The force of gravity on a spacecraft some distance from the earth is 800N What would that force be if its distance to the earth's centre were one half as great?

gravitational attraction between two bodies conforms to an inverse square law, so the force applied to the spacecraft by the earth is proportional to one over the displacement squared

in this case, one quarter squared is one sixteenth, so the force would be multiplied by one over one sixteenth, which is sixteen

the force would therefore be 800 x 16 = 12800N