Remixed your code:
Graphics3D 640, 480, 0, 2 cam = CreateCamera() cube = CreateCube() MoveEntity cube, 0, 1, 20 ScaleEntity cube, 1,1.9,1 cube2 = CreateCube() MoveEntity cube2, 3, 0, 20 cube_velocity_y# = 0 Const gravity# = 9.81 ; acceleration = meters(units) per second per second Const jump_velocity# = 2.0 ; velocity = meters(units) per second previous_time = MilliSecs() Repeat this_time = MilliSecs() elapsed_time# = (previous_time - this_time) / 1000.0 previous_time = this_time cube_velocity_y = cube_velocity_y + gravity * elapsed_time If KeyHit(57) Then cube_velocity_y = jump_velocity MoveEntity cube, 0, cube_velocity_y, 0 If EntityY(cube) < 1 Then PositionEntity cube, 0, 1, 20 cube_velocity_y = 0 EndIf RenderWorld Text 0, 0, cube_velocity_y Flip Until KeyHit(1) End