Presentation on the topic mechanical energy. Presentation on "mechanical work and energy". Potential energy of a body raised above the Earth
LESSON TOPIC: ???
Let's solve the crossword puzzle
2? The reason for the change in body speed?
3? Product of the "reason" for change
the speed per distance traveled is called...?
4? The ability of a body to do work is called...?
MECHANICAL ENERGY
Lesson type. Learning new material.
Lesson objectives: To introduce the concept of energy as the body’s ability to do work; Define potential and kinetic energy.
- Updating previously acquired knowledge. Formation of new concepts. Application of new knowledge to solving practical problems.
Metasubject
- Personal: accept and maintain the learning goal and task.
- Regulatory: ability to set new educational goals and objectives
- Cognitive: formation of ideas about energy, kinetic and potential energies.
- Communicative: the ability to argue your point of view, skills to work in a group: the ability to listen to your interlocutor, discuss issues that have arisen..
- Basic concepts: Energy; kinetic energy; potential energy of a body raised above the Earth; potential energy of an elastically deformed body.
Energy is the work that a body can do when transitioning from a given state to zero.
The term “energy” was introduced into physics by the English scientist T. Young in 1807.
Translated from Greek, the word “energy” means action, activity.
Since mechanics studies the movement of bodies and their interaction, then
POTENTIAL
KINETIC
motion energy
interaction energy
Kinetic energy
Let us determine the kinetic energy of a body moving with speed v
energy is the work that needs to be done to transfer a body from the zero state (υ 0 =0) to the given one (υ ≠0).
Let's transform this expression:
According to Newton's Law
Path with uniformly accelerated motion:
Potential energy
Let us determine the potential energy of interaction of the body with the Earth at a height h.
Energy is the work that needs to be done to transfer a body from the zero state (h 0 = 0) to the given one (h).
Energy is the work that needs to be done to transfer a body from the zero state (h 0 = 0) to the given one (h).
Let us determine the work done by force F:
Derive the formula yourself
Let's check:
potential energy:
We got acquainted with two types of mechanical energy
KINETIC
POTENTIAL
motion energy
interaction energy
However, in the general case, a body can have both kinetic and potential energy at the same time.
called
Total mechanical energy
This concept was introduced in 1847 by the German scientist G. Helmholtz.
Study of free falling bodies
(in the absence of friction and resistance forces) shows that any decrease in one type of energy leads to an increase in another type of energy.
LAW OF CONSERVATION MECHANICAL ENERGY
Let us denote the initial energy of the body
And the final
Then the law of conservation of energy can be written as
Suppose that at the beginning of the movement the speed of the body was equal to υ 0, and the height was h 0, then:
And at the end of the movement, the speed of the body became equal to υ, and the height h, then:
The total mechanical energy of a body, which is not affected by friction and resistance forces, remains unchanged during movement.
example
A stone weighing 2 kg flies at a speed of 10 m/s. What is the kinetic energy of the stone?
Kinetic energy of stone
Answer: 100 J.
A brick weighing 4 kg lies at a height of 5 m from the surface of the earth. What is the potential energy of the brick?
Potential energy of a brick
Let's substitute the numerical values of the quantities and calculate:
Answer: 200 J.
Which of these moving bodies has more kinetic energy?
At the plane
In which places of the river - at the source or at the mouth - does each cubic meter of water have more potential energy?
Justify your answer.
Waterfall in the tropics
Which of these two planes has more potential energy?
At the top
Test
1. The energy that a body possesses as a result of its movement is called... energy.
- potential
- kinetic
- Don't know
1) potential
2) kinetic
3) I don't know
- Raise the helicopter higher;
- Lower the helicopter below;
- Land the helicopter on the ground.
- Only kinetic;
- Only potential;
- No;
- Don't know.
Checking the test.
1 . The energy that a body possesses as a result of its movement is called... energy.
- potential
- kinetic
- Don't know
2. The energy of a compressed spring is an example of... energy.
1) potential
2) kinetic
3) I don't know
3. Two balls of the same size, wooden and lead, had the same speed at the moment they fell to the ground." Did they have the same kinetic energy?
1) The lead ball had more energy.
2) The wooden sha had more energy
3) Identical, since their speeds and sizes are the same
- Lower the helicopter below;
- Raise the helicopter higher;
- Increase the speed of the helicopter;
- Reduce helicopter speed;
- Land the helicopter on the ground.
- Only kinetic;
- Only potential;
- Potential and kinetic;
- No;
- Don't know.
The robbers took the victim's money and documents, stripped him naked and, deciding that there was nothing more to take from him, they threw him from the bridge into the river. What did the victim still have halfway to the cold water?
Answer: potential energy, gradually turning into kinetic energy.
Homework:
- Read § 14.15
- Learn basic concepts, formulas, definitions.
- Prepare a short summary
§ 16 for level I,
abstract presentation on the topic
Presentation on the topic "Energy. Kinetic and potential energy. Derivation of the law of conservation of mechanical energy"
Download:
Preview:
To use presentation previews, create a Google account and log in to it: https://accounts.google.com
Slide captions:
Energy. Kinetic and potential energy. Derivation of the law of conservation of mechanical energy
A ball weighing 100 g, flying at a speed of 1.5 m/s, is caught in mid-flight. What is the average force with which the ball acts on the hand if its speed decreases to zero in 0.03 s.
A load weighing 80 kg fell from a boat weighing 240 kg, moving without a rower at a speed of 1 m/s. What was the speed of the boat?
In water, a stone with a volume of 0.6 m 3 is raised to the surface from a depth of 5 m. The density of the stone is 2500 kg/m3. find a job lifting stones.
If a body or system of bodies can do work, then they say that they have energy.
ENERGY IS DESIGNATED: E ENERGY IS MEASURED: J
Mechanical energy is a physical quantity that characterizes the ability of a body to do work. Mechanical energy Kinetic (capable of moving) Potential (force)
Kinetic energy is the energy of a moving body.
Potential energy is the energy of interaction.
Potential energy of elastic deformation.
Law of energy conservation. In a closed system in which conservative forces operate, energy does not appear from anywhere and does not disappear anywhere, but only passes from one type to another.
h E p= max E k=0 Ep=0 Ek= max Ep=Ek Ep Ek
A=-(E p -E p 0) (1) A=-(E to -E to 0) (2) E to 0 + E p 0 = E to + E p E=E to + E p – full mechanical energy
Helmholtz Hermann Ludwig Ferdinand (1821-1824)
In physics, conservative forces (potential forces) are forces whose work does not depend on the shape of the trajectory (depends only on the starting and ending points of application of the forces). This leads to the following definition: conservative forces are those forces whose work along any closed trajectory is equal to 0.
Types of impacts Absolutely elastic impact Absolutely inelastic impact Elastic impact Inelastic impact
Mechanical energy is not converted into internal energy. All mechanical energy is converted into internal energy. A small part of the mechanical energy is converted into internal energy. Almost all mechanical energy is converted into internal energy.
Problem No. 1. With what initial speed must a ball be thrown down from a height h so that it jumps to a height 2h? Consider the impact to be absolutely elastic. Given: h Find: Solution: h 2h Epo + Eko Ep Ek
Epo + Eko Ek Ep
Task No. 2. A sled with a rider with a total mass of 100 kg slides down a mountain 8 m high and 100 m long. What is the average force of resistance to movement if at the end of the mountain the sled reached a speed of 10 m/s, the initial speed is 0. h L Epo Ek
Given: m=100 kg h=8 m L=100 m Find: Fc- ? Solution: Epo Ek+Ac
What is ENERGY? In our lives, we often come across the concept of energy. Cars and airplanes, diesel locomotives and ships operate by consuming the energy of burning fuel. People, in order to live and work, replenish their energy reserves with food... So what is energy?
For example: A body raised relative to the surface of the Earth has potential energy, because energy depends on the relative position of this body and the Earth and their mutual attraction. The water that is raised by the dam of the power plant, falling down, drives the turbines of the power plant. When a spring is stretched or compressed, work is done. In this case, the individual parts of the spring change position relative to each other.
Qualitative tasks. 1. Which of the two bodies has greater potential energy: a brick lying on the surface of the earth, or a brick located in the wall of a house at the level of the second floor? 2.Which of the two bodies has greater potential energy - a steel ball or a lead ball of the same size, lying on the fifth floor balcony? 3.Under what condition will two bodies raised to different heights have the same potential energy? 4.At track and field competitions, athletes put the shot put. Men - a core weighing 7 kg, women - a core weighing 4 kg. Which nucleus has more kinetic energy at the same flight speed? 5. Which of the two bodies has greater kinetic energy: the one moving at a speed of 10 m/s, or the one moving at a speed of 20 m/s? 6.What is the physical meaning of the Finnish proverb “What you spend going uphill, you get back on the way down”? To contents
Challenges for ingenuity. 1. Two identical barrels were loaded onto a car. One barrel was loaded using an inclined plane, and the second was raised vertically. Are the potential energies of the barrels on the car equal? 2.When does a car consume more fuel: when driving evenly or when driving in stops and starts? 3.Can potential energy be negative? Give examples. To contents
Test. 1.Which of the following is a unit of kinetic energy? A) N B) J B) Pa D) W 2. What mechanical energy does an extended or compressed spring have? A) Kinetic B) Potential C) Does not have mechanical energy 3. Energy, which is determined by the position of interacting bodies or parts of the same body, is called... A) potential energy. B) kinetic energy. 4.The notebook is on the table. What mechanical energy does it have relative to the floor? A) Kinetic B) Potential C) Does not have mechanical energy 5. What does the kinetic energy of a body depend on? A) On the mass and speed of the body. B) From the speed of the body. B) From the height above the Earth’s surface and body weight. 6. The energy that a body possesses due to its motion is called... A) potential energy. B) kinetic energy. 7.What does the potential energy of a body raised above the ground depend on? A) On the mass and speed of the body. B) From the speed of the body. B) From the height above the Earth’s surface and body weight. 8. What mechanical energy does a car moving along the road have? A) Kinetic B) Potential C) Does not have mechanical energy To the table of contents
Mechanical work and energy:
- KINETIC ENERGY
- AND MECHANICAL WORK
- WORK OF GRAVITY AND POTENTIAL ENERGY
- LAW OF CONSERVATION OF MECHANICAL ENERGY
- Let's begin the path to another conservation law.
- It is necessary to introduce several new concepts so that they do not seem to you to have fallen “from the ceiling,” but reflect the living thoughts of people who first pointed out the usefulness and meaning of new concepts.
- Let's begin.
- Let's solve the problem using Newton's laws: a body of mass m moves with acceleration under the influence of the three forces indicated in the figure. Determine the speed at the end of the path S.
- F1 + F2 + F3 = m×a,
- in projection onto the OX axis:
- F1cos - F3 = m×a
- F1cos - F3 = m × (υ²–υо²)
- F1S cos - F3S = mυ² –mυо²
- F1S cos - F3S = mυ²– mυо²
- Let's discuss the result obtained.
- The influence of all forces on the change in ΔΕк can be described in a unified way by introducing the value A=Fs cosα, called mechanical work:
- A1= F1S cos,
- A2= F2S cos 90°=0,
- A3 = F3S cos180°=F3S,
- and together A1 + A2 + A3= Ek Eko
- or: the change in the kinetic energy of a body is equal to the work of forces acting on the body.
- The resulting expression is the theorem on kinetic energy: ΣA=ΔΕk.
- =1J
- [A]=1J
- Please note that Ek and A are scalar quantities!
- Let's consolidate information about new concepts.
- Which body has more kinetic energy: a calmly walking person or a flying bullet?
- The speed of the car doubled (tripled). How many times did its kinetic energy change?
- During which of the following movements does the kinetic energy of bodies change: RPD, RUD, RDO?
- Express the kinetic energy in terms of the modulus of momentum of the body and the modulus of momentum in terms of kinetic energy.
- 3) Threshold υ=υ0+at υ
- (velocity module increases), m = const
- .
- Body impulse module:
- Kinetic energy:
- Work is a scalar quantity, expressed as a number. A 0, if 0≤90°; A0, if 90° ≤ 180°.
- If a force acts on a body at an angle of 90° to the direction of instantaneous velocity, say, the force of gravity when a satellite moves in a circular orbit or the elastic force when the body rotates on a thread. A=Fs cos90 °=0.
- According to the theorem 0 = Ek – Eko Ek = Eko force does not change the speed!!!
- Let's also remember about momentum: are there any bodies in the picture that have the same momentum?
- The numbers in the circles indicate the masses of the bodies, the numbers next to the vector indicate the velocities of the bodies. All quantities (mass and velocity) are expressed in SI units.
- IMPULSE - VECTOR!
- Indicate with an arrow the direction of speed such that:
- A1 0, A2 0, A3 0;
- A1 0, A2 0, A3 =0;
- A1 0, A2 0, A3 =0;
- A1 0, A2 0, A3 0.
- Is it possible to have such a combination of work signs for which it is generally impossible to select the direction of velocity?
- In which of the following cases is the work of the resultant positive, negative, or zero:
- The bus departs from the stop, moves uniformly and in a straight line, turns at a constant absolute speed, and approaches the stop;
- You are going down a hill; do you ride on a carousel or on a swing?
- The concept of kinetic energy was first introduced by the Dutch physicist and mathematician Christiaan Huygens, whom I. Newton himself called great. Studying the collisions of elastic balls, Huygens came to the conclusion: “When two bodies collide, the sum of the products of their magnitudes and the squares of their velocities remains unchanged before and after the impact” (“magnitudes” - read “mass”). From a modern point of view, Huygens' discovery is nothing more than a special case of the manifestation of the law of conservation of energy. Huygens, a handsome man from an old family in which “talents, nobility and wealth were hereditary,” not only first defined kinetic energy, but also pointed out the vector nature of the impulse. He invented pendulum clocks and performed a number of brilliant works in mathematics and astronomy. “A finely disciplined genius...respecting his abilities and striving to use them to the fullest.”
- In everyday life, we constantly have the need to change the direction and speed of various bodies (movement of fingers, eyelids, etc.). To change the speed module, it is necessary to perform mechanical work: A=ΔΕk. This work is done by your muscles.
- Let's consider the most common phenomenon - climbing stairs. You stand on a step, put your foot on the next one, strain your muscles, a support reaction occurs, compensating for the force, the force does positive work A0, the speed of your body increases: ΔΕk 0, you rise one step. At the same time, gravity does negative work, since =180°. The work done by the muscle tension force must be at least slightly greater than the work done by gravity (in absolute value), otherwise it will not be possible to increase Εk.
- AA, otherwise it will not be possible to increase the kinetic energy Ek = A + A, (A 0). Since the movement of the body under the influence of these forces is the same, it is clear that , and