A lever is a simple machine, a device built by man to win by means of a force, called a motor, another force called a resistant one.
The purpose of the primordial levers was to amplify the human strength allowing to perform jobs not allowed by the simple use of the muscular force.
The levers obey a fairly simple physical principle:
the system is in equilibrium if the resultant of the moments of the two forces is zero.
In physics the moment of a force with respect to a center is expressed by a module vector equal to the product of the intensity of the force for the length of its arm (distance of the line of action of the force from the fulcrum).
M OMENTO = F ORZA xb raccio
If the resultant of the moments must be null (so that the system is in equilibrium) then:
M FORCE IN ENTRY = M FORCE OUTPUT
F xb = F 'x b'
Or expressing the relationship in other terms we can say:
F: F '= b': b
Therefore the ratio between the forces must be equal to the inverse relationship between the arms.
If b is 10 times greater than b '(b = 10b') so that the system remains in equilibrium F must be 10 times smaller (F = 1 / 10F ').
We have therefore created an advantageous lever: imagine that the force F 'is represented by the weight force of a 100 kg boulder, to lift it you will need to simply apply a force F> 10 kg (> 1/10 of the weight force):
Let us now define the advantageous leverage: a lever is said to be advantageous when in equilibrium conditions the driving force is less than the resistant one (just as in the case just described).
However, a lever can also be disadvantageous if the driving force is greater than the resistant one.
Finally a lever is neutral or indifferent when the driving force is equal to the resistant one.
LEVER OF FIRST TYPE:
a lever is of the first type or first type if the fulcrum is between the driving force and the resistant force. In turn the first type lever can be advantageous if the driving force is further away from the fulcrum of the resistant force or, in the opposite case, disadvantageous.
They are levers of the first type the crowbar, the scissors, the tongs, etc.
SECOND TYPE LEVER:
a lever is said to be of the second type or of the second kind if the fulcrum is on the same side of the motive force and of the resistant force, at the same time it is necessary that the motive force be more distant from the fulcrum than the resistant one. It can therefore be deduced that second-order levers are always advantageous.
The wheelbarrow is the second type of nutcracker the bottle opener.
THIRD TYPE LEVER:
a lever is of the third type or of the third kind if the fulcrum is on the same side of the driving force and of the resistant force, at the same time it is necessary that the driving force is closer to the fulcrum than the resistant one. It can therefore be deduced that third-order levers are always disadvantageous.
The calipers are the third type of lever.
Also the muscles of our body, inserted on the bones, are from the physical point of view of the levers. The figure shows the flexion of the forearm by the biceps brachialis muscle.
This is a classic example of a third type lever, which as we have said is always disadvantageous. As the lever arm is shorter, the force developed by the biceps muscle must be far greater than the weight force of the ball held on the hand. This type of lever, however, allows a great amplitude and rapidity of movement.
In fact, in physics we speak of a static advantage and a dynamic advantage. There is a static advantage, when using a smaller driving force you can win a greater resistant force (advantageous lever), but in this case the speed and the amplitude of the movement are small, so there is a dynamic disadvantage.
Automatically a static disadvantage (disadvantageous lever) allows a greater speed and amplitude of movement, ie a dynamic advantage.
STATIC ADVANTAGE = DYNAMIC DISADVANTAGE
STATIC DISADVANTAGE = DYNAMIC ADVANTAGE
Plantar flexion of the foot with extended lower limbs is an example of second-order lever.
Resistance (weight) and force (muscle) are on the same side with respect to the fulcrum, the driving force is more distant and therefore the lever is advantageous. The very backward insertion of the triceps sural therefore facilitates the movement.
The whole musculoskeletal system is based on a system of levers. Therefore, whenever there is movement, a lever is produced that can be of first, second or third type. In the human body the most frequent levers are those of the first and third type, while the advantageous third-order levers are very rare.
However the human body uses some elements that can facilitate the action of the muscles. This is the case, for example, of the patella which, by displacing the quadriceps traction force anteriorly, increases their effectiveness.
