Centre of Gravity.
What is the centre of gravity in a body?
The centre of gravity in a body, is the point on which the body may be balanced, as is shown in [Fig. 30].
Fig. 30.
Where is the centre of gravity in a circle?
It is the centre of the circle.
Where is the centre of gravity in a ball?
It is the centre of the ball.
What influence does the earth have on the centre of gravity?
The earth draws the centre of gravity in a body as near to itself as possible.
Why will an egg, when placed on either end, fall over on its side?
Because the centre of gravity is nearer the earth when the egg is placed upon its side than when upon either end, as is shown in [Fig. 31] and [Fig. 32].
Fig. 31.
Fig. 32.
Why will a ball remain in any position when placed on the earth?
Because its centre of gravity is always the same distance from the earth, whatever may be its position.
When will a body change its position if left unsupported?
A body will change its position whenever by so doing its centre of gravity will be lowered.
When may a body be readily moved about?
When its centre of gravity always remains at the same distance from the earth while being moved.
Why is a round body, like a ball, easily rolled about the floor?
Because its centre of gravity remains at the same distance from the floor.
When is the position of a body difficult to change?
Whenever the centre of gravity in it has to be lifted up in order to make the change, as is shown in [Fig. 35].
Fig. 33.
Fig. 34.
Fig. 35.
The centre of gravity, in [Fig. 33], may be lowered; hence, it will change its position if unsupported. The centre of gravity, in [Fig. 34], can neither be lowered nor lifted up; hence, it may be readily rolled about. The centre of gravity, in [Fig. 35], must be lifted up; hence, it remains firm in its place.
When does an object stand firm?
It stands firm when it has a broad base, and its centre of gravity is near the earth.
What effect is produced by elevating the centre of gravity?
The higher the centre of gravity in any body is raised above the earth, the greater is its tendency to fall.
Why is a load of hay more easily overturned than a load of iron, when upon the same wagon?
Because the centre of gravity in the load of hay is much higher than it is in the load of iron.
Why do passengers on top of a stage cause it to overturn more easily?
Because their weight on the top raises the centre of gravity in the stage, and it is more readily overturned.
Why is a person more likely to fall when standing up than when sitting down?
Because the centre of gravity is higher in a person when standing than when sitting.
Why is a boy more likely to fall when raised on stilts?
Because the centre of gravity in the boy is farther from the earth when he is raised on stilts.
When will a load of hay be overturned?
It will be overturned whenever the line from the centre of gravity to the earth falls outside of the wheels, as is shown in [Fig. 36].
Fig. 36.
Why should lamps have large bottoms?
Lamps should have large bottoms so that the line of gravity may not fall outside of the bottom.
Where must the line of gravity in a body always fall?
It must always fall within the base of that body, or the body will be overturned.
How must a leaning tower be built so as not to fall?
It must be built so that the line of gravity will fall within its base.
Why do we lean back when carrying a load in front of us?
We lean back so that the line of gravity may be kept directly over our feet.
Why does a person standing up in a wagon fall when the wagon is suddenly moved?
Because the feet, being in contact with the wagon, move with it before the motion extends to the rest of the body, thus throwing the line of gravity behind the feet; hence, the person must fall, unless supported.
Why does a person standing up in a wagon fall when the wagon is suddenly stopped?
Because the feet, being in contact with the wagon, are suddenly stopped, while the body still moves forward, thus throwing the line of gravity ahead of the feet; hence, the person must fall, unless supported.
Why do animals with four feet stand firmer than those with only two feet?
Because the four feet afford a larger base to stand upon than the two feet do, and, therefore, support the centre of gravity better.
Why can a horse or a cow remain standing for many hours without seeming to be weary?
Because their four feet afford a good support to the centre of gravity.
Why does a person who is standing soon become weary?
Because it requires a constant effort to keep the line of gravity over the feet, and this soon produces weariness.
Why can a person sit longer than he can stand?
Because the centre of gravity is lower, when he is sitting, than when he is standing; hence, it requires less strength to support the body in a sitting position.
Why can a person lie down longer than he can sit or stand?
Because the centre of gravity is nearest the earth when he is lying down, and it requires no effort to remain in that position.
How do we measure time?
We measure time by means of a pendulum, such as is used in a common clock.
How does the pendulum move?
The pendulum moves regularly to and fro, from the line of gravity.
Does the pendulum vary any in its vibrations, or the time of moving to and fro?
The pendulum does not vary any in its vibrations.
Fig. 37.
Why does the pendulum keep moving?
When the pendulum is at A, in [Fig. 37], gravity causes it to descend to C, and the motion it gains while falling to C carries it up to B; gravity then causes it to descend to C, and the motion it gains while falling to C carries it back to A again.
What helps to keep the pendulum in motion?
The works inside of the clock help to keep the pendulum in motion.
How long must the pendulum be, so as to tick once in every second?
The pendulum must be thirty-nine inches long.
How does it tick when the pendulum is shortened?
It ticks more rapidly.
How does it tick when the pendulum is lengthened?
It ticks more slowly.
How then does cold affect a clock?
Cold makes the clock go faster by shortening the pendulum.
How does heat affect a clock?
Heat makes the clock go more slowly by lengthening the pendulum.
How may the pendulum always be kept of the same length?
By having the weight at the bottom of the pendulum, so that it may be raised or lowered by means of a screw.