Of Motion, Etc.

body fall freely towards the earth, by the attraction of gravitation, it will descend in a line perpendicular to its surface. In the first second it will fall through 16 feet; in the second second, having the velocity already acquired and being still acted on by the force of gravity, it will descend through 32 feet; in the third second, it will descend through 48 feet; in the fourth second, through 64 feet, and so on, adding to its velocity in every additional second. This is a motion uniformly accelerated, for the velocity is equally increased in each second of

time.

5. MOMENTUM is the force with which a body in motion would strike against another body. If a body of a given weight, say 10 pounds, were moving at the rate of 30 feet per second, and another body of the same weight were to move twice as fast, the last would have double the momentum of the first. Hence, when the bodies are equal the momentum will depend on the velocity. But if two unequal bodies move with the same velocity, their momentum will depend upon their weight. Hence, the momentum of a body will depend on its weight and ve

QUEST.-How far will it fall in the first second of time? How far in the second second? How far in the third second, etc. What kind of a motion will the body have? 5. What is momentum? If two equal bodies move with different velocities, what will their momentums be proportional to? If the velocities are equal and the bodies unequal, what will the momentum depend on? Generally what is the momentum of a body equal to ?

Of Motion, Etc.

locity that is, it will be equal to the weight mutiplied by the velocity.

If the weight of a body be represented by 5 and its velocity by 6, its momentum will be 5×6=30.

If the weight of a body be represented by 8, and its velocity by 2, its momentum will be represented by 16x2=32.

6. ACTION and REACTION.-When a body in motion strikes against another body, it meets with resistance. The force of the moving body is called action, and the resistance offered by the body struck is called reaction; and it is a general principle, that action and reaction are equal. Thus, if you strike a nail with a hammer, the action of the hammer against the nail, is just equal to the reaction of the nail against the hammer. Also, if a body fall to the earth, by the attraction of gravitation, the action of the body when it strikes the earth, is just equal to the reaction of the earth against the body.

7. CENTRE OF GRAVITY.-The centre of gravity is that point of a body about which all the parts will exactly balance each other. Hence, if the centre of gravity be supported, the body will not fall, for all the parts will balance each other about the centre of gravity.

The centre of gravity of a body is not changed by

QUEST.-6. What is action? What is reaction? Are action and reaction equal? 7. What is the centre of gravity of a body? If the centre of gravity is supported will the body fall? Is the centre of gravity changed by altering the position of the body?

Centre of Gravity.

changing its position. Thus, if a body be suspended by a cord, attached at its centre of gravity, it will remain balanced, in every position of the body.

8. If we have two equal bodies A and B, connected

C

B

together by a bar AB, the centre of gravity will be at C, the middle point of AB, and about this point the bodies will exactly balance each other.

and just as much nearer as the larger body exceeds the smaller. Thus, if B is three times greater than A, then BC will be one third of AC.

10. If one of

the bodies is very large in comparison with the oth

er, the centre of

B

QUEST.-8. If two equal bodies be connected by a bar, at what point of the bar will the centre of gravity be found? 9. If the bodies are unequal, where will the centre of gravity be found? How much nearer will it be to the larger body? 10. May one body be so much larger than another as to bring the common centre of gravity within the larger body?

Centre of Gravity.

gravity may fall within the larger body. Thus the centre of gravity of the bodies A and B, falls at C.

11. The vertical line drawn through the centre of gravity, is called the line of direction of the centre of gravity.

If the line of direction of the centre of gravity falls within the base on which the body stands, the body will be supported; but if the line falls without the base, the body will fall.

Thus, if in a wine glass, the centre of gravity be at C, the glass will fall the moment the line CD falls without the base.

12. Let us suppose a cart on inclined ground, to be loaded with stone, so that the centre of gravity of the mass shall fall at C. In this position the line of direction CD, falls within the base, and the cart will stand.

BD

But if the cart be loaded with hay, so as to bring the centre of gravity at A, the line of direction AB, will fall without the base, and the cart will be upset.

QUEST.-11. What is the line of direction of the centre of gravity? When will a body be supported, and when will it fall? 12. If the centre of gravity is near the base, is the body less, or more likely to fall than when it is further from the base? Give the illustration.

Of the Mechanical Powers.

SECTION III.

OF THE MECHANICAL POWERS.

1. There are six simple machines, which are called Mechanical powers. They are, the Lever, the Pulley, the Wheel and Axle, the Inclined Plane, the Wedge, and the Screw.

2. To understand the power of a machine, four things must be considered.

1st. The power or force which acts. This consists in the effort of men or horses, of weights, springs, steam, &c.

2d. The resistance which is to be overcome by the power. This generally is a weight to be moved.

3d. We are to consider the centre of motion, or fulcrum, which means a prop. The prop or fulcrum is the point about which all the parts of the machine move.

4th. We are to consider the respective velocities of the power and resistance.

3. A machine is said to be in equilibrium when the

QUEST.-1. How many simple machines are there? What are they called? 2. What things must be considered in order to understand the power of a machine? 3. When is a machine said to be in equilibrium?