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Mechanical energy is one of the most mentioned concepts in Physics program 10. So what is the function? How is the law of conservation of mechanical energy? Mầm non Cát Linh will summarize all the necessary knowledge in the following article.

What is mechanical energy?

What is mechanical energy? Mechanical energy is a physical quantity that shows the ability to produce any object. Specific understanding, an object with mechanical energy when it has the ability to perform mechanical efforts. In physics, the energy is equal to the total potential and kinetic energy. The greater the performance, the greater the function of the object.

• The symbol of the function is W.

• The unit is Jun (J).

For example: Place a brick on a glass panel. Initially, the brick was unable to perform the glass on the glass. However, when bringing it to a height H compared to the glass and dropped, the brick can cause the glass to break. At that time, we said the brick was able to produce. Therefore, when bringing bricks to height H, the brick has mechanical energy.

How many types of mechanics have?

The function consists of two main forms that are energy and kinetic energy.

About potential

The function of an object placed at a certain height is called potential. Attractive potential is the function of an object at an H height compared to the ground or compared to another position chosen as a mold. When the object lies on the ground, the distance from the object to the ground is zero, so the attractive potential is 0. The higher the object and the larger the volume, the greater the attractive potential.

Besides, the elastic potential is also defined as a form of function of the object, it depends on the deformation of the spring. To determine whether an object is elastic, we will consider whether it is deformed or elastic.

For example, when pulling the bowstring, we have changed the shape of the bowstring, which means that we have provided the bow with an elastic potential.

About kinetic energy

The function of the object is made up of motion called kinetic energy. The faster the moving object and the greater the volume, the greater the kinetic energy. If the object stands still, the kinetic energy is 0.

The transformation between kinetic energy and potential

The kinetic energy and potential are the two basic types of energy of the object, and they can transform each other. Specifically:

• The kinetic energy can be transformed into potential. For example, when a ball is thrown, the kinetic energy of the ball will decrease and convert into a gravity.
• The potential can convert into kinetic energy. For example, when a ball falls from above, the gravity of the ball will decrease and convert into kinetic energy.

The metabolism between kinetic energy and potential occurs continuously in the movements of the object. For example, when a pendulum fluctuates, the kinetic energy and potential of the pendulum transforms each other.

In fact, due to always friction and other obstacles, part of the energy will be lost in the form of heat. However, in mechanical problems, we often ignore obstacles to simplify the problem.

The law of preserving mechanical energy

The law of conservation of mechanical energy: The potential and kinetic energy of an object can change back and forth when the object moves inside the gravity. However, because the function is equal to the total potential and kinetic energy, this total is always constant.

We have mechanical calculation formula:

W = WĐ + WT = const

Consequences:

• When an object moves in gravity, the kinetic energy of the object will increase if the potential of the object decreases and vice versa.
• At the position where the potential is in the minimum, the kinetic energy is at the maximum and vice versa when the function is at the maximum, the kinetic energy will be at the minimum.

Note: The law of conservation of mechanical energy (or mechanical conservation) is only applied when the object moves in the gravity and is not under the effect of friction but only the effect of gravity or elastic force.

The function of the object moves in gravity

What is the energy of the moving object in the gravity? How to calculate? Let’s find out in the following section:

Define

When an object moves in gravity, the total potential and kinetic energy of the object is called mechanical function.

The formula for calculating the function of the object moves in the gravity

Recipe:

The preservation of the energy of the object moves in the gravity

When an object of gravity moves and only bears the effect of gravity (ignoring friction, resistance, …), the function of the object is constant (a preserving quantity).

From there, we have:

The function of the object is under the effect of elastic force

The conservation of the function of the object is under the effect of elastic force

When only the effect of elastic force is caused by the deformation of an elastic springs acting on the object, in the movement process, the function is determined by the total potential and elastic kinetic energy of the object is a preserving quantity.

Note: The law of preserving the function is only true when the object is under the effect of two forces is gravity and elastic force and not adding the impact of any other force such as resistance, friction force, …

Formula for calculating the function of the object is subject to elastic force

Answering questions about potential and kinetic energy

1. When does the function be equal to the function?

When is the energy function? The energy is equal to the function when the object is only energy and no kinetic energy. Because, the function of an object is the sum of the potential and kinetic energy (W = WT + WĐ), so when the kinetic energy of the object is zero, the energy of the object is equal to energy.

For example, a ball is hung at a height H compared to the ground. When the ball is released, the energy of the ball will decrease and convert into kinetic energy. When the ball touches the ground, the energy of the ball is 0 and the kinetic energy of the ball reaches the maximum value.

2. Speaking about the law of preserving kinetic energy:

In a closed system, if there is no friction force or other obstacles acting on objects in the system, the kinetic energy of the system will be preserved. However, in reality, due to always friction and other obstacles, the kinetic energy of the system is not fully preserved.

See also: What is kinetic energy? Dynamic expressions & exercise exercises (Physics 10)

Functional exercises (Physics 10)

Here are some exercises on the mechanical theme, with a detailed solution so that they can check and compare the results after completion.

Question 1: A small object is thrown from a point M above the ground; The object is up to the point N then stops and falls. Ignore the resistance of the air. During MN

A. Increased kinetic energy

B. Power reduction

C. Maximum mechanical function at n

D. The function does not change

Answer: Choose D.

Explanation: Because of ignoring the resistance of the air, in the process of constant mechanical function.

Question 2: From point M (with a height of the ground equal to 0.8m) throw a object with a speed of 2m/s. Know the weight of the object is 0.5kg, take g = 10m/s^2. What is the function of the object?

A. 4 j

B. 1 j

C. 5 j

D. 8 j

Answer: Select C

Explain:

Select the potential landmark at the ground. At the throw point we have:

So the function of the object is equal to 5J

Question 3: The function of an object with a mass of 2kg falls from a height of 5m to the ground is:

A. 10 j

B. 100 j

C. 5 j

D. 50 j

Answer: Select B

Explain:

Because the function of the system is preserved, the function is equal to the initial potential, or:

W = W (T = 0) = Wđ + WT = MGH = 100 (J).

Question 4: An object is thrown upright at a speed of 6 m/s. Take g = 10 m/s^2. Calculate its maximum height.

A. H = 1.8 m.

B. H = 3.6 m.

C. H = 2.4 m

D. H = 6 m

Answer: Select A

Explain:

When the object reaches the maximum height, V = 0.

The law of preserving the energy for 2 positions to start throwing objects and the maximum height:

W1 = W2 ⇔ WĐ1 + WT1 = Wđ2 + WT2.

Question 5: A sliding object without initial velocity from the top to the foot of a inclined plane has a length of 5m, and tilted at an angle of 30 ° from the horizontal plane. The friction force between the object and the inclined plane is as large as a quarter of the weight of the object. Take g = 10m/s^2. The velocity of the object at the bottom of the inclined plane has a magnitude of

A. 4.5 m/s. B. 5 m/s. C. 3.25 m/s. D. 4 m/s.

Answer: Select B

Explain:

Apply theoretical theorem for 2 positions that start moving and when the object stops, we have: we have:

Question 6: A 650N athletes jump at the initial velocity v0 = 2 m/s from the jumping bridge at an altitude of 10 m into the water in a vertical direction to the bottom. Take g = 10 m/s^2, after touching the water, the person moves an additional 3m movement in the water vertically, stop. The person’s energy variation is:

A. – 8580 J

B. – 7850 j

C. – 5850 j

D. – 6850 j

Answer: Select A

Explain:

Choose the potential root at the separation of the water and the air.

The function of the person at the beginning is:

At the stop position, the coordinates are h ‘-3 m.

The function when that person stopped:

WSAU = – MGH ‘= -1950 (J)

Mechanical variation: ΔW = WSAU – WORKS = – 8580 (J).

Question 7: An object is released freely, during the falling process

A. The kinetic energy of the object is constant.

B. The function of the object is constant.

C. The total kinetic energy and potential of the object does not change.

D. The total kinetic energy and potential of the object always changes.

Answer: Choose C.

Explanation: An object is released freely, in the process of falling velocity and height of the object changes, the potential and kinetic energy change, but the total potential and dynamic of the object does not change.

Question 8: A skiing athlete from the cliff slides, the sliding speed is increasing. So for athletes

A. Increased kinetic energy, increasing potential.

B. Increased kinetic energy, decreasing potential.

C. Dynamic kinetic energy, reduced energy.

D. The kinetic energy decreases, the energy increases.

Answer: Choose B.

Explanation: A skiing athlete from the cliff slides so the height decreases and speeds. Therefore the potential decreases, the kinetic energy increases.

Question 9: During the oscillation of a single pendulum, at the equilibrium position

A. The kinetic energy reaches the maximum value.

B. Potence reaches maximum value.

C. The function is zero.

D. Perfect with kinetic energy.

Answer: Select A.

Explanation: In the process of oscillation of a single pendulum, at the position of a single pendulum equilibrium with the lowest coordinates, so the smallest function, the biggest kinetic energy.

Question 10: When releasing a sliding object without the head velocity on the inclined plane with friction

A. The function of the object is equal to the maximum value of the kinetic energy.

B. The dynamic variation equals by the friction force.

C. The reduction of energy equal to the gravity.

D. The decrease in energy is equal to dynamic increase.

Answer: Choose C.

Explanation: When dropping a slide, the head velocity on the inclined plane has a reduced potential friction due to the gravity. Therefore reducing the potential of the gravity.

Question 11: An object is released freely from a height of 3m. The height of the object when the kinetic energy is twice

A. 1.5 m.

B. 1.2 m.

C. 2.4 m.

D. 1.0 m.

Answer: Select D

Explanation: Select the potential landmark at the ground.

Free falling objects should be preserved: W1 = W2

⇒ W1 = Wđ2 + WT2 = 2.WT2 + WT2 ⇒ H2 = H/3 = 1 (m).

Question 12: An object is thrown upright from the ground at a speed of 4 m/s. Ignore air resistance. The speed of the object when the kinetic energy is equal to the potential

A. 2√2 m/s.

B. 2 m/s.

C. √2 m/s.

D. 1 m/s.

Explanation: Select the potential landmark at the ground.

Ignore air resistance, so the function is preserved: W1 = W2

WĐ1+0 = WĐ2+WT2 = 2.WĐ2

v2 = v12 = 42 = 22 (m/s)

Conclusion:

Hopefully the information about the function and the law of preserving the function that Mầm non Cát Linh has provided will help them in the process of learning and solving related physical exercises. Besides, they can also refer to many other useful articles at the website Mầm non Cát Linh.edu.vn.

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