What is the rule of lever equilibrium? Lever equilibrium condition

Leverage is called solid, which can rotate around a fixed point. The fixed point is called fulcrum. The distance from the fulcrum to the line of action of the force is called shoulder this power.

Lever equilibrium condition: the lever is in equilibrium if the forces applied to the lever F 1 And F 2 tend to rotate it in opposite directions, and the modules of the forces are inversely proportional to the shoulders of these forces: F 1 /F 2 = l 2 /l 1 This rule was established by Archimedes. According to legend, he exclaimed: Give me a foothold and I will lift the Earth .

For the lever it is fulfilled « golden rule» mechanics (if friction and mass of the lever can be neglected).

By applying some force to a long lever, you can use the other end of the lever to lift a load whose weight greatly exceeds this force. This means that by using leverage you can gain power. When using leverage, a gain in power is necessarily accompanied by an equal loss along the way.

Moment of power. Rule of Moments

The product of the force modulus and its shoulder is called moment of force.M = Fl , where M is the moment of force, F is the force, l is the leverage of the force.

Rule of Moments: A lever is in equilibrium if the sum of the moments of forces tending to rotate the lever in one direction is equal to the sum of the moments of forces tending to rotate it in the opposite direction. This rule is valid for any rigid body capable of rotating around a fixed axis.

The moment of force characterizes the rotating action of the force. This action depends on both the force and its leverage. That is why, for example, when wanting to open a door, they try to apply force as far as possible from the axis of rotation. With the help of a small force, a significant moment is created, and the door opens. It is much more difficult to open it by applying pressure near the hinges. For the same reason, a nut is easier to unscrew with a longer wrench, a screw is easier to remove with a screwdriver with a wider handle, etc.

The SI unit of moment of force is newton meter (1 N*m). This is the moment of a force of 1 N having a shoulder of 1 m.

Municipal budgetary educational institution
Mikheykovskaya high school
Yartsevo district, Smolensk region
Lesson on the topic
« Simple mechanisms.
Application of the law of equilibrium
lever to block"
7th grade
Compiled and conducted
Physics teacher of the highest category
Lavnyuzhenkov Sergey Pavlovich

2016 – 2017 academic year

Lesson objectives (planned learning outcomes):
Personal:
developing skills to manage your educational activities;
formation of interest in physics during analysis physical phenomena;
formation of motivation by setting cognitive tasks;
developing the ability to conduct dialogue on the basis of equal relations and mutual respect;
development of independence in acquiring new knowledge and practical skills;
development of attention, memory, logical and creative thinking;
students' awareness of their knowledge;
Metasubject:
developing the ability to generate ideas;
develop the ability to determine goals and objectives of activities;
conduct an experimental study according to the proposed plan;
formulate a conclusion based on the results of the experiment;
develop communication skills when organizing work;
independently evaluate and analyze your own activities from the perspective of received
results;
use various sources to obtain information.
Subject:
developing an understanding of simple mechanisms;
developing the ability to recognize levers, blocks, inclined planes, gates, wedges;
do simple mechanisms provide gains in strength;
developing the ability to plan and conduct an experiment based on the results
experiment to formulate a conclusion.
Lesson progress
№
p.p.
1
2
3
4
5
6
7
8
9
Teacher activities
Student activity
Notes
Organizational stage
Preparing for the lesson
Repeat and test phase
mastering the material covered
Working with pictures, working in
parah - oral story
According to plan
mutual verification
knowledge
Stage of updating knowledge,
goal setting
stage: assistance and control over work
students
Fizminutka
Organizational activity
stage: practical work,
actualization and goal setting
Practical consolidation stage
knowledge gained: problem solving
Stage of consolidation of what has been learned
material
Introduction of the concept of “simple”
mechanisms", according to
Working with a textbook, compiling
schemes
Self-esteem
Exercise
Installation assembly
Introduction of the concept of “leverage”
goal setting
Introduction of the concept of “shoulder strength”
Experimental
confirmation of the equilibrium rule
lever
Self-esteem
Solve problems
Peer review
Answer questions
Homework discussion stage
Write down homework

10
Reflection stage:
students are asked to highlight
new, interesting, difficult in the lesson
Share your impressions in
orally and in writing
Teacher:
Today in the lesson we will look into the world of mechanics, we will learn to compare and analyze. But
First, let's complete a number of tasks that will help open the mysterious door wider and show all
the beauty of a science like mechanics.
There are several pictures on the screen:
What do these people do? (mechanical work)
The Egyptians build a pyramid (lever);
A man lifts water (with the help of a gate) from a well;
People roll a barrel onto a ship (inclined plane);
A man lifts a load (block).
Teacher:

Plan your story:
1. What conditions are necessary to perform mechanical work?
2. Mechanical work- This …………….
3. Symbol mechanical work
4. Work formula...
5. What is the unit of measurement for work?
6. How and after which scientist is it named?
7. In what cases is work positive, negative or zero?
Teacher:
Now let's look at these pictures again and pay attention to how these people do their work?
(people use a long stick, a collar, an inclined plane device, a block)
Teacher:

Students: Simple mechanisms
Teacher:

Right! Simple mechanisms. What topic do you think we will be talking about in the lesson?
How can you call these devices in one word?
talk today?
Students: About simple mechanisms.
Teacher: Correct. The topic of our lesson will be simple mechanisms (writing the topic of the lesson in a notebook,
slide with the topic of the lesson)
Let's set the goals of the lesson:
Together with children:
study what simple mechanisms are;
consider types of simple mechanisms;
lever equilibrium condition.
Teacher: Guys, what do you think simple mechanisms are used for?
Students: They are used to reduce the force we apply, i.e. for her
transformations.
Teacher: Simple mechanisms are found both in everyday life and in all complex factory machines, etc.
Guys, in what household appliances and devices have simple mechanisms.
Students: Lever scales, scissors, meat grinder, knife, axe, saw, etc.
Teacher: What a simple mechanism does a crane have?
Students: Lever (boom), blocks.

Teacher: Today we will take a closer look at one of the types of simple mechanisms.
It is on the table. What kind of mechanism is this?
Students: This is a lever.
We hang weights on one of the arms of the lever and, using other weights, balance the lever.
Let's see what happened. We see that the shoulders of the weights are different from each other.
Let's swing one of the lever arms. What do we see?
Students: After swinging, the lever returns to its equilibrium position.
Teacher: What is called a lever?
Students: A lever is a rigid body that can rotate around a fixed axis.
Teacher: When is the lever in balance?
Students:
Option 1: the same number of weights at the same distance from the axis of rotation;
Option 2: more load – less distance from the axis of rotation.
Teacher: What is this dependence called in mathematics?
Students: Inversely proportional.
Teacher: With what force do the weights act on the lever?
Students: Body weight due to the gravity of the Earth. P = Fstrand = F
F 
1
F
2
l
2
l
1
where F1 is the module of the first force;
F2 – module of the second force;
l1 – shoulder of the first force;
l2 – shoulder of the second force.
Teacher: This rule was established by Archimedes in the 3rd century BC.
Task: Using a crowbar, a worker lifts a box weighing 120 kg. What strength is he
applies to the larger arm of the lever if the length of this arm is 1.2 m, and the smaller arm is 0.3 m.
What will be the gain in power? (Answer: Strength gain is 4)
Solving problems (independently with subsequent mutual verification).
1. The first force is equal to 10 N, and the shoulder of this force is 100 cm. What is the second force equal to if its shoulder
equals 10 cm? (Answer: 100 N)
2. A worker uses a lever to lift a load weighing 1000 N, while he applies a force of 500 N.
What is the arm of the greater force if the arm of the lesser force is 100 cm? (Answer: 50 cm)
Summing up.
What mechanisms are called simple?
What types of simple mechanisms do you know?
What is a lever?
What is leverage?
What is the rule for lever equilibrium?
What is the significance of simple mechanisms in human life?
D/z
1. Read the paragraph.
2. List the simple mechanisms that you find at home and those that people
uses in everyday life, recording them in the table:
A simple mechanism in everyday life, in technology
Type of simple mechanism
3. Additionally. Prepare a report about one simple mechanism used in everyday life,

§ 03-i. Lever balance rule

Even before our era, people began to use levers in construction business. For example, in the picture you see the use of a lever to lift weights during the construction of the pyramids in Egypt.

Lever called a rigid body that can rotate around a certain axis. A lever is not necessarily a long and thin object. For example, any wheel is a lever, since it can rotate around an axis.

Let's introduce two definitions. Line of action of force let's call a straight line passing through the force vector. Shoulder of strength let's call shortest distance from the axis of the lever to the line of action of the force. From geometry you know that the shortest distance from a point to a line is the distance perpendicular to the line.

Let us illustrate these definitions. In the picture on the left the lever is the pedal. Its axis of rotation passes through the point ABOUT. Two forces are applied to the pedal: F 1 – the force with which the foot presses on the pedal, and F 2 – the elastic force of the tensioned cable attached to the pedal. Passing through the vector F 1 line of action of the force (depicted by a dotted line), and by constructing a perpendicular to it from the so-called ABOUT, we will get segment OA – arm of force F 1

With strength F 2, the situation is simpler: the line of its action need not be drawn, since its vector is located more successfully. Having built from so. ABOUT perpendicular to the line of action of the force F 2, we get segment OB – arm of force F 2 .

Using a lever, a small force can balance a large force.. Consider, for example, lifting a bucket from a well (see figure in § 5-b). The lever is well gate– a log with a curved handle attached to it. The axis of rotation of the gate passes through the log. The lesser force is the force of the person's hand, and the greater force is the force with which the chain pulls down.

On the right is a diagram of the gate. You see that the arm of greater force is the segment O.B., and the shoulder of lesser force is the segment O.A.. It is clear that OA > OB. In other words, the shoulder of lesser force is larger than the shoulder of greater force. This pattern is true not only for the gate, but also for any other lever.

Experiments show that when the lever is in balance The shoulder of the smaller force is as many times greater than the shoulder of the larger force, how many times the greater force is greater than the smaller one:

Let us now consider the second type of lever - blocks. They can be movable or immobile (see figure).

Even before our era, people began to use levers in construction. For example, in the picture you see the use of leverage in the construction of the pyramids in Egypt. A lever is a rigid body that can rotate around a certain axis. A lever is not necessarily a long and thin object. For example, a wheel is also a lever, since it is a rigid body rotating around an axis.

Let us introduce two more definitions. The line of action of a force is a straight line passing through the force vector. The shortest distance from the axis of the lever to the line of action of the force will be called the shoulder of the force. From your geometry course, you know that the shortest distance from a point to a line is the perpendicular distance to this line.

Let us illustrate these definitions with an example. In the picture on the left, the lever is the pedal. The axis of its rotation passes through point O. Two forces are applied to the pedal: F1 is the force with which the foot presses on the pedal and F2 is the elastic force of the tensioned cable attached to the pedal. Drawing through vector F1 the line of force action (shown blue), and by lowering a perpendicular from point O onto it, we get the segment OA - the arm of force F1.

With force F2 the situation is even simpler: the line of its action need not be drawn, since the vector of this force is located more successfully. Dropping a perpendicular from point O to the line of action of force F2, we obtain segment OB—the arm of this force.

With the help of a lever, a small force can balance a large force. Consider, for example, lifting a bucket from a well. The lever is a well gate - a log with a curved handle attached to it. The axis of rotation of the gate passes through the log. The lesser force is the force of the person's hand, and the greater force is the force with which the bucket and the hanging part of the chain are pulled down.

The drawing on the left shows the gate diagram. You can see that the arm of greater force is segment OB, and the arm of lesser force is segment OA. It is clearly seen that OA > OB. In other words, the lower-strength arm is larger than the higher-strength arm. This pattern is true not only for the gate, but also for any other lever. In more general view it sounds like this:

When a lever is in equilibrium, the arm of the smaller force is as many times larger than the arm of the larger force, how many times the larger force is greater than the smaller one.

Let's illustrate this rule using a school lever with weights. Take a look at the picture. In the first lever, the arm of the left force is 2 times greater than the arm of the right force, therefore, the right force is twice as great as the left force. The second lever has a right-hand force arm that is 1.5 times larger than a left-hand force arm, that is, the same number of times as left force more right power.

So, when two forces are in balance on a lever, the larger of them always has a smaller leverage and vice versa.