High school physics, in the law of conservation of momentum, how to calculate relative velocity

Taking one object as a reference, the relative velocity of another object to it is the difference from it.

Explanation of the law. The conclusion that a system is not affected by external forces or the sum of the external forces is zero, and the total momentum of the system remains the same, is called the law of conservation of momentum.

1.The law of conservation of momentum is one of the most important and universal conservation laws in nature, which is an experimental law, which can also be deduced from Newton's third law in combination with the momentum theorem.

2.The system of things that has forces with each other is called a system, and the objects in the system can be two, three or more, and the system should be reasonably selected according to the needs and the convenience of solving the problem when solving practical problems.

For example: a flatbed car with mass m moves along a smooth level to the right, a person with a mass m stands on the car, the car moves to the right at speed v0, and now the person runs to the left with speed u relative to the car, ask the person what is the speed v before leaving the flatbed?

Let the velocity of the car be v, and the velocity of the person relative to the car is u, then the velocity relative to the ground is v person=v-u

Conservation of momentum: (m+m)v0=mv-m(v-u).

Find v=[(m+m)v0-mu] (m-m).

Generally, after the positive direction is specified, the physical quantities that can determine the direction are all expressed as "+" or "-" only the magnitude is substituted in the physical quantity: the physical quantity that cannot determine the direction can be represented by letters, if the calculation result is "+", it means that its direction is the same as the specified positive direction, and if the calculation result is "-", it means that its direction is opposite to the specified positive direction.

Extended information: no external force or zero combined external force; Although the resultant external force of the system is not zero, the internal force of the system is much greater than the external force, such as collision, ** and other phenomena, the momentum of the system can be regarded as approximately conserved.

If the system does not meet any of the above conditions in general, the total momentum of the system is not conserved. However, if the system meets any of the above conditions in a certain direction, the momentum of the system is conserved in that direction.

I didn't see a specific topic, so let's give an example.

A flatbed cart with mass m moves along a smooth level facing right, and a person with mass m stands on the cart. The car moves to the right at speed v0. Now the person is running to the left at the speed u relative to the car, and ask the person what is the speed v of the car before leaving the flatbed?

Let the velocity of the car be v, and the velocity of the person relative to the car is u, then the velocity relative to the ground is v man = v-u Conservation of momentum: (m+m)v0=mv-m(v-u) to obtain v=[(m+m)v0-mu] (m-m).

b.The velocity relationship between the three vehicles is VC>VA>VBAB

C and the child's composition of the system momentum is conserved, the initial momentum is 0, the child jumps away from C when it moves to the right, jumps away from B and jumps to A, the ground speed is the same, so the B car finally stops, and when it reaches A, it remains stationary relative to A, at this time, the momentum of A and the child to the left is equal to the momentum of C and the direction is opposite, so VC>VA>VB

The object is moving, then it has some properties of its motion, momentum is a physical quantity that represents the nature of motion, the conservation of momentum can be regarded as the property of motion (magnitude, direction) unchanged, and the time accumulation effect of force will change momentum, therefore, if the time accumulation of force is zero, momentum is unchanged, that is, conservation. For example, if the net external force of a bicycle moving in a straight line at a uniform speed is zero, then the accumulation of time by the net external force is zero, and the momentum is conserved.

The simplest understanding is: just like the Chinese New Year, your parents give you New Year's money, no matter how much they give you (or ask for it), the total amount of money in your family is the same.

And the unchanging premise is that you don't give money to outsiders or earn money from outsiders.

Momentum is conserved.

The initial momentum of ball A and ball B is zero.

A throws the ball v=4m s has 60v=10*4, and obtains that A's v = B catches the ball (70+10)*v=10*4, and obtains B's v=