! David Vann
! Given user input of a celestial body (Earth, Moon, Ceres, or Jupiter) and a list of
! needed dimensions, calculate and display the escape velocity
! Input Variables: body, used to choose which celestial body's escape velocity to calculate
! Output Variables: e_velocity_mantissa, e_velocity_exponent, used to calculate the
! mantissa and exponent of the escape velocity separately because of odd compiler
! errors which shouldn't happen that were caused by using numbers that are too large
PROGRAM escape_velocity
! Declare variables and initialize to zero
IMPLICIT NONE
! Will not compile if an undeclared variable is used (such as a typo)
INTEGER, PARAMETER :: DBL = SELECTED_REAL_KIND(p=15, r=100)
INTEGER :: body = 0
INTEGER, PARAMETER :: Earth = 1, Moon = 2, Ceres = 3, Jupiter = 4
REAL(KIND=DBL) :: e_velocity_mantissa = 0, e_velocity_exponent = 0, e_velocity_exponent_extra = 0,  mass_mantissa = 00
REAL(KIND=DBL) :: mass_exponent = 0, radius_mantissa = 0, radius_exponent = 0
REAL(KIND=DBL), PARAMETER :: G_mantissa = 6.673, G_exponent = -11
! G = Gravitational constant in Nm**2 * kg**-2
! Tell user how to enter input
WRITE(*,*) 'Enter a celestial body:'
WRITE(*,*) 'Earth   =', Earth
WRITE(*,*) 'Moon    =', Moon
WRITE(*,*) 'Ceres   =', Ceres
WRITE(*,*) 'Jupiter =', Jupiter
! Get input
READ(*,*) body
! Determine what the input means
SELECT CASE (body)
CASE (Earth)
mass_mantissa = 6.0
mass_exponent = 24
! mass in kilograms, broken down to avoid too large a number in an arithemetic operation error
radius_mantissa = 6.4
radius_exponent = 6
! radius in meters
WRITE(*,*) 'Earth selected.'
CASE (Moon)
mass_mantissa = 7.4
mass_exponent = 22
radius_mantissa = 1.7
radius_exponent = 6
WRITE(*,*) 'Moon selected.'
CASE (Ceres)
mass_mantissa = 8.7
mass_exponent = 20
radius_mantissa = 4.7
radius_exponent = 5
WRITE(*,*) 'Ceres selected.'
CASE (Jupiter)
mass_mantissa = 1.9
mass_exponent = 27
radius_mantissa = 7.1
radius_exponent = 7
WRITE(*,*) 'Jupiter selected.'
CASE DEFAULT
WRITE(*,*) 'USER ERROR: INVALID INPUT'
STOP
END SELECT
! calculate using set values
e_velocity_mantissa = SQRT( 2 * G_mantissa * mass_mantissa / radius_mantissa )
e_velocity_exponent = ( mass_exponent / 2 ) - ( radius_exponent / 2 ) + ( G_exponent / 2 )
DO WHILE ( e_velocity_mantissa < 1 )
e_velocity_mantissa = e_velocity_exponent * 10
e_velocity_exponent = e_velocity_exponent + 1
END DO
DO WHILE ( e_velocity_mantissa >= 10 )
e_velocity_mantissa = e_velocity_exponent / 10
e_velocity_exponent = e_velocity_exponent - 1
END DO
WRITE(*,*) 'With a mass of', mass_mantissa, '* 10 ^', mass_exponent
WRITE(*,*) 'and a radius of', radius_mantissa, '* 10 ^', radius_exponent
WRITE(*,*) 'the escape velocity is:', e_velocity_mantissa, '* 10 ^', e_velocity_exponent, 'meters per second or'
e_velocity_mantissa = e_velocity_mantissa * 10**e_velocity_exponent
WRITE(*,*) e_velocity_mantissa, 'meters per second'
END PROGRAM escape_velocity

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