questions after the texts

Rotational motion is an important part of everyday life. The rotation of the Earth creates the cycle of day and night, the rotation of wheels enables easy vehicular motion, and modern technology depends on circular motion in a variety of contexts, from the tiny gears in a Swiss watch to the operation of lathes and other machinery. The concepts of angular speed, angular acceleration, and centripetal acceleration are central to understanding the motions of a diverse range of phenomena, from a car moving around a circular race track to clusters of galaxies.

Rotational motion, when combined with Newton's law of universal gravitation and his laws of motion, can also explain certain facts about space travel and satellite motion, such as where to place a satellite so it will remain fixed in position over the same spot on the Earth. The generalization of gravitational potential energy and energy conservation offers an easy route to such results as planetary escape speed. Finally, we present Kepler's three laws of planetary motion, which formed the foundation of Newton's approach to gravity.

Angular speed and angular acceleration

In the study of linear motion, the important concepts are displacement, velocity, and acceleration. Each of these concepts has its analog in rotational motion: angular displacement, angular velocity and angular accel-

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eration. The radian, a unit of angular measure, is essential to the understanding of these concepts. Generally, angular quantities in physics must be expressed in radians. Be sure to set your calculator to radian mode; neglecting to do this is a common error.

Rotation motion under constant angular acceleration

A number of parallels exist between the equations for rotational motion and those for linear motion. The procedure used to develop the kinematic equations for linear motion under constant acceleration can be used to derive a similar set of equations for rotational motion under constant angular acceleration.

Relations between angular and linear quantities

Angular variables are closely related to linear variables. The tangential speed of a point on a rotating object equals the distance of that point from the axis of rotation multiplied by the angular speed. Every point on the rotating object has the same angular speed. The tangential acceleration of a point on a rotating object equals the distance of that point from the axis of rotation multiplied by the angular acceleration.

Centripetal acceleration

For circular motion at constant speed, the acceleration vector always points toward the center of the circle. Such an acceleration is called a centripetal (center-seeking) acceleration. Geometric result relating the centripetal acceleration to the angular speed, but physically an acceleration can occur only if some force is present. For example, if a car travels in a circle on flat ground, the force of static friction between the tires and the ground provides the necessary centripetal force.

The acceleration itself is always directed towards the center of rotation. Because the tangential and centripetal components of acceleration are perpendicular to each other, we can find the magnitude of the total acceleration with the Pythagorean theorem.

Fictitious Forces

Anyone who has ridden a merry-go-round as a child (or as a fun-loving grown-up) has experienced what feels like a "center-fleeing" force. Holding onto the railing and moving toward the center feels like a walk up a steep hill.

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Actually, this so-called centrifugal force is fictitious. In reality, the rider is exerting a centripetal force on his body with his hand and arm muscles. In addition, a smaller centripetal force is exerted by the static friction between his feet and the platform. If the rider's grip slipped, he wouldn't be flung radially away; rather, he would go off on a straight line, tangent to the point in space where he let go of the railing. The rider lands at a point, that is further away from the center, but not by "fleeing the center" along a radial line. Instead, he travels perpendicular to a radial line, traversing an angular displacement while increasing his radial displacement.

Newtonian gravitation

Prior to 1686, a great deal of data had been collected on the motions of the Moon and planets, but no one had a clear understanding of the forces affecting them. In that year, Isaac Newton provided the key that unlocked the secrets of the heavens. He knew from the first law that a net force had to be acting on the Moon. If it were not, the Moon would move in a straight-line path rather than in its almost circular orbit around Earth. Newton reasoned that this force arose as a result of an attractive force between Moon and Earth, called the force of gravity, and that it was the same kind of force that attracted objects - such as apples - close to the surface of the Earth. In 1687, Newton published his work on the law of universal gravitation.

The gravitational force exerted by a uniform sphere on a particle outside the sphere is the same as the force exerted if the entire mass of the sphere were concentrated at its center. This is called Gauss's law, after the German mathematician and astronomer Karl Friedrich Gauss. The gravitational constant was first measured in an important experiment by Henry Cavendish in 1798.

1.Why does physics study the rotational motion?

2.What analogies of linear motion exist in rotational motion?

3.What is the unit of angular measure?

4.Give the definition of tangential acceleration.

5.Which acceleration is called a centripetal?

6.Give an example of fictitious force.

7.Which law is called Gauss's law?

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