Rolling friction force

The rolling friction force is much less than the sliding friction force, so it is much easier to roll a heavy object than to move it.

The force of friction depends on the relative velocity of the bodies. This is its main difference from the forces of gravity and elasticity, which depend only on distances.

Resistance forces during the motion of solid bodies in liquids and gases

When a solid body moves in a liquid or gas, it is acted upon by the resistance force of the medium. This force is directed against the speed of the body relative to the medium and slows down the movement.

This leads to the fact that with the force of the hands it is possible to move a heavy body, for example, a floating boat, while it is simply impossible to move, say, a train with the force of the hands.

The modulus of the resistance force F c depends on the size, shape and state of the surface of the body, the properties of the medium (liquid or gas) in which the body moves, and, finally, on the relative velocity of the body and the medium.

An approximate nature of the dependence of the module of the resistance force on the module of the relative velocity of the body is shown in Figure 3.25. At a relative velocity equal to zero, the resistance force does not act on the body (F c = 0). As the relative speed increases, the resistance force first grows slowly, and then faster and faster. At low speeds of movement, the resistance force can be considered directly proportional to the speed of the body relative to the medium:

F c = k 1 υ, (3.12)

where k 1 - resistance coefficient, depending on the shape, size, state of the surface of the body and the properties of the medium - its viscosity. It is not possible to calculate the coefficient k 1 theoretically for bodies of any complex shape; it is determined empirically.

At high speeds of relative motion, the drag force is proportional to the square of the speed:

F c = k 2 υ 2 , υ, (3.13)

where k 2 is a drag coefficient other than k 1 .

Which of the formulas - (3.12) or (3.13) - can be used in a particular case is determined empirically. For example, for a passenger car, it is desirable to apply the first formula at approximately 60-80 km / h, at high speeds, the second formula should be used.

Questions for the paragraph

1. Look around you. Do you see the beneficial effect of friction forces?

2. Why are notches made on the jaws of a vise and pliers?

3. Why is a relief pattern (tread) made on car tires?

4. Under what conditions do friction forces appear?

5. What determines the modulus and direction of the static friction force?

6. Within what limits can the static friction force change?

7. What force imparts acceleration to a car or locomotive?

8. Can the force of sliding friction increase the speed of a body?

9. What is the main difference between the resistance force in liquids and gases and the friction force between two solid bodies?

10. Give examples of the beneficial and harmful effects of friction forces of all kinds.

Sample assignments for the exam A1. A box of mass 20 kg is placed on a horizontal floor. The coefficient of friction between the floor and the box is 0.3. A force of 36 N is applied to the box in the horizontal direction. What is the friction force between the box and the floor? 1) 0 2) 24 N 3) 36 N 4) 60 N A2. The area of ​​the first side face of the bar on the table is 2 times less than the area of ​​the second face, and the coefficient of friction on the table surface is 2 times greater. When turning the bar from the first side to the second, the frictional force of sliding the bar on the table 1) will not change 3) will decrease by 4 times 2) will decrease by 2 times 4) will increase by 2 times A3. How does the force of friction change when the rod slides off the surface of an inclined table? The speed is directed along the rod. 1) does not change 2) changes linearly 3) gradually decreases 4) remains constant until the middle of the rod, and then becomes equal to zero A4. The body moves uniformly along the plane. The pressure force of the body on the plane is 8 N, the friction force is 2 N. The slip coefficient is 1) 0,16 2) 0,25 3) 0,75 4) 4 A5. A skater with a mass of 70 kg slides on ice. What is the friction force acting on the skater if the friction coefficient of sliding skates on ice is 0.02? 1) 0.35 N 2) 1.4 N 3) 3.5 N 4) 14 N

>>Physics: Resistance forces during the motion of solids in liquids and gases

???
1. Under what conditions do friction forces appear?
2. What determines the modulus and direction of the static friction force?
3. Within what limits can the static friction force change?
4. What force imparts acceleration to a car or locomotive?
5. Can the force of sliding friction increase the speed of a body?
6. What is the main difference between the resistance force in liquids and gases and the friction force between two solid bodies?
7. Give examples of the beneficial and harmful effects of friction forces of all kinds.

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics Grade 10

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Ministry of Education and Science of the Samara Region
GBOU SPO "Syzran Medical and Humanitarian College"

Methodical development
lesson on the topic: "Forces in mechanics: universal gravitation, elasticity, friction"

physics teacher

GBOU SPO "SMGC"

Syzran, 2013

1. Explanatory note. 3

2. An extended lesson plan. 4

3. Literature. eight

4. Conclusions on the lesson. nine

4. Application.

1. Questions to prepare for the seminar. ten
2. Table for systematization of knowledge on the topic "Forces in mechanics". eleven
3. Variants of the test on the topic "Forces in mechanics". 12

Explanatory note.

The cyclical commission on social disciplines is working on the problem of creating conditions for students to adapt to new educational conditions, so I devoted this methodological work to the same problem. It contains a practical part and an application.

The purpose of the methodological work: the use of an innovative form of lesson to create conditions for students to adapt to the new educational environment.

To innovative lesson forms i ty include: lessons-seminars, lessons-ko n conferences, lessons using game methods, and n integrated lessons and others

I often use seminars in my work. What characterizes their didactic features and methodological foundations?

The purpose and specificity of the seminar lessons (from the Latin seminarium - a hotbed, in this case - a hotbed of knowledge) is to activate the independent work of the student. a studying on educational and additional literature and thus encourage them to a deeper understanding and enrichment of knowledge on the topic under study.

I conduct seminars, as a rule, on topics that have rich additional material and are easy for independent study. For example, when studying the topics “Free fall of bodies”, “Forces in mechanics”, “Application of the first law of thermodynamics to isoprocesses”, etc. When preparing a lesson-seminar, the teacher develops questions in advance and indicates lit e rature for independent work, allocating the necessary time for its study. This is followed by the seminar itself.

Students discuss in detail the questions posed, using for this both the material of the textbook and information from additional literature, clarifying, expanding and deepening their knowledge, the teacher should encourage e standard judgments, original thoughts, the search for new approaches to understanding the topic under study. It is not difficult to understand that such lessons not only activate about cognitive activity of students, but also allow them to acquire the ability to independently obtain science h knowledge, develop their speech and thinking, adapt to new educational conditions. This is where their value lies.

Methodical work can be used by teachers of social disciplines of technical schools and colleges in their work.

Lesson topic: "Forces in mechanics: universal gravitation, elasticity, friction"

Goals:

educational: concretize students' ideas about forces in mechanics, systematize students' knowledge on this topic;

educational: to promote the formation of skills to independently acquire knowledge, to continue work on the development of thinking;

developing: to form motivation by setting cognitive tasks, revealing the connection between experience and theory, to form the ability to perform experimental tasks: select equipment, plan and perform actions, describe results and draw conclusions.

Type of occupation: lesson-seminar.

Lesson layout: posters , made by students, containing information on forces in mechanics;experimental equipment, which allows to measure any mechanical force.

Lesson stages	Time, min	Methods and forms
Organizational moment. Formulation of the purpose of the lesson. Concretization of theoretical knowledge. Development of experimental skills. Practical application of knowledge. Independent work. Summarizing. Homework.	2-3 5 25 15 15 15 5	Officer's report. Introductory speech of the teacher. Students' answers at the blackboard and from their seats. Solution of experimental problems. Solving quality problems. Individual work. The final word of the teacher. Teacher's message, writing on the board.

Two weeks before the lesson, students were informed about the topic of the seminar and sample questions, and they were also asked to select high-quality tasks on this topic. Before the start of the lesson, students received a "skeleton" of the table, which they will have to fill out during the lesson.

During the classes.

1) The duty officer submits a report, names those who are absent and the reasons for their absence, the teacher greets the students, paying special attention to appearance and readiness for the lesson.

2) The teacher formulates the goals and objectives of the lesson, presents the children with an expert commission (these are two students who answered all questions on the topic the day before the seminar), introduces students to the types of forces in nature.

Gravitational forces- act between all bodies, but are so small that they can be neglected if both bodies have a small mass, or are at a great distance from each other.

Electromagnetic forces- act between particles that have electric charges. The scope of their activities is especially extensive and varied.

nuclear forces - the strongest interaction in nature, but their actions are very limited by distance.

Weak interactions- cause transformations of elementary particles into each other.

Only gravitational and electromagnetic interactions can be considered as forces in the sense of Newtonian mechanics.

3) Students answer questions of a theoretical nature.

1. Define strength.

Force is a quantitative measure of the action of one body on another, as a result of which acceleration is imparted to the body.

The force of universal gravitation manifests itself between any two bodies, depends on the mass of both bodies and on the distance between these bodies. The forces of universal gravitation are directed along a straight line passing through the centers of gravity of two interacting bodies. Determined by the formula F = G m 1m2 / R2

The gravitational constant is numerically equal to the force of universal gravitation between bodies of mass 1 kg each, if the distance between them is 1 m.

G \u003d 6.67 10 -11 N m 2 / kg 2 It was first measured by G. Cavendish (1731-1810) using a torsion balance.

Gravity is the force with which the Earth acts on any body located near its surface. The force of gravity is always directed towards the center of the Earth, is determined by the formula F = mg, where g is the free fall acceleration, equal to 9.8 m/s 2 .

5. What is the reason that the Earth imparts the same accelerations to all bodies, regardless of their masses?

The free fall acceleration does not depend on the mass of the body, but depends on the distance to the center of the Earth. Distanceto the center of the Earth is assumed to be equal to the radius of the Earth, so all bodies located near the surface are given the same acceleration.

6. What is the condition for elastic forces?

Elastic forces arise when a body is deformed. However, they are always directed against deformation.

The English physicist R. Hooke (1635-17703) experimentally established that for small deformations, the relative elongation is directly proportional to the stress (the elongation of a rod during elastic deformation is proportional to the force acting on the rod).

8. Define body weight.

The weight of a body is the elastic force that arises as a result of the deformation of the body and is directed to the support or suspension from the side of this body. The modulus of body weight depends on the projection of the body acceleration on the vertical axis.

9. When do friction forces occur?

Friction forces arise when two bodies in contact interact.

Reduce friction by lubricating or polishing between rubbing engine parts; increase the friction force with the help of branches and sand when the car slips.

The modulus of the friction force depends on the modulus of the reaction force of the support and on the coefficient of friction between these surfaces.

Maybe rolling friction.

4) Students solve problems with the help of experimental equipment.

1.How to measure gravity?

2. How to measure the spring force?

3. How to determine the elasticity coefficient of a rubber cord?

4.How to measure the force of sliding friction?

5) Students explain the solution of pre-prepared problems.

In a vessel with water there are two bars of the same mass - wooden and copper. Which of the bars is affected by the greatest force of gravity?

Does a weight hanging from a string have weight? What will be the weight of the kettlebell if the string is cut?

Does gravity act on a swift flying in the air?

Will the skydiver be in a state of weightlessness during the jump?

Explain why the use of springs reduces the shaking of the car?

Why are notches made on the jaws of a vise and pliers?

Why are embossed patterns made on car tires?

What force imparts acceleration to a car or locomotive?

6) To check the assimilation of the material, independent work is carried out in the form of testing in three versions for 15 minutes (see the tasks in Appendix 3)

7) At this stage, the floor is presented to the expert committee. The guys evaluate the answers of other students, make comments, give recommendations for the future. In the final speech, the teacher indicates whether the objectives of the lesson have been achieved, emphasizes that the systematization of knowledge on this topic is necessary for repetition.

8) Homework is written on the blackboard and read out by the teacher.

Textbook "Physics 10" p. 32,33,35,37,38,39; table.

Literature.

1. Zhdanov L.S., Zhdanov G.L. Physics. Textbook for secondary specialized educational institutions.

1. Martynova N.K. Physics. The book for the teacher.

1. Myakishev G.Ya., Bukhovtsev B.B., Sotsky N.N. Physics grade 10.

1. Razumovsky V.G. Physics lesson in modern school.

1. Collection of questions and problems in physics. Ed. Stepanova T.A.

Lesson conclusions.

The lesson-seminar was held in group 1 stand, the lesson was attended by 24 students, three were absent due to illness. The educational goal of the lesson: to concretize students' ideas about forces in mechanics, to systematize students' knowledge on this topic was achieved. Almost all students (about 83%) actively worked at the lesson, the guys prepared posters on all mechanical forces, prepared experiments during consultations, worked with additional literature. Independent work in the lesson was approximately 18% of the time, which is slightly more than planned, the quality of knowledge during the test was 62.5%, academic performance was 96%. During the answers, the students simultaneously filled out a table on the systematization of knowledge on this topic, 17 people completed the task, 3 students did not complete it, they were given additional homework on the table. The solution of qualitative problems made it possible to reveal the connection between experience and theory, experimental tasks allow the formation of experimental skills.

The disadvantage of the lesson is that a small amount of equipment in the "Mechanics" section does not allow frontal laboratory work, which would be much more effective in such lessons.

Questions to prepare for the seminar.

"Forces in mechanics: universal gravitation,

elasticity, friction

1. Define strength.
2. Describe the force of gravity.
3. Give the definition of the gravitational constant, by whom and when was it first measured?
4. Define gravity. What is the free fall acceleration?
5. How to measure gravity?
6. Under what conditions do elastic forces appear?
7. Formulate Hooke's law, under what conditions is it fulfilled?
8. What is called the state of weightlessness?
9. How to measure the elastic force of a spring?

1. When do friction forces occur?
2. How can the friction force be reduced or increased and in what situations?
3. What determines the modulus and direction of the friction force?
4. Can the force of friction increase the speed of a body?
5. How to measure the force of sliding friction?

Additional questions.

1. Will the skydiver be in a state of weightlessness during the jump?

2. Why do drops of rain fall on the ground, grains of snow?

3. In a vessel with water there are two bars of the same mass - wooden and copper. Which of the bars is affected by the greatest force of gravity?

4. Does a weight hanging on a thread have weight? What will be the weight of the kettlebell if the string is cut?

5. Does gravity act on a swift flying in the air?

6. Explain why the use of springs reduces vehicle shaking.

7. Why are notches made on the jaws of a vise and pliers?

8. Why is a relief pattern made on car tires?

9. What force imparts acceleration to a car or locomotive?

Table for systematizing knowledge on the topic "Forces in mechanics"

The name of the force	Definition	Formula	Under what conditions does	The image of the force in the figure
The force of gravity
Gravity
Elastic force
Support reaction force
Thread tension force
Body weight
sliding friction force
Rolling friction force

"Forces in mechanics" Option 1.

1 . A car is moving along a straight stretch of highway with constant acceleration. What conclusion can be drawn about the resultant F of all forces acting on the car?

A. F=0, directed upwards. B. F=0, directed downwards. B. F=0. G. F=0, directed horizontally. D. None of the answers are correct.

2 . The elastic force arising from the deformation of the spring, 20N. Spring stiffness 200N/m.

What is its elongation?

A. 0.1m. B. 0.2m C. 0.3m. G. 0.5m. D. None of the answers are correct.

3. The student measures the strength of his hand with a spring force meter. In this case, the connection of force with ... a) acceleration of bodies is used; b) the magnitude of the deformation of the bodies;

1. A; 2. B; 3. A and B; 4. Neither A nor B.

1. What determines the modulus of the force of universal gravitation?

A) on the sum of the masses of both bodies; b) on the distance between the bodies; c) from the product of the masses of both bodies; d) from the environment; e) on the sizes of both bodies.

1. What force causes the formation of rockfalls in the mountains?

6. There is a stack of 10 identical books on the table. What is the ratio between the modules of the force F1, which must be applied to move the top five books, and the force F2, which must be applied to pull the fifth book from the top out of the pile?

1) F1 > F2 ; 2) F1=F2; 3) F1>F2.

7. Sketch a body on an inclined plane. Mark in this figure the friction force and the reaction force of the support acting on this body.

"Forces in mechanics" Option 2.

1. When all the forces acting on the car are compensated, its speed remains unchanged. What is the name of this phenomenon?

A. Gravity. B. Inertia. B. Weightlessness. D. Friction. D. None of the answers are correct.

2 . The elastic force arising from the deformation of the spring, 30N. Determine the spring constant if the elongation is 0.2 m.

A. 150 N/m; B. 300 N/m; V. 100 N/m; D. 200 N/m. D. None of the answers are correct.

3. A person measures body weight with the help of spring scales (steelyard). In this case, the connection of body mass with .. ... a) acceleration of bodies is used; b) the magnitude of the deformation of the bodies;

1. A; 2. A and B; 3. B; 4. Neither A nor B.

4. What does the modulus of friction force depend on?

A) from the environment; b) from body weight; c) on the coefficient of friction; d) from body deformation; e) on the size of the body.

1. What force keeps a tourist on a steep mountain road?

A) the force of friction b) the force of gravity; c) the force of universal gravitation; d) elastic force.

1. The moon and earth interact by gravitational forces. What is the ratio between the force modules F1 of the Earth's action on the Moon and F2 of the Moon's action on the Earth?

1) F1=F2; 2) F1 >F2 ; 3) F1>F2.

1. Draw a diagram of a ball hanging from a thread. Note in this figure the tension of the thread and the gravity of the ball.

"Forces in mechanics" Option 3

1 . What physical law states that the action of one body on another is mutual?

A. In Newton's first law. B. In Newton's second law. B. In Newton's third law. D. In the law of conservation and transformation of energy. D. None of the answers are correct.

2 . Determine the elastic force that occurs when the spring is deformed if the spring stiffness is 40 N/m and its elongation is 5 cm?

A. 1H. B. 2 N. C. 3 N. D. 5 N. D. None of the answers are correct.

3. On a dirt road washed out by rain, a loaded car slips less than an unloaded one. At the same time, we see the connection between the friction force and .... a) speed, b) body mass;

1) A; 2) B; 3) A and B; 4) Neither A nor B.

4. What does the modulus of elasticity depend on?

A) from the environment; b) from body weight; c) on the stiffness coefficient; d) from body deformation; e) on the size of the body.

5. What force makes the Earth and other planets move around the Sun?

A) friction force; b) gravity; c) the force of universal gravitation; d) elastic force.

6. On the table in the train car are a box of chocolates and an apple. Why, at the beginning of the movement, the apple rolled back (relative to the car), but the box of sweets remained in place?

A) the box is heavy and the apple is light; b) sliding friction is less than rolling friction;

c) sliding friction is greater than rolling friction;

D) the box has a large contact area, while the apple has a small one.

7. Sketch the Earth and the Moon. Mark in this picture the forces of universal gravitation that act between these bodies.

Key to the test.

No. p / p	Option 1	Option 2	Option 3
	B, c	B, c	B, g