Momentum (mv) and kinetic energy () are both physical quantities that reflect the motion state of an object, and both depend on the mass and speed of the moving object, but these two physical quantities are essentially different.
1. Momentum and kinetic energy are physical quantities that respectively reflect two different abilities of a moving object.
Momentum only expresses the ability of mechanical motion transmission, and it is a physical quantity that describes the mechanical motion state of an object. The mechanical motion transmits not the speed, but the momentum of the object. For a given object (constant mass), if its motion speed is different. The ability of mechanical motion transmission is also different; For objects with different masses, even if the moving speed is the same, their mechanical motion transmission ability will be different. Therefore, the mechanical motion transmission ability of an object is not expressed by speed, but by momentum. Even if the momentum is equal, the result of mechanical motion transmission will be different because of the different direction of motion, so momentum is a vector, and its direction is consistent with the direction of instantaneous velocity. Because speed is a state quantity, momentum is also a state quantity. General momentum always refers to the momentum of an object at a certain moment or position.
Kinetic energy only indicates the ability of an object to do work at a certain moment, and it is also a physical quantity that describes the motion state of an object. For a given object (with constant mass), if its moving speed is different, its ability to do work is also different; For objects with different masses, even if they move at the same speed, their ability to do work is different. Therefore, the ability of a moving object to do work can not be expressed by speed, but only by kinetic energy. For a given object (constant mass), when the moving speed of the object changes. Its kinetic energy also changes, and the kinetic energy of an object at a certain moment is only determined by the speed of the object at that moment, and has nothing to do with the speed change process. No matter what the motion direction of an object is, as long as its speed and mass are constant, its ability to do work is the same, so kinetic energy is scalar. When the momentum of an object changes, its kinetic energy does not necessarily change, but when the kinetic energy of an object changes, its momentum must change.
2. Momentum and kinetic energy are two different basic physical quantities to measure the motion of an object.
16 ~ 17 th century, based on the philosophy that the total amount of motion is always conserved, people began to look for suitable physical quantities to measure mechanical motion to express the conservation of motion. Although speed is a physical quantity describing the motion state of an object. If the mechanical motion is measured by speed, it obviously cannot reflect the conservation of motion, so two methods, momentum and kinetic energy, are proposed to measure the mechanical motion from different angles.
Momentum is a measure of the motion of an object, which is measured from the perspective of mechanical motion transmission. In the process of mechanical motion transmission, the transmission of mechanical motion follows the law of conservation of momentum. Objects with equal momentum may have completely different velocities. Although momentum is related to speed, it is different from speed. Simple speed does not reflect the difficulty of letting an object get this speed or stopping an object from moving at this speed. Momentum, as a measure of the motion of an object, can reflect how much force and how long it takes for a given object to obtain a certain speed.
Kinetic energy is also a measure of the motion of objects. It measures mechanical motion from the perspective of energy conversion by transforming mechanical motion into a certain amount of other forms of motion. In the process of kinetic energy transformation, the transformation of kinetic energy follows the laws of energy transformation and conservation. Kinetic energy, as a measure of the motion of an object, can reflect how much force is needed to make a given object obtain a certain speed. How far to move in the direction of force.
Third, the change of momentum and kinetic energy respectively corresponds to the cumulative effect of two different forces.
Momentum theorem describes that impulse is a measure of momentum change of an object. Momentum is a quantity representing the state of motion, the increment of momentum represents the change of the state of motion of an object, and impulse is the cause of the change of the state of motion and a measure of the change of momentum. The momentum theorem describes the process that the momentum of an object changes due to the action of impulse.
The theorem of kinetic energy reveals that the change of kinetic energy is realized by doing work, and the change of kinetic energy is measured by doing work. The kinetic energy theorem reveals this relationship. The same relationship exists between work and various forms of energy change, that is, work is a measure of energy change. Various forms of energy can be transformed into each other, and this transformation is also achieved by doing work, which is measured by doing work. This shows that. The fundamental difference between momentum and kinetic energy lies in their different descriptions of the characteristics of physical processes and conservation laws. Every moving object has certain momentum and kinetic energy, but the change of momentum and the transformation of energy completely obey different laws. Therefore, to understand and distinguish these two concepts, we must consider the process of physical change.
The change of momentum shows the cumulative effect of force on time, and the change of momentum is equal to the impulse of external force; The change of kinetic energy shows the cumulative effect of force on space, and the change of kinetic energy is equal to the work done by external force. Momentum and impulse are two closely related but essentially different physical quantities. Momentum determines how long an object can resist resistance movement; Kinetic energy and work are also closely related. It is also an essentially different physical quantity. Kinetic energy determines how far an object can move against resistance.
Example 1 An object moves in a straight line in a horizontal plane from rest under the action of a constant force. If t is used to represent the time of object motion and s is used to represent the displacement of object motion, the following statement is correct ().
A. the kinetic energy of a particle at time t is proportional to t.
The kinetic energy of a particle at time t is proportional to s2.
The momentum of a particle at time t is proportional to t2.
The momentum of a particle at time t is equal to.
According to the theorem of kinetic energy and momentum, under the action of constant force, the kinetic energy of an object moving in a straight line from rest at time t is equal to the change of kinetic energy of the object in time t, that is, the work done by external force in time t, so the kinetic energy of the object at this time reflects the accumulation effect of force on space, and the magnitude of kinetic energy is proportional to displacement; Similarly, under the action of constant force, the momentum of an object moving in a straight line from rest at time t is equal to the change of the momentum of the object at time t, that is, the corresponding impulse of external force at time t, so the momentum of the object at this time reflects the effect of force on time accumulation, and the momentum is proportional to time. Among the options given in the title, option ABC is the opposite of the above relationship. Therefore, according to the law of kinematics, option D is correct.
Although momentum and kinetic energy are fundamentally different. The two can neither be confused nor replaced with each other, but momentum and kinetic energy are related to the mass of an object. They describe the characteristics of moving objects from different angles, all of which are state quantities, and there is a relationship between size.
refer to
1 Wang Peiqing. Problems easily confused in middle school physics teaching. Changsha: Hunan Education Press, 1983+062 ~ 164.
People's Education Press. The first teaching reference book of physics in normal school. Beijing: People's Education Press,1998.5438+080 ~1438+0.
3 Guan Jingcheng. Law of conservation. Jinan: Shandong Education Press, 200 1.9 1 ~ 93.