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11

To get a clear notion of what this means, let us consider the case of falling bodies.

First of all, we observe that the force of gravity pulling the body down is proportional

to the mass of 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 eight-pound weight, dropped at the same

moment from a height, will keep level and reach the ground together, because the

force on the 8-pound mass is four times that on the 2-pound mass.

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.

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, a grain of dust to a planet.

Comprehension check

Read the text carefully and say whether these statements are true or false. Correct

false statements.

1. Newton’s first law of motion can be stated as follows: if a body is at rest, it will

remain at rest unless acted upon by an inside force.

2. The size of friction doesn’t depend upon the weight of the body.

3. The change of motion of a body is proportional to the force acting on the body and

takes place against the direction of the force.

4. “Momentum” is mass divided by velocity.

5. “Acceleration” is the rate of increase of velocity

6. Reaction is always equal and opposite to action.

11 To get a clear notion of what this means, let us consider the case of falling bodies. First of all, we observe that the force of gravity pulling the body down is proportional to the mass of 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 eight-pound weight, dropped at the same moment from a height, will keep level and reach the ground together, because the force on the 8-pound mass is four times that on the 2-pound mass. 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. 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, a grain of dust to a planet. Comprehension check Read the text carefully and say whether these statements are true or false. Correct false statements. 1. Newton’s first law of motion can be stated as follows: if a body is at rest, it will remain at rest unless acted upon by an inside force. 2. The size of friction doesn’t depend upon the weight of the body. 3. The change of motion of a body is proportional to the force acting on the body and takes place against the direction of the force. 4. “Momentum” is mass divided by velocity. 5. “Acceleration” is the rate of increase of velocity 6. Reaction is always equal and opposite to action.

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