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!  Program file name: motion.f90                                          !
!                                                                         !
!  © Tao Pang 2006                                                        !
!                                                                         !
!  Last modified: June 2, 2006                                            !
!                                                                         !
!  (1) This F90 program is created for the book, "An Introduction to      !
!      Computational Physics, 2nd Edition," written by Tao Pang and       !
!      published by Cambridge University Press on January 19, 2006.       !
!                                                                         !
!  (2) No warranties, express or implied, are made for this program.      !
!                                                                         !
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!
PROGRAM MOTION
!
! An example of studying the motion of a particle in
! one dimension under an elastic force.
!
  IMPLICIT NONE
  INTEGER, PARAMETER :: N=10000, J=200
  INTEGER :: I
  REAL :: PI, DT, T, JDT
  REAL, DIMENSION (N+1):: X, V
!
! Assign time step and initial position and velocity
!
  PI   = 4*ATAN(1.0)
  DT   = 2*PI/N
  X(1) = 0
  V(1) = 1
!
! Calculate other position and velocity recursively
!
  DO I = 1, N
    X(I+1) = X(I)+V(I)*DT
    V(I+1) = V(I)-X(I)*DT
  END DO
!
! Output the result in every J time steps
!
  T = 0
  JDT = J*DT
  DO I = 1, N, J
    WRITE (6,"(3F16.8)") T, X(I), V(I)
    T = T + JDT
  END DO
!
END PROGRAM MOTION