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OCR: TITLE grav1. pde} 'Earth -Moon Gravitational Field' SELECT ngrid= DEFINITIONS units 3.84e8 Lx= d Ly= 2*d m1 98e24 m2= 7 35e22 Masses of Earth and Moon .67e-11* [m 1/sqrt(x^2+y^2+ m2/sqrtx^2+y-d)^2] gx= dx(U) -dy(U) {Field strength vector( gy gm= sqrt( gx^2 gy^2) Magnitude BOUNDARIES region start( -LX, -Ly/4 line to Lx,-Ly/4 to Lx, 3/4*Ly) to(-Lx,3/4*Ly) to finish PLOTS contour( U contour( log10(-U)) contour( log10( gm)) contour( abs( gx)) log contour( abs( gy)) log vector( gx/gm, gy/gm) Gravitational field' contour( arctan( gy/gx) pi*180) field direction (degrees) surface( -U) log END contourt