Материал: Андрианова English Thesaurus of Student-Physicist 2013

Negative acceleration doesn't necessarily mean an object is slowing down. If the acceleration is negative and the velocity is also negative, the object is speeding up. The word deceleration means a reduction in speed, a slowing down. Some confuse it with a negative acceleration, which can speed something up.

Velocity and acceleration are sometimes confused with each other, but they're very different concepts, as can be illustrated with the help of motion diagrams. If the car moves the same distance in each time interval, this means that the car moves with constant positive velocity and has zero acceleration.

Freely falling objects

According to legend, Galileo discovered the law of falling objects by observing that two different weights dropped simultaneously from the Leaning Tower of Pisa hit the ground at approximately the same time. Although it's unlikely that this particular experiment was carried out, we know that Galileo performed many systematic experiments with objects moving on inclined planes. In his experiments, he rolled balls down a slight incline and measured the distances they covered in successive time intervals. The purpose of the incline was to reduce the acceleration and enable Galileo to make accurate measurements of the intervals. Galileo's achievements in the science of mechanics paved the way for Newton in his development of the laws of motion.

1.What does dynamics study?

2.What is the part of dynamics that describes motion without regard to its causes?

3.What do you know about geocentric and heliocentric models?

4.What is a frame of reference?

5.What is the difference between terms speed and velocity?

6.Give the definition of the average speed.

7.Give the definition of acceleration.

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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:

average velocity, average speed, deceleration, constant positive velocity, zero acceleration, displacement, free-fall acceleration, distance

1.The ball travels a _____ equal to twice the maximum height reached, but its _____ is zero.

2.If you run from x = 0 m to x = 25 m and back to you starting point in a time interval of 5 s, the _____ is zero, while the _____ is 10 m/s.

3.The world _____ means a reduction in speed, a slowing down.

4.The car moves the same distance in each time interval, if it moves with _____ and has _____.

5.An object falling in the presence of Earth’s gravity exhibits a

_____ directed towards Earth’s center.

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UNIT 3

VECTORS AND TWO-DIMENSIONAL MOTION

Read the following texts. Study the thesaurus on fig. 3, answer the questions after the texts

In discussion of one-dimensional we used the concept of vectors only to limited extent. In our further study of motion, manipulating vector quantities will become increasingly important, so much of this Unit is devoted to vector techniques. We’ll then apply these mathematical tools to twodimensional motion, especially that of projectiles, and to the understanding of relative motion.

Vectors and their properties

Each of physical quantities can be categorized as either a vector quantity or a scalar quantity. Vector has both direction and magnitude (size). A scalar can be completely specified by its magnitude with appropriate units; it has no direction. Displacement, velocity, and acceleration are vector quantities. Temperature is an example of a scalar quantity. If the temperature of an objects is -5°C, that information comple tely specifies the temperature of the object; no direction is required. Masses, time intervals, and volumes are scalars as well. Scalar quantities can be manipulated with the rules of ordinary arithmetic. Vector can also be added and subtracted from each other, and multiplied, but there are a number of important differences.

Equality of Two Vectors. Two vectors are equal if they have the same magnitude and the same direction. This property allows us to translate a vector parallel to itself in a diagram without affecting the vector. In fact, for most purposes, any vector can be moved parallel to itself without being affected.

Adding Vectors. When two or more vectors are added, they must all have the same units. For example, it doesn't make sense to add a velocity vector, carrying units of meters per second, to a displacement vector, carrying units of meters. Scalars obey the same rule: It would be similarly meaningless to add temperatures to volumes or masses to time intervals.

Vectors can be added geometrically or algebraically. The resultant vector R = A + В is the vector drawn from the tail of A to the tip of B. This procedure is known as the triangle method of addition. When two vectors are added, their sum is independent of the order of the addition: A + В = В + A. This relationship is called the commutative law of addition.

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Negative of a Vector. The negative of the vector A is defined as the vector that gives zero when added to A. This means that A and -A have the same magnitude but opposite directions.

Displacement, velocity, and acceleration in two dimensions

In one-dimensional motion the direction of a vector quantity such as a velocity or acceleration can be taken into account by specifying whether the quantity is positive or negative. The velocity of a rocket, for example, is positive if the rocket is going up and negative if it's going down. This simple solution is no longer available in two or three dimensions. Instead, we must make full use of the vector concept.

Motion in two dimensions

Anyone who has tossed any kind of object into the air has observed projectile motion. If the effects of air resistance and the rotation are neglected, the path of a projectile in Earth's gravity field is curved in the shape of a parabola. The most important experimental fact about projectile motion in two dimensions is that the horizontal and vertical motions are completely independent of each other. This means that motion in one direction has no effect on motion in the other direction.

In general, the equations of constant acceleration developed in Unit 2 follow separately for both the x-direction and the y-direction. An important difference is that the initial velocity now has two components. We assume that at t = 0, the projectile leaves the origin with an initial velocity. If the velocity vector makes an angle Q with the horizontal, Q is called the projection angle.

For example:

A water fountain. The individual water streams follow parabolic trajectories. The horizontal range and maximum height of a given stream of water depend on the elevation angle of that stream's initial velocity as well as its initial speed.

Acceleration at the Highest Point

The acceleration in the y-direction is not zero at the top of a projectile's trajectory. Only the y-component of the velocity is zero there. If the acceleration were zero, too, the projectile would never come down!

Relative velocity

Relative velocity is all about relating the measurements of two different observers one moving with respect to the other. The measured

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