PROGRAM trebuchet

!  Filename:  trebuchet.f90
!
!  Programmer:  Br. David Carlson
!
!  Date:  January 26, 2006
!
!  This program performs some calculations related to the performance of a
!  trebuchet.  The user is asked to input the angle of elevation (in degrees)
!  for the rock that is thrown by the trebuchet.  The starting velocity is
!  assumed to always be 127 miles per hour.  The program calculates and outputs
!  the number of seconds that the rock is in the air as well as the horizontal
!  distance travelled by the rock (in yards).  Air resistance is ignored.

! Constants:
REAL, PARAMETER::INITIAL_VELOCITY = 127.0 ! Initial velocity of rock (miles per hour).
REAL, PARAMETER::MINUTES_PER_HOUR = 60.0  ! Minutes per hour.
REAL, PARAMETER::SECONDS_PER_MIN = 60.0   ! Seconds per minute.
REAL, PARAMETER::FEET_PER_MILE = 5280.0   ! Number of feet in a mile.
REAL, PARAMETER::FEET_PER_YARD = 3.0      ! Number of feet in a yard.

! Variables:
REAL::InitialFeetPerSec     ! Initial velocity in feet per second.
REAL::InitialVertVelocity   ! Initial vertical component of velocity in feet per second.
REAL::InitialHorizVelocity  ! Initial horizontal component of velocity in feet per second.
REAL::Time                  ! Time the rock is in air in seconds.
REAL::HorizDistanceFt       ! Horizontal distance rock is thrown in feet.
REAL::HorizDistanceYd       ! Horizontal distance rock is thrown in yards.
REAL::AngleDegrees          ! Angle of elevation in degrees.
REAL::AngleRadians          ! Angle of elevation in radians.
REAL::Pi                    ! PI
REAL::RadiansPerDegree      ! Radians per degree to convert from degrees to radians.

WRITE (*,*) 'Enter the angle of elevation for the rock that is thrown by the trebuchet:'
READ (*,*) AngleDegrees
WRITE (*,*) 'Assuming an initial velocity of ', INITIAL_VELOCITY, ' miles per hour.'

Pi = 2.0 * ASIN(1.0)
RadiansPerDegree = Pi / 180.0 
InitialFeetPerSec = INITIAL_VELOCITY * FEET_PER_MILE / (MINUTES_PER_HOUR * SECONDS_PER_MIN)
AngleRadians = AngleDegrees * RadiansPerDegree
InitialVertVelocity = InitialFeetPerSec * SIN(AngleRadians)
InitialHorizVelocity = InitialFeetPerSec * COS(AngleRadians)
Time = InitialVertVelocity / 16.0
HorizDistanceFt = InitialHorizVelocity * Time
HorizDistanceYd = HorizDistanceFt / FEET_PER_YARD

WRITE (*,*) 'The rock is in the air for about ', Time, ' seconds.'
WRITE (*,*) 'The rock travels horizontally about ', HorizDistanceYd, ' yards.'

END PROGRAM