Concept: product of mass and velocity of an object

Original formula: p=mv

Unit: kg·m/s

Direction: Same as the direction of velocity [vector]

relationship with kinetic energy

Unit: N·s

Direction: Same as the force direction [vector]

Original formula: I=Ft

Concept: The product of force and force application time

①Content: The change in momentum of an object at the beginning and end of a process is equal to the impulse of the force it experiences during the process.

②Formula: p'-p=I

③Scope of application: single object or system composed of multiple objects

→Newton's second law is an instantaneous expression while the momentum theorem studies the cumulative effect over time

vertical gravity

Application of Holistic Approach

Determination of speed relationship (kinematic formula, especially s=0.5v at the end of t)

particle model

expression

①The system is not subject to external forces or the resultant force of the external forces is zero.

②The internal force acting on each object in the system is much greater than the external force it receives (approximately)

③If the net external force on the system in a certain direction is zero, then momentum is conserved in that direction.

Distinguish between what is internal force and what is external force

Pay attention to the conservation of directionality

Pay attention to vectoriality

Pay attention to whether the mechanical energy before and after the collision is conserved

The momentum of each object in the system before and after interaction must be relative to the same inertial frame

p1, p2 or p1', p2' must be the momentum of each object in the system at the same time

Cart and slider

Arc groove

spring critical

Objects moving in opposite directions do not collide

Derivation of the formula for elastic collision

Special case

Perfectly inelastic collision: momentum is conserved and kinetic energy loss is maximum

Three sets of relationship constraints

At the moment of collision, due to the extremely short time, the impulse suffered by the connecting body (rope, spring, etc.) is negligible, that is, the speed remains unchanged.

Pay attention to whether the mechanical energy is conserved after the collision (if it is not conserved, the energy conservation sequence cannot be started from the untouched state, but the energy conservation sequence should be started from the collision state)

Two basic limits of scope problems

Inclined plane problem (straight, curved)

bullet hits wood block

Different parts of an object move in opposite directions under the action of internal forces

If the internal force is much greater than the external force, it can be considered that momentum is conserved.

System mechanical energy increases

The original mass of the rocket is M. After ejection, m remains. The speed of the gas ejected is u.

The velocity obtained by the rocket v=(M/m-1)u

In the man-ship model, the centers of mass of the two objects must move relative to each other, and the original centers of mass will be displaced in different directions.

relativity of speed

The application of holistic approach

man ship model

Crash and explosion

jet model

*Speed ​​loss