Equilibrium in physics is a state of the system, in which it is in relative rest to the surrounding objects. Statics is the study of equilibrium conditions. One of the mechanisms, knowledge of the equilibrium conditions for the operation of which is of fundamental importance, is the lever. Consider in the article what types of leverage are.
What is it in physics?
Before talking about the types of levers (in physics, grade 7 passes this topic), let's define this device. A lever is a simple mechanism that allows you to convert force into distance and vice versa. The lever has a simple device, it consists of a beam (board, rod), which has a certain length, and one support. The position of the support is not fixed, so it can be located both in the middle of the beam and at its end. We note right away that the position of the support generally determines the type of lever.
The latter has been used by man since time immemorial. So, it is known that in ancient Mesopotamia or in Egypt, with the help of it, they raised water from rivers or moved huge stones duringconstruction of various structures. Actively used the lever in ancient Greece. The only written evidence that has survived of the use of this simple mechanism is Plutarch's "Parallel Lives", where the philosopher gives an example of the use of the system of blocks and levers by Archimedes.
The concept of torque
Understanding the principle of operation of different types of levers in physics is possible if you study the issue of equilibrium of the mechanism under consideration, which is closely related to the concept of moment of force.
The moment of force is the value that is obtained by multiplying the force by the distance from the point of its application to the axis of rotation. This distance is called the "shoulder of the force". Let's denote F and d - the force and its shoulder, respectively, then we get:
M=Fd
The moment of force provides the ability to rotate around this axis of the entire system. Vivid examples in which you can observe the moment of force in action are unscrewing a nut with a wrench or opening a door with a handle that is far from the door hinges.
Torque is a vector quantity. In solving problems, one often has to take into account its sign. It should be remembered that any force that causes the system of bodies to rotate counterclockwise creates a moment of force with the sign +.
Lever balance
The figure above shows a typical lever and the forces that act on it are marked. Later in the article it will be said that it is -leverage of the first kind. Here, the letters F and R denote an external force and a certain weight of the load, respectively. You can also see that the support is offset from the center, so the lengths of the arms dF and dR are not equal to each other.
In statics it is shown that the lever does not move as a whole mechanism, the sum of all the forces that act on it must be equal to zero. We have noted only two of them. In fact, there is also a third one, which is opposite to these two and equal to their sum - this is the support reaction.
In order for the lever not to make rotational movements, it is necessary that the sum of all moments of forces be equal to zero. The shoulder of the reaction force of the support is zero, so it does not create a moment. It remains to write down the moments of forces F and R:
RdR- FdF=0=>
RdR=FdF
Recorded lever equilibrium condition as a formula, also given:
dR/dF=F/R
This equality means that in order for the lever not to rotate, the external force must be as many times greater (less) than the weight of the load being lifted, how many times the shoulder of this force is less (greater) than the shoulder on which the weight acts cargo.
The given wording means that how many times we win on the way with the help of the mechanism under consideration, we lose the same amount in strength.
Lever of the first kind
It was shown in the previous paragraph. Here we just say that for a lever of this type, the support is located between the acting forces F and R. Depending on the ratio of the lengths of the arms, such a lever canbe used both for lifting weights and for giving the body acceleration.
Mechanical scales, scissors, a nail puller, a catapult are examples of levers of the first kind.
In the case of a balance, we have two arms of the same length, so the balance of the lever is achieved only when the forces F and R are equal to each other. This fact is used to weigh bodies of unknown mass by comparing it with a reference value.
Scissors and a nail puller are prime examples of gaining strength but losing along the way. Everyone knows that the closer to the axis of the scissors a sheet of paper is laid, the easier it is to cut it. On the contrary, if you try to cut paper with the tips of scissors, then there is a high probability that they will begin to "chew" it. The longer the handle of the scissors or nail puller, the easier it is to perform the corresponding operation.
As for the catapult, this is a vivid example of gaining with the help of a lever on the way, and therefore in the acceleration that its shoulder imparts to the projectile.
Lever of the second kind
In all levers of the second kind, the support is located near one of the ends of the beam. This arrangement leads to the presence of only one shoulder at the lever. In this case, the weight of the load is always located between the support and the external force F. The arrangement of forces in the lever of the second kind leads to the only useful result: gain in strength.
Examples of this type of leverage are the wheelbarrow, which is used to carry heavy loads, and the nutcracker. In both cases, the loss along the way does not have any negative value. So, in the case of manualwheelbarrows, it is only important to keep the load on weight while it is moving. In this case, the applied force is several times less than the weight of the load.
Lever of the third kind
The design of this type of lever is in many ways similar to the previous one. The support in this case is also located at one of the ends of the beam, and the lever has a single arm. However, the location of the acting forces in it is completely different than in a lever of the second kind. The point of application of force F is between the weight of the load and the support.
Shovel, barrier, fishing rod and tweezers are striking examples of this type of leverage. In all these cases, we win on the way, but there is a significant loss in strength. For example, to hold a heavy load with tweezers, you need to apply a large force F, so using this tool does not mean holding heavy objects with it.
In conclusion, we note that all types of levers work on the same principle. They do not give a gain in the work of moving goods, but only allow you to redistribute this work in the direction of its more convenient implementation.
