On the relationship between work and kinetic energy in the conservation of mechanical energy

1) Regarding this problem, you can first find the highest altitude reached by a person, then find the initial velocity when jumping, and then use the kinetic energy theorem and subtract the work done by gravity when jumping to the highest height to find the work done by the springboard, right?

Answer: The method is wrong, only the initial velocity at the time of jumping is required, and the kinetic energy is calculated directly by this initial velocity, which is the work done by the springboard.

2) However, if the conservation of mechanical energy is used, the water surface is regarded as a zero potential surface, and the kinetic energy at the moment of jumping can be obtained, but how can the kinetic energy be successfully converted here? Is finding kinetic energy here an algebraic sum of the work done for the springboard and gravity? But at this moment, gravity doesn't seem to have done work, so what is the relationship between work and kinetic energy here?

Answer: It is true to use the conservation of mechanical energy to do, gravity does not do work at the moment of jumping, because the meaning of this question is that it takes only a small amount of time to jump off the springboard on the springboard, and the springboard (1 meter should be the jump) shakes a very small distance, and the size of gravity can be ignored relative to the jumping force.

In summary, the work done by the springboard (ignoring gravity) at the time of jumping is the kinetic energy when jumping out.

The answer is 2000j work.

The first question is according to your algorithm, after a person takes off to the highest point, it is completely the transformation of mechanical energy itself, and the kinetic energy during the jump is equal to the height of the rise, and the result is equal to zero.

The first problem should be: when a person jumps on a springboard, the person changes from speed 0 to velocity, and the kinetic energy at this time can only be provided by the springboard (ignore the work done by gravity first), so his kinetic energy is the work done by the springboard.

First of all, it is necessary to admit that gravity must do the work at the moment of jumping. So why ignore gravity?

1.Assuming that the person displaces 40 cm at the moment of jumping, it can be calculated that 50*10*hypothetical 2000j is the resultant force of the springboard and gravity, then the work done by the springboard can be found as: 2000+200=2200j, and the work done by gravity is only 11/11 of the work done by the springboard, which can be ignored.

The smaller the displacement, the less gravity acts.

2.In addition, within the scope of high school physics, all objects are just particles, without factors such as the up and down bounce of the springboard, and the bending of people, the springboard is a jumping platform, and the person is a small ball. According to this point of view, the ball suddenly gains velocity, and the displacement tends to be infinitesimal as calculated in the previous question, and the displacement is infinitesimal and the force of the foot is very large, even greater than the gravitational force.

Gravity needs to be ignored even more.

If you don't understand anything, just text me. Thank you**.

The initial velocity when jumping, at this time kinetic energy = the work done by the springboard, jumping to the highest height, the person is out of the board, and the board does not do work.

Stunned me.,Hehe.,It's been a long time since I've been exposed to physics.。。。

The main meaning of this question is to ask the springboard to do the work of the person, if the person does not jump, the height itself is one meter high from the water level, according to your understanding that now the person does not understand the springboard has done the work of 1 meter height to the person. His problem is obvious, referring to what the springboard does to people!!

This is a profound problem, first of all, it is affected by friction, that is, dissipative force, the total mechanical energy of the system must be constantly decreasing, and converted into internal energy, which is the principle of entropy increase, that is, the movement of matter tends to be disordered, and various energies tend to be transformed into internal energy; However, the total momentum is constant, because the universe is an isolated system, and its internal dissipative forces have no effect on the overall momentum, and if it is considered that the center of mass of the universe is stationary, then according to the big ** theory, its total momentum is zero (momentum is the superposition of vectors, and the non-absolute values are added).

As a simple example, in the study of the two-body problem, since there is a certain recovery coefficient e, the total kinetic energy after the collision should be reduced, but the conservation of momentum is still true, because the two centroids are not impulsed by an external force.

Returning to the dissipation problem, since it is said that the combined momentum is always zero, the mechanical energy is constantly lost....Isn't the cosmic heat death just around the corner? In fact, the gravitational force between the stars is equivalent to negative entropy, and the increase in entropy cancels it out, and the heat death of the universe becomes unfounded!

Momentum Conservation Dynamic Collision Static Formula: v1'=(m1-m2)v1/(m1+m2)v2'=2m1v1/(m1+m2)。

From the occurrence of elastic collisions, it can be seen that

Conservation by dynamic search: MV1=MV2+MV3.

It is conserved by mechanical energy.

You can solve it:

v3=2m/(m+m)*v1。

v2=(m-m)/(m+m)*v1。

The law of conservation of momentum and the law of conservation of energy.

Together with the law of conservation of angular momentum, they become the three fundamental conservation laws in modern physics. Initially, they were inferences of Newton's laws, but later it was found that they were far more applicable than Newton's laws, and were more fundamental laws of matter or reason than Newton's laws, and were a reflection of the nature of time and space. Among them, the law of conservation of momentum is derived from the translational invariance of space, the law of conservation of energy is derived from the translational invariance of time, and the law of conservation of angular momentum is derived from the rotational symmetry of space.

Physics at the secondary level is almost always ideal, in which case mechanical energy is conserved. But it's not that simple.

I remember that it seems that it cannot be said that kinetic energy is conserved, because kinetic energy is generally not conserved, an object or a system, kinetic energy is conserved only when no external force is done work, generally momentum is conserved, (kinetic energy is a scalar quantity, momentum is a vector quantity).

Mechanical energy includes kinetic energy and potential energy, and conservation of mechanical energy refers to an object or a system, in which the kinetic energy and potential energy are unchanged no matter how they change.

In general, the conservation of kinetic energy and the conservation of mechanical energy have nothing to do with each other, of course, in the horizontal plane and in the air, ignoring the drag force and friction, the object does a uniform motion "conservation of kinetic energy" and "conservation of mechanical energy" are both conserved.

There is also a small mistake on the 3rd floor to correct it, not "the conservation of mechanical energy is the conversion of kinetic energy and potential energy in the absence of external forces." Rather, "the conservation of mechanical energy is the conversion of kinetic energy and potential energy without external force 'doing work'." ”(

Hee-hee......, when the force is perpendicular to the direction of motion, or in equilibrium, there are external forces that are also conserved.

Hello. Let me remind you that there is no conservation of kinetic energy, only mechanical energy. But there is the kinetic energy theorem.

Kinetic energy theorem, the work done by the combined external force is equal to the change in the kinetic energy of the object.

Gravitational potential energy, elastic potential energy, and kinetic energy are collectively referred to as mechanical energy.

Conservation of mechanical energy is the conversion of kinetic energy and potential energy in the absence of external forces.

There is no conservation of kinetic energy, only the theorem of kinetic energy and the conservation of momentum.

The kinetic energy theorem states that the work done by the combined external force is equal to the amount of change in kinetic energy, which is different from the premise of conservation of mechanical energy.

The conditions for the two conservations are not the same.

The conservation condition for conservation of mechanical energy is ......Only gravity or the elastic force of the spring do the work, so that there can be conservation of mechanical energy.

The condition for the conservation of momentum is that there is no external force in the system, or the external force is zero or negligible. Only then can momentum be conserved, or in a certain direction, momentum can also be conserved, so there is no necessary connection between them.

If you really want to ask why there is no impact, then it is as ...... as your neighbor's name and your nameThere is only a contact at a certain time, and the other ......There is no necessary connection.

Conservation of momentum requires that the impulse of the external force experienced by the system be zero, or in the event of a collision, the internal force is much greater than the external force at the moment. However, the conservation of mechanical energy requires that the work done by the external forces on the system is zero, and the internal forces are only conservative forces, such as gravity, elastic force, and gravitational force. There are two conservation laws, one of which observes the accumulation of force over time, i.e., impulse.

One observation is the accumulation of force on space, that is, work. Therefore, the conservation conditions of the two are inconsistent.

For example, when two small balls collide completely inelastically, the momentum is conserved because there is only internal force acting, but there is a loss of energy, so the mechanical energy is not conserved.

Conservation of momentum is one of the earliest conservation laws discovered. If a system is not subjected to an external force or the vector sum of the external forces is zero, then the total momentum of the system remains the same, and this conclusion is called the law of conservation of momentum. The law of conservation of momentum is one of the most important and universal conservation laws in nature, which applies to both macroscopic objects and microscopic particles; It is suitable for both low-speed moving objects and high-speed moving objects; It applies to both conservative and non-conservative systems.

Conservation of mechanical energy means that the potential energy (gravitational potential energy or elastic potential energy) and kinetic energy of an object are converted into each other when the work is done by a conservative force such as gravity (or the elastic force of a spring), but the total mechanical energy remains unchanged.

From the point of view of work: if all the external forces and non-hail forces of a system do not do work or the sum of their work is zero, then the kinetic energy and potential energy of the objects in the system can be converted into each other, but the mechanical energy of the system always remains a conserved quantity!

Condition (i.e. note): All external and non-conservative internal forces of a system do not do work or the sum of their work is zero. This condition must be met for the mechanical energy of the system to be conserved!

Conservative source and force and non-conservative force (judging from the work): The work of conservative force is related to the beginning and end position, and the path is dust-free! The work done by the non-conservative force is related to the path!

Common conservative forces are gravitational force, gravitational force, elastic force, and Coulomb force for electrostatic fields. The most common non-conservative force is frictional force.