The work done by gravity does not cause a change in the mechanical energy of the object

For example, if you lift an object from a height of h1 to a height of h2 with force f, the velocity of the object changes from v1 to v2

f work w1 = f* (h2-h1), gravitational work w2 = -m*g* (h2-h1), the kinetic energy of the object changes δek = m*v2 2 2-m*v1 2 2, by the kinetic energy theorem, w1 + w2 = δek, that is, f*(h2-h1)-m*g*(h2-h1)=m*v2 2 2-m*v1 2 2, the arrangement can obtain f*(h2-h1) = (m*v2 2 2 + m*g*h2)-(m*v1 2 2 + m*g*h1)

The left end of the above equation is f (force other than gravity) and the right end is the change in the mechanical energy of the object. The meaning of this equation is that the work done by forces other than gravity changes with the mechanical energy of the object, which is the functional principle. From this, it can be seen that the work done by gravity does not change the mechanical energy.

The reason for this is because the work done by gravity will change the gravitational potential energy, and the mechanical energy includes potential energy and kinetic energy, if only the work done by gravity, only the conversion between kinetic energy and gravitational potential energy occurs, and the mechanical energy remains unchanged, and the mechanical energy changes only when there is work done by other forces other than gravity. Therefore, the work done by gravity does not cause a change in the mechanical energy of the object.

Because mechanical energy is the sum of kinetic energy and potential energy, the work done by gravity will only convert energy under these two forms of energy. For example, if I have $5 in my left pocket and $10 in my right pocket, whether I put the money in my left pocket or put the money in my right pocket in my left pocket, although the money in both pockets has changed, my total money is still $15.

Mechanical energy includes gravitational potential energy and kinetic energy, and only when gravity does work, the mechanical energy does not change; If there are other forces doing work besides gravity, the mechanical energy will also change. Because only when gravity does work, it is the mutual conversion of gravitational potential energy and kinetic energy, and there is no change in mechanical energy.

Mechanical energy is conserved.

Gravitational potential energy is converted into kinetic energy.

Only the work done by gravity or elastic force is a condition for the conservation of mechanical energy.

The total work exerted on the object by forces other than gravitational elasticity causes a change in the mechanical energy of the object.

The gravitational potential energy is completely converted into kinetic energy, and the mechanical energy is the sum of the kinetic energy and the potential energy, so it does not change.

Here Mechanical Energy = Gravitational Potential Energy + Kinetic Energy @ while Kinetic Energy = w Gravity (work done by gravity) = Amount of reduction of gravitational potential energy @中一增一减大小相等 so the mechanical energy does not change.

How it works. The increment of mechanical energy of an object system is equal to the algebraic sum of the total work done by the external forces on the system and the work done by the dissipative forces within the system.

The mechanical energy of an object includes the kinetic energy and gravitational potential energy of the object The work done by gravity is to convert the gravitational potential energy of the object into kinetic energy The sum of kinetic energy and gravitational potential energy is constant i.e., mechanical energy is conserved!

Because gravity is a conservative force ...

Mechanical energy is the sum of kinetic energy and gravitational potential energy.

Gravity does the work. The gravitational potential energy changes.

The mechanical energy also changes.

1. Because the effect of gravity to do work is kinetic energy and gravitational potential energy.

The effect of elastic work is the conversion between kinetic energy and elastic potential energy, and kinetic energy, gravitational potential energy and elastic potential energy are all called mechanical energy.

Therefore, the work done by gravity and the elastic work done by the spring do not change the mechanical energy.

2. In addition to gravity and the elastic force of the spring, the effect of other forces doing work is that kinetic energy and other energy (such as internal energy, electrical energy, etc.) are converted, and internal energy and electrical energy are not mechanical energy. So the force other than the gravitational force and the elastic force of the spring does positive work, and the mechanical energy increases.

3. The total work is related to the change of kinetic energy, which is the kinetic energy theorem.

It is the mutual conversion or transfer between kinetic energy and other energy.

When the force other than gravity and elastic force (when elastic force acts, the force applied by elastic force and the force object are counted in the system) do positive work, and the mechanical energy of the system increases.

On the contrary, if negative work is done, the mechanical energy decreases.

This is deduced from the kinetic energy theorem.

Kinetic energy theorem: Work done by an external force = change in the kinetic energy of an object.

In the work done by the combined external force, the work done by gravity is equal to the decrease of potential energy.

So: work done by combined external force - work done by gravity = change in kinetic energy - decrease in potential energy.

Work done by external force - work done by gravity = change in kinetic energy + increase in potential energy (positive work done by gravity increases, negative work decreases).

That is, the work done by forces other than gravity is equal to the change in the mechanical energy of the object. (If there is spring elastic potential energy, elastic force and elastic potential energy also need to be added).

In a gravitational field, for a shed compass to move in a straight line, its mechanical energy includes kinetic energy and potential energy. If the object falls freely, then its mechanical energy will gradually be converted into kinetic energy to complete the acceleration of the object. Conversely, if the object is decelerated by an external force, its kinetic energy will be converted into potential energy, and the pure trillion will eventually reduce the velocity of the object.

In the case of deceleration and deceleration, the object moves downward along the gravitational field, and the gravitational force and the deceleration force are in opposite directions. Thus, the work done by gravity will gradually reduce the kinetic energy of the object and at the same time increase the potential energy of the object so that its height gradually decreases. But the total mechanical energy is conserved, i.e., the total amount of mechanical energy does not change, so the kinetic energy that is reduced by the work done by gravity will be converted into potential energy, but the total mechanical energy of the object remains the same.

In addition, it is important to note that the air resistance experienced by the object during deceleration and descent may also have an effect on the conversion of mechanical energy. However, as long as the air resistance does not undergo work, the total mechanical energy will remain the same.

When an object slows down, gravity exerts a force downward, while the object's velocity gradually slows down. Due to the law of conservation of energy, the mechanical energy of the object remains constant in this process, i.e., the initial value of mechanical energy is equal to the base value of the last round of mechanical energy.

In the process of deceleration and descent, the work done by gravity is negative, and its magnitude is equal to the cosine value of the modulus length of gravity multiplied by the distance of the object falling and the angle of the direction of gravity. However, the total mechanical energy of the system remains the same as the kinetic energy lost by the object during the deceleration descent is converted into other forms of energy such as potential energy or thermal energy. Thus, even though gravity does negative work, the total amount of mechanical energy of the object does not change due to the law of conservation of energy.

The mechanical energy of the object decreases when decelerating and descending, and the problem should be analyzed from the whole, and the amount of work done by gravity and the amount of mechanical energy change cannot be considered.

First of all, let's look at what is mechanical energy, as you said, mechanical energy is equal to the sum of kinetic energy and potential energy. The potential energy here includes gravitational potential energy, elastic potential energy, etc.

Let's look at the law of conservation of mechanical energy: in a system of objects where only gravity or elastic force does work, kinetic energy and potential energy can be converted into each other, while the total mechanical energy remains the same. The elastic work mentioned here refers to the spring elastic work in high school.

Let's take the following situation as an example

The falling process of the object in Figure A belongs to free fall, so only gravity does work during the movement, that is, the gravitational potential energy and kinetic energy are converted into each other, and the mechanical energy is conserved. In Figure B, the object is subject to gravity and drag, that is, both gravity and drag force do work. At this time, part of the gravitational potential energy is converted into kinetic energy, and part of it is converted into work done to overcome resistance, that is, other forces other than gravity (here referring to resistance) do work.

It is evident that the mechanical energy is not conserved in this process. At this time, the mechanical energy will change, that is, the amount of change in mechanical energy can be regarded as the amount of work done by other forces other than gravity. This statement can be considered problematic for this model.

But in general, it is better for us to discuss the problem comprehensively, so we say that the amount of mechanical energy change is equal to the work done by forces other than gravity and elastic force, which is more scientific.

Gravity and elastic force are both conservative forces, which I will tell you about in high school, that is, the work done by these forces on an object, if it passes through a closed curve, then the work done is zero. For example, gravity does work on an object, and if the object descends a certain distance from an exit point and then goes back up to its original position, then the work done by gravity is zero. In the same way, elastic force and gravity are both conservative forces, then the work done by him is zero, and the influence condition of its mechanical energy as these objects is mainly the work done on the object by a force other than the running hand force, then the mechanical energy of the object will change.

For example, the sliding friction force on an object, when the object moves forward under the action of power, the frictional force always hinders the motion of the object, then the work done by this frictional force on the object will cause the mechanical energy of the object to decrease.

1) The work of gravity = initial gravitational potential energy - final gravitational potential energy, mechanical energy contains gravitational potential energy, and cannot be counted as both the work of gravity and the change of gravitational potential energy.

The work of the combined external force = the work of the gravitational force + the work of the other force.

Work of the resultant external force = increment of kinetic energy.

Kinetic energy increment = work of gravity + work of other forces = reduction of potential energy of gravity + work of other forces.

Gravitational potential increment + work of other forces.

Work of other forces = kinetic energy increment + gravitational potential energy increment = mechanical energy increment.

2) The work of all forces, including gravity, should be considered when applying the kinetic energy theorem.

When f lifts the object at a constant speed, gravity does negative work, f does positive work, the total work is 0, and the kinetic energy does not change. But the object overcomes gravity and does work, that is, gravity does negative work, and the object rises, and of course the gravitational potential energy increases.

Finally, emphasize:1When using the kinetic energy theorem, the work of all forces is calculated; 2.

When using the functional principle (the work of a force other than gravitational elasticity = the increment of mechanical energy), do not calculate the work of gravity and elastic force; 3.As long as gravity does work, whether it is positive or negative, the gravitational potential energy changes.

When an object overcomes gravity to do work, the object's (c gravitational potential energy must increase, kinetic energy may not change) aThe gravitational potential energy must be reduced and the mechanical energy may remain the same.

Mistake! Because the title tells that the work is done to overcome gravity, it can be seen that the direction of displacement is upward.

b The gravitational potential energy must increase, and the mechanical energy must increase.

Mistake! Gravitational potential energy must increase – that's right; However, "mechanical energy must increase" is not necessarily, for example, the vertical upward throwing motion does not change.

c The gravitational potential energy must increase, and the kinetic energy may remain the same.

That's right! Gravitational potential energy must increase – that's right; "Kinetic energy may not change" is also true, for example, under the action of a vertical upward pulling force, the kinetic energy is unchanged in a straight line at a uniform speed.

d The gravitational potential energy must be reduced, and the kinetic energy may be reduced.

Mistake! From the above analysis, it can be seen that this option is wrong.

I hope it helps you, and if you have any questions, you can ask them

I wish you progress in your studies and go to the next level! (*

Mechanical energy is the sum of potential energy and kinetic energy, when the object is thrown vertically, the object moves upward and decelerate, the gravitational potential energy increases, and the kinetic energy decreases. It is the kinetic energy that is converted into gravitational potential energy, and the mechanical energy of this process does not change. B false.

Suppose that a spring moves vertically upwards after compression, and the spring velocity remains unchanged when gravity and elastic force are balanced. The gravitational potential energy increases, while the kinetic energy does not change. c Pair.

A and D are clearly wrong.

This can be proved: first of all, we must know the relationship between the work of gravity wg and the gravitational potential energy increment δep wg=-δep.........1）

Then there is the kinetic energy theorem: w = δek.........2）

2)-(1) w-wg = δep + δek

The left w-wg of the above equation is the work of the combined external force minus the work of gravity, that is, the work done by other forces other than gravity, and the right δep + δek is the increment of mechanical energy, and the meaning of this equation is that the increment of mechanical energy is equal to the work done by other forces except gravity.

This is a functional relationship of universal significance.