A diver performed parachute jumping, left the plane for free fall, opened the parachute 125m away from the ground, decelerated at an acceleration of 14.3m/s, and reached the ground speed of 5m / s (1) How high is the diver from the plane to the ground? (2) How long does it take to get to the ground after leaving the aircraft? (g = 10m / s)

A diver performed parachute jumping, left the plane for free fall, opened the parachute 125m away from the ground, decelerated at an acceleration of 14.3m/s, and reached the ground speed of 5m / s (1) How high is the diver from the plane to the ground? (2) How long does it take to get to the ground after leaving the aircraft? (g = 10m / s)

Set the speed from 125 meters to the ground as V2 and the speed from 125 meters to the ground as v3
V3 ^ 2-v2 ^ 2 = 2As, V2 = 60m / s, t = 3.85s
The vertical displacement at 125 m away from the aircraft is as follows:
S=V1T+1/2at^2
Time t = (v2-v1) / g = 6 seconds
S = 0.5 * 10 * 30 = 150m
(1) The height from the ground is 275 meters
(2) 85 seconds

Mechanics analysis of a high school
There is one of the following situations
A board of mass m is placed on a smooth table, and an object of mass 2m is placed on the board. There is direct friction between the board and the object. The maximum static friction = F
When there is an external force from 0 to 2F acting on the board
Please help me analyze the dynamic situation from 0 to 2F
I'm sorry... Yes, the external force level
Both can produce slippage

Level of external force? I'll do level analysis! If not, please add!
First of all, we should consider whether there is relative sliding between them
First of all, let's see what the maximum external force is if there is no sliding between them: the maximum acceleration of the object is f / 2m
Because the acceleration of the whole body is f / 2M and the external force is (M + 2m) * f / 2m = 1.5f
1.5f

I would like to know the application conditions of the integral method and the isolation method, especially the integral method. You can use the integral method and the isolation method respectively to judge the direction of the friction force on the inclined plane when the wooden block slides at a constant speed, accelerates or decelerates, while the inclined plane does not move!

When Newton's second law is applied: the acceleration of the integral method must be the same (including the direction) -- special case: fixed pulley, the direction is different
We can use the whole method
When applying functional relationship or energy conservation, we should pay attention to whether there are different forms of energy conversion inside
M is v & # 186; when a plank is driven onto a rough horizontal plane, the kinetic energy theorem can not be applied only considering the external friction of the horizontal plane
Because of the internal friction between the two, the mechanical energy is transformed into internal energy... And so on
M. After M stacking, there is (M + m) GSIN θ = μ (M + m) GCOS θ along the inclined plane
The lower acceleration is (M + m) GSIN θ - μ (M + m) GCOS θ = (M + m) a
There is μ (M + m) GCOS θ - (M + m) GSIN θ -) = (M + m) a in deceleration
As a whole, the internal forces of a particle need not be considered

Holistic approach and segregation approach
Tell me how to use it. I feel dizzy when I see that kind of question

When solving mechanical problems, the vast majority of objects are interrelated and interact with each other. Therefore, in order to solve the problem, we often isolate the research object from other objects, analyze the stress of isolated objects separately, and apply the corresponding laws to analyze and solve them. This method is called isolation method