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9

the body. For instance, doubling the mass means double the momentum and double the

impulse, for a given time. So that the rate of fall at any moment after the start is the same

whatever the mass: neglecting the air resistance, we see that a 2-pound weight and an 8-pound

weight, dropped at the same moment from a height, will keep level and reach the ground

together.

Since the increase in velocity is proportional to the force, which is unchanging, the rate

of increase of fall must be unchanging. The rate of increase of velocity is called acceleration,

so that the man of science says that the acceleration during free fall under gravity is constant.

Thus to be quite clear, a body falling freely from rest will have a velocity of 32.2 feet per

second (ft/sec) at the end of 1 second; double that velocity, namely 64.4 ft/sec, at the end of 2

seconds; treble that velocity, namely 96.6 ft/sec, at the end of 3 seconds, and so on. Of course,

as the velocity increases the distance travelled per second also increases.

The acceleration of gravity (g) is the most important figure in science, and comes into all

kinds of calculations.

Newton’s third law of motion is that reaction is always equal and opposite to action. That

is, if two bodies, A and B, act upon one another, the action of A on B is always equal in

magnitude and opposite in direction to the action of B on A.

Let us make this clear. When a gun is fired the bullet pushes the gun back with a force

equal to that with which the gun pushes the bullet forward. Owing to its much greater mass it

does not move nearly so fast as the bullet, but the speed can be exactly calculated.

Another example is given by the rocket. The downward rush of gases at very high speed

results in the body of the rocket being pushed upward.

We have now considered Newton’s three laws of motion. They are of immense

importance, governing as they do the movements of every machine and engine and of every

object large enough to be visible, from a grain of dust to a planet.

Comprehension check:

True or False.

1) When we move a body there is a force of friction.

2) The size of the frictional force depends upon the time

3) The first law of motion isn’t true if we take all the forces into account.

4) The force multiplied by the time of action is called the impulse.

5) The rate of fall at any moment after the start depends upon the mass.

6) As the velocity increases the distance travelled per second also increases.

7) When a gun is fired the bullet pushes the gun back with a force different from that with

which the gun pushes the bullet forward.

III Grammar:

Future Simple; First Conditional

Future Simple: Will + inf. (without ‘to’); will not = won’t; (-), (?) No’ Do, Does’

Use: a) spontaneous decisions: I will give you your book back next week.

b) predictions: It will rain tomorrow.

First Conditional: If + Present Simple, Will + inf. (without ‘ to’)

Use: - possible condition and a probable result in the future:

If it rains, I’ll stay at home. What will you do, if it rains?

1. Put the verb in brackets into the correct tense. If there is no verb, use ‘ if’.

a) If a body (be) at rest, it (remain) at rest unless acted upon by an outside force.

9 the body. For instance, doubling the mass means double the momentum and double the impulse, for a given time. So that the rate of fall at any moment after the start is the same whatever the mass: neglecting the air resistance, we see that a 2-pound weight and an 8-pound weight, dropped at the same moment from a height, will keep level and reach the ground together. Since the increase in velocity is proportional to the force, which is unchanging, the rate of increase of fall must be unchanging. The rate of increase of velocity is called acceleration, so that the man of science says that the acceleration during free fall under gravity is constant. Thus to be quite clear, a body falling freely from rest will have a velocity of 32.2 feet per second (ft/sec) at the end of 1 second; double that velocity, namely 64.4 ft/sec, at the end of 2 seconds; treble that velocity, namely 96.6 ft/sec, at the end of 3 seconds, and so on. Of course, as the velocity increases the distance travelled per second also increases. The acceleration of gravity (g) is the most important figure in science, and comes into all kinds of calculations. Newton’s third law of motion is that reaction is always equal and opposite to action. That is, if two bodies, A and B, act upon one another, the action of A on B is always equal in magnitude and opposite in direction to the action of B on A. Let us make this clear. When a gun is fired the bullet pushes the gun back with a force equal to that with which the gun pushes the bullet forward. Owing to its much greater mass it does not move nearly so fast as the bullet, but the speed can be exactly calculated. Another example is given by the rocket. The downward rush of gases at very high speed results in the body of the rocket being pushed upward. We have now considered Newton’s three laws of motion. They are of immense importance, governing as they do the movements of every machine and engine and of every object large enough to be visible, from a grain of dust to a planet. Comprehension check: True or False. 1) When we move a body there is a force of friction. 2) The size of the frictional force depends upon the time 3) The first law of motion isn’t true if we take all the forces into account. 4) The force multiplied by the time of action is called the impulse. 5) The rate of fall at any moment after the start depends upon the mass. 6) As the velocity increases the distance travelled per second also increases. 7) When a gun is fired the bullet pushes the gun back with a force different from that with which the gun pushes the bullet forward. III Grammar: Future Simple; First Conditional Future Simple: Will + inf. (without ‘to’); will not = won’t; (-), (?) No’ Do, Does’ Use: a) spontaneous decisions: I will give you your book back next week. b) predictions: It will rain tomorrow. First Conditional: If + Present Simple, Will + inf. (without ‘to’) Use: - possible condition and a probable result in the future: If it rains, I’ll stay at home. What will you do, if it rains? 1. Put the verb in brackets into the correct tense. If there is no verb, use ‘if’. a) If a body (be) at rest, it (remain) at rest unless acted upon by an outside force.

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