Lever.
What is a lever?
A Lever is a rod or bar of some kind used for raising weights.
Of what are levers made?
Levers are made of any solid substance, such as wood and iron.
When a spoon is balanced on a cup, as in [Fig. 40], what may the spoon be called?
The spoon may then be called a lever.
Upon what does the spoon rest?
It rests upon the edge of the cup.
Fig. 40.
What is the cup on which the spoon rests called?
It is called a Fulcrum.
What is a fulcrum?
A fulcrum is any solid body against which a lever rests.
When the spoon is called a lever, what are the parts on each side of the fulcrum called?
They are called the arms of the lever.
What is necessary, so that a lever may balance?
The arms of the lever must be of equal weight, or they will not balance.
If the bowl of the spoon be filled with water, what will result?
The bowl will sink, and the handle of the spoon will rise.
What will cause the bowl of the spoon to sink?
The weight of the water in it will cause it to sink.
How can the bowl of the spoon be prevented from sinking?
By applying a force to the handle of the spoon, equal to the weight of the water put in the bowl.
What is the force applied to the handle called?
It is called the Power.
What three things are always necessary when the lever is used?
The power, the fulcrum, and the weight, are always necessary when the lever is used.
Which of these three things are movable?
The power and the weight are movable.
Which one does not move?
The fulcrum does not move.
How many kinds of levers are there?
There are three kinds of levers.
What is the first kind of lever?
It is where the fulcrum is between the power and the weight, as is shown in [Fig. 41].
Fig. 41.
When the fulcrum is under the middle of the lever, how can the lever be kept balanced?
The lever will remain balanced, so long as the power and the weight used are equal to each other.
When the lever is moved, so that the fulcrum is nearer the weight, which arm of the lever is the heavier?
The arm towards the power, because it is the longer arm.
Will the power applied to the long arm of the lever be as heavy as the weight?
No; the nearer the fulcrum is to the weight, the less will be the power needed to balance that weight.
How will it be when the fulcrum is nearer to the power than it is to the weight?
Then the power must be heavier than the weight.
When the long arm of the lever is twice the length of the short arm, what will be the difference in the weights?
One pound applied to the long arm will then balance two pounds applied to the short arm.
Upon what does the power of a lever depend?
It depends upon how much nearer the fulcrum is placed to one end of the lever, than to the other end.
Of what use is the lever?
It is very useful in raising heavy bodies.
Fig. 42.
What is the weight to be lifted in [Fig. 42]?
The stone is the weight to be lifted.
Where is the fulcrum placed?
The fulcrum is placed near the stone.
Where is the power applied?
The power is applied by the hand to the long arm of the lever.
How may a see-saw be made?
A see-saw may be made by placing a plank or a board across a rail in a fence, so that it will balance.
Is the see-saw, as shown in [Fig. 43], an example of the lever?
Yes; the see-saw is an example of the lever.
Fig. 43.
What are the parts of the plank on each side of the fence-rail called?
They are called the arms of the lever.
What is the fulcrum in the see-saw?
The fence-rail on which it rests, is the fulcrum.
What is the power in the see-saw?
The power is the boy on one end of the lever.
What is the weight in the see-saw?
The weight is the boy on the other end of the lever.
How do children play at see-saw?
By pushing against the earth with their feet, the arms of the lever are alternately forced to rise and fall in the air, thus making a see-saw.
Where must the fulcrum be when the two boys are of the same weight?
The fulcrum must be under the middle of the lever.
Where must the fulcrum be when one boy is heavier than the other?
The fulcrum must then be nearest to the heavier boy, so that they may balance.
Does it require much force to play at see-saw?
No; when the see-saw is balanced, a very little force will cause it to move up and down.
Of what use is a pair of scales?
A pair of scales is very useful in weighing different substances.
Fig. 44.
Are the scales, as shown in [Fig. 44], an example of the lever?
The scales are an example of the lever.
Where is the fulcrum of the scales?
The fulcrum is the point on which the lever rests.
What is the weight?
The weight is the lead put in one scale.
What is the power?
The power is the substance put in the other scale, so as to balance the weight.
When the lead put in one scale weighs exactly a pound, how much sugar must be put in the other scale to balance the lead?
Exactly one pound of sugar must be put in to balance the lead.
For what is a balance mostly used?
A balance is mostly used for weighing heavy bodies.
Is the balance a lever?
Yes; the balance is a lever.
Which is the fulcrum in the balance?
The fulcrum is the hook that holds the balance up.
Fig. 45.
What is the weight?
The weight is the large body hanging from the short arm of the lever.
What is the power?
The power is the smaller body hanging from the long arm of the lever.
In [Fig. 45], the distance from the fulcrum to the power is ten times the distance from the fulcrum to the weight; now, if the small body weighs one pound, how much must the large body weigh, so as to balance it?
The large body must weigh ten pounds.
When the small body weighs ten pounds, how much must the large body weigh, so as to balance it?
The large body must weigh ten times as much, or one hundred pounds.
If the smaller body be moved half-way to the fulcrum, how much weight will then balance it?
It will then take only half the weight, or fifty pounds to balance it.
What other familiar example of this kind of lever may be mentioned?
The pump-handle is a lever of this kind.
Fig. 46.
What is the power?
The power is the force applied to the pump-handle.
What is the fulcrum?
The fulcrum is the screw that fastens the handle to the pump.
What is the weight?
The weight is the water raised by the bucket.
Are scissors an example of the lever?
Yes; scissors are a double lever.
What is the fulcrum in the scissors?
The fulcrum is the rivet which holds the two levers together.
What is the power?
The power is the force applied by the fingers.
What is the weight?
The weight is the paper, or other substance, to be cut by the scissors.
Can scissors be made very strong?
Yes; some scissors are made so strong that they are used for cutting iron, tin, and zinc.
What other useful instruments are made upon the same principle as the scissors?
Such instruments as nippers, pincers, tweezers, and blacksmith’s tongs.
What is the second kind of lever?
It is where the weight is between the power and the fulcrum, as is shown in [Fig. 47].
Fig. 47.
Of what use is this kind of lever?
It is very useful in lifting heavy weights a short distance from the earth.
What example is there of this kind of lever?
The door on its hinges is a lever of this kind.
What is the fulcrum?
The hinges are the fulcrum.
What is the weight?
The door is the weight.
What is the power?
The force that opens or shuts the door is the power.
What other examples of this kind of lever have we?
We have an example in the lid of the tea-kettle, and in the cover of the ink-stand, when they are fastened on by hinges.
What is the third kind of lever?
It is where the power is between the fulcrum and the weight, as is shown in [Fig. 48].
Fig. 48.
What examples of this kind of lever have we in common use?
The shovel, the spade, and the pitch-fork, are levers of this kind.
What is the fulcrum?
One hand is the fulcrum.
What is the power?
The other hand is the power.
What is the weight?
The earth raised on the shovel is the weight.
What other familiar example have we of this kind of lever?
The fishing-rod is an example of this kind. One hand is the fulcrum, the other hand is the power, and the line, with the fish, is the weight. This is made plain by [Fig. 49], representing a lad just drawing a fish out of the water.
Fig. 49.
There are several other examples of this kind of lever; will you name some of them?
Yes; brooms, hoes, rakes, and axes are all levers of this kind.
What is the first kind of lever mentioned?
It is where the fulcrum is between the power and the weight, as in [Fig. 50].
Fig. 50.
What is the second kind of lever mentioned?
It is where the weight is between the power and the fulcrum, as in [Fig. 51].
Fig. 51.
What is the third kind of lever mentioned?
It is where the power is between the fulcrum and the weight, as in [Fig. 52].
Fig. 52.