# Vật Lí 8 Bài 16: Cơ năng – Giải bài tập SGK Vật Lí 8 Bài 16

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Physics 8 Lesson 16: Ability compiled by Le Hong Phong High School hopes to be a useful document to help students master the lesson knowledge and achieve good results in exams and tests in class.

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## Summary of Physics Theory 8 Lesson 16

### Function

The greater the ability to do work, the greater the mechanical energy of the object.

The unit of mechanical energy is the joule (J).

Example: A stone placed on a glass, it is not able to do work on the glass. But if you put it to a height h above the glass, when it falls, it can break the glass, meaning it has the ability to do work. So when the stone is raised to a height h, the stone has some potential energy.

Note: 1 kJ = 1000 J

### Potential energy

a) Gravitational potential

– The mechanical energy of an object depends on the position of the object relative to the ground or compared to another location chosen as a landmark to calculate the height called gravitational potential energy.

– The higher the mass and the higher the object, the greater the gravitational potential energy.

An object will have different gravitational potential energy if a different height datum is chosen

For example:

– If we choose the landmark to calculate the altitude as the ground, then the height h is the distance from the ground to the box).

– If we choose the landmark to calculate the height as the 3rd step, then we have the height h’ which is the distance from the 3rd ladder to the box).

Seeing that h > h’, the gravitational potential energy of the cartridge when falling from a height h will be greater than the gravitational potential energy of the cartridge when falling from a height h’.

Note: When the object is on the ground and the ground is chosen as a landmark, the gravitational potential energy of the object is zero.

b) Elastic potential energy

The mechanical energy of an object that depends on the deformation of the object is called elastic potential energy.

Example: When we pull the chord, we give the arc an elastic potential energy. When letting go, the bowstring does the work of making the arrow fly away. ### Kinetic energy

The mechanical energy of an object due to its motion is called kinetic energy.

– The more massive an object is and the faster it is moving, the more kinetic energy it has.

If the object is at rest, the kinetic energy of the object is zero.

Example: The Space Shuttle is being launched into orbit. The ship has a very large mass, when launched at high speed, its kinetic energy is also very large. ### Magnitude of mechanical energy

Potential energy and kinetic energy are two forms of mechanical energy.

The mechanical energy of an object is equal to the sum of its kinetic and potential energy.

### Solution method

Identify objects with potential energy

To know whether an object has potential energy or not, we must consider:

– Is the location of that object high above the ground or another landmark? If so, then the object has gravitational potential energy.

– Is the object elastic and deformed? If so, the object has elastic potential energy.

Identify objects with kinetic energy

To know whether an object has kinetic energy or not, we must see if the object is moving relative to the reference object. If so, the object has kinetic energy.

Identify objects with potential

If an object has only potential energy or only kinetic energy or both kinetic and potential energy, the object has mechanical energy.

Compare the gravitational potential energy of two objects

– Two objects with the same mass, the one at the higher altitude has the greater gravitational potential energy.

– When two objects are at the same height, the object with the greater mass has the greater gravitational potential energy.

Compare the kinetic energy of two objects

You are watching: Physics 8 Lesson 16: Mechanical Energy – Exercises in Physics Textbook 8 Lesson 16

– Two objects have the same mass, the object with the greater velocity has the greater kinetic energy.

– Two objects have the same non-zero velocities, the object with the greater mass has the greater kinetic energy.

## Solve exercises in Physics textbook 8 Lesson 16

### Lesson C1 (page 55 of Physics Textbook 8)

If the weight is raised to a certain height (H.16.1b), does it have mechanical energy? Why? Have. If the weight is raised to a certain height and then released, object A will move downwards, causing the string to stretch. The tension in the string makes object B move, so object A has done work, so it has mechanical energy.

### Lesson C2 (page 56 Physics Textbook 8)

There is a spring made of steel bent into a circle (H.16.2a). The spring is compressed by tying a rope, on top of which is a piece of wood (H.16.2b). Now the spring has a potential. How do we know if the spring has potential energy? To know if the spring has potential, we just need to cut or burn the wire and observe that the spring opens and the piece of wood above the spring is pushed up, so the spring has done work, which means it has a chance. power.

### Lesson C3 (page 56 of Physics Textbook 8)

Roll down the steel ball A from position 1 on the inclined chute and hit the piece of wood B (H.16.3). How will the phenomenon happen? Ball A rolls from position (1) on an inclined chute and hits wood B, it will collide with wood B and cause wood B to move.

### Lesson C4 (page 56 of Physics Textbook 8)

Prove that a moving sphere A is capable of doing work.

The ball hits the piece of wood, causing the piece of wood to move, so that ball A is able to do work.

### Lesson C5 (page 56 Physics Textbook 8)

From the results of the above experiment, find suitable words to fill in the blanks of the following concluding sentences:

A moving object has the ability……that is, has potential energy.

A moving object has the ability to do work, that is, it has potential energy.

### Lesson C6 (page 57 of Physics Textbook 8)

How does the magnitude of the velocity of the ball change compared to experiment 1? Compare the work done by ball A now with before. From this, how does the kinetic energy of sphere A depend on its velocity?

– The magnitude of the ball’s velocity increases compared to its velocity in experiment 1.

– The work done by sphere A is greater than before.

Thus, as speed increases, kinetic energy increases. Exact experiments show that kinetic energy increases proportionally to the square of velocity.

### Lesson C7 (page 57 of Physics Textbook 8)

What happens when the phenomenon is different from experiment 2? Compare the work done by two spheres A and A’. From this, how does the ball’s kinetic energy depend on its mass?

– When a ball A’ is replaced with a larger mass, the piece of wood B is pushed further away upon impact.

– The work done by sphere A’ is greater than the work done by sphere A.

The kinetic energy of an object depends on the mass of the object. The greater the mass of an object, the greater the kinetic energy of the object.

Thus, the kinetic energy of an object is directly proportional to the mass of the object.

### Lesson C8 (page 57 of Physics Textbook 8)

The above experiments show that kinetic energy depends on what factors and how?

The above experiments show that kinetic energy depends on two factors: the mass of the object and the speed of the object:

– When the mass of the object remains the same, if the velocity increases, the kinetic energy also increases (kinetic energy is proportional to the square of the velocity).

When velocity is constant, kinetic energy is proportional to mass.

### Lesson C9 (page 57 of Physics Textbook 8)

Give an example of an object that has both kinetic and potential energy.

An airplane is flying high, the plane has altitude so it has potential energy, and at the same time it has speed so it also has kinetic energy.

### Lesson C10 (page 57 of Physics Textbook 8)

What kind of mechanical energy does the mechanical energy of each object in Figure 16.4a, b, c belong to? The bow is bent: Elastic potential energy.

Water flowing from above: Kinetic and potential energy.

Water trapped on a high dam: Gravitational potential energy.

## Physics Quiz 8 Lesson 16 (with answers)

Lesson 1: An object has potential when:

A. Objects have the ability to do work.

B. Massive object.

C. Objects have great inertia.

D. The object is at rest.

When an object has the ability to do work, we say the object has potential energy

Lesson 2: On what factors does gravitational potential energy depend? Choose the most complete answer.

A. Volume.

B. Specific gravity.

C. The mass and position of the object relative to the ground.

D. The mass and velocity of the object.

– The mechanical energy of an object depends on the position of the object relative to the ground or compared to another location chosen as a landmark to calculate the height called gravitational potential energy.

– The higher the mass and the higher the object, the greater the gravitational potential energy.

⇒ Gravitational potential energy depends on the object’s position relative to the ground and its mass.

Lesson 3: Elastic potential energy depends on what factors?

A. Volume.

B. Deformation of elastic body.

C. Mass and substance.

D. Velocity of the object.

The mechanical energy of an object that depends on the deformation of the object is called elastic potential energy.

Exercise 4: If the ground is chosen as a reference point to calculate potential energy, which of the following objects does not have potential energy?

A. The bullet is flying.

B. The spring is left at a natural height above the ground.

C. The marble is rolling on the ground.

D. The spring is forced to be placed directly on the ground. The ball is rolling on the ground Has no potential energy and has kinetic energy

The spring is forced to rest on the ground ⇒ Has elastic potential energy

Lesson 5: Which of the following objects has no potential energy (relative to the ground)?

A. The table is stationary on the floor.

B. The leaf is falling.

C. A person standing on the third floor of the building.

D. The ball is flying overhead.

The table has no potential energy because it is at rest on the ground

Lesson 6: On what factors does the kinetic energy of an object depend?

A. Volume.

B. Velocity of the object.

C. Mass and substance.

D. The mass and velocity of the object.

The mechanical energy of an object due to motion is called kinetic energy. The more massive an object is and the faster it moves, the more kinetic energy it has

Lesson 7: Which of the following objects has no kinetic energy?

A. The marble rests on the floor.

B. The marble rolls on the floor.

C. The plane is flying.

D. The bullet is flying.

The ball is at rest on the floor and does not move, so it has no kinetic energy

Lesson 8: Which of the following is true about mechanical energy? Choose the most correct sentence.

A. Mechanical energy that depends on the deformation of an object is called elastic potential energy.

B. The mechanical energy that depends on the position of the object relative to the ground is called gravitational potential energy.

C. The mechanical energy of an object due to motion is called kinetic energy.

D. Both A, B and C.

Mechanical energy that depends on the deformation of an object is called elastic potential energy.

Mechanical energy that depends on the position of an object relative to the ground is called gravitational potential energy.

The mechanical energy of an object due to its motion is called kinetic energy.

Lesson 9: A spring made of steel is being compressed. Now the spring has a potential. Why do springs have potential?

A. Because springs have many turns.

B. Because springs are capable of doing work.

C. Because springs have mass.

D. Because springs are made of steel.

A spring made of steel that is being compressed has energy because the spring is capable of doing work

Lesson 10: In the following cases, which of the objects has both kinetic and potential energy? Choose a landmark at the earth’s surface.

A. An airplane is moving on the runway of an airport.

B. A car is parked in the parking lot.

C. An airplane is flying overhead.

D. A car is moving on the road.

– An airplane is moving on the runway of the airport, a car is moving on the road ⇒ has kinetic energy.

– A car parked in a station ⇒ has no kinetic energy because the car is parked.

– An airplane flying high has both kinetic and potential energy.

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Above is the content of Physics lesson 8 Lesson 16: Mechanical energy compiled by Le Hong Phong High School, including theory, exercise solutions and multiple-choice questions with full answers. Hope you will master the knowledge of Mechanical. Wish you all the best in your studies and always get high scores in class tests.

Compiled by: Le Hong Phong High School

Category: Physics 8

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