杭电 1155 Bungee Jumping（物理题）

Problem Description

Once again, James Bond is fleeing from some evil people who want to see him dead. Fortunately, he has left a bungee rope on a nearby highway bridge which he can use to escape from his enemies. His plan is to attach one end of the rope to the bridge, the other end of the rope to his body and jump off the bridge. At the moment he reaches the ground, he will cut the rope, jump into his car and be gone.

Unfortunately, he had not had
enough time to calculate whether the bungee rope has the right length, so it is
not clear at all what is going to happen when he jumps off the bridge. There are
three possible scenarios:
The rope is too short (or too strong), and James
Bond will never reach the ground.
The rope is too long (or too weak), and
James Bond will be going too fast when he touches the ground. Even for a special
agent, this can be very dangerous. You may assume that if he collides at a speed
of more than 10 m/s, he will not survive the impact.
The rope's length and
strength are good. James Bond touches the ground at a comfortable speed and can
escape.
As his employer, you would like to know whether James Bond survives
or whether you should place a job ad for the soon-to-be vacant position in the
local newspaper. Your physicists claim that:
The force with which James is
pulled towards the earth is
9.81 * w,
where w is his weight in kilograms
and 9.81 is the Earth acceleration in meters over squared seconds.
Mr. Bond
falls freely until the rope tautens. Then the force with which the bungee rope
pulls him back into the sky depends on the current length of the rope and is

k * Δl,
where Δl is the difference between the rope's current length and
its nominal, unexpanded length, and k is a rope-specific constant.
Given the
rope's strength k, the nominal length of the rope l in meters, the height of the
bridge s in meters, and James Bond's body weight w, you have to determine what
is going to happen to our hero. For all your calculations, you may assume that
James Bond is a point at the end of the rope and the rope has no mass. You may
further assume that k, l, s, and w are non-negative and that s <
200.

The input contains several test cases, one test case per line. Each
test case consists of four floating-point numbers (k, l, s, and w) that describe
the situation. Depending on what is going to happen, your program must print
"Stuck in the air.", "Killed by the impact.", or "James Bond survives.". Input
is terminated by a line containing four 0s, this line should not be processed.

Sample Input

350 20 30 75

375 20 30 75

400 20 30 75

425 20 30 75

450 20 30 75

400 20 30 50

400 20 30 80

400 20 30 85

0 0 0 0

Sample Output

Killed by the impact.

James Bond survives.

Stuck in the air.

James Bond survives.

Killed by the impact.

物理题，求一个人从桥上用绳子跳下的三种状态，绳子有弹性，劲度系数为k，若人落地的速度大于10就会摔死。

思路：

　　先求出人从桥到地的重力势能Eg，速度为10的动能Ev，绳拉到地（绳不一定能到地面，假设到地 ) 的弹性势能Ek，先比较绳的长度和桥的高度，若绳长大于桥高，判断Eg和Ev的大小；

若绳长小于桥高，比较Eg和Ek的大小,Eg < Ek说明人不会落地，否则比较Eg-Ek和Ev的大小。

 /*

 k 绳子劲度系数

 l 绳长

 s 桥的高度

 w 体重

 */

 #include<cstdio>

 #define g 9.81

 int main()

 {

     double k,l,s,w;

     double Eg,Ek,Ev;

     while(scanf("%lf %lf %lf %lf",&k,&l,&s,&w)&&(k+l+s+w))

     {

         Eg=w*g*s;

         Ev=*w/;

         Ek=k*(s-l)*(s-l)/;

         if(l >= s)

         {

             if(Eg <= Ev)

                 printf("James Bond survives.\n");

             else

                 printf("Killed by the impact.\n");

         }

         else

         {

             if(Eg < Ek)

                 printf("Stuck in the air.\n");

             else if(Eg-Ek <= Ev)

                 printf("James Bond survives.\n");

             else

                 printf("Killed by the impact.\n");

         }

     }

 }