How gravity gets weaker with distance

Two masses of $800\ \text{kg}$ and $1200\ \text{kg}$ pull on each other. You will make the program find the force at $1\ \text{m}$, then at every distance from $1\ \text{m}$ to $6\ \text{m}$, and find the first distance where the force has fallen below a tenth of its value at $1\ \text{m}$. Then it draws the force against the distance.

The idea. $F = \dfrac{Gm_1m_2}{r^2}$: twice the distance gives a quarter of the force, three times the distance a ninth.

Your tasks.

1. Work out F1, the force at $1\ \text{m}$, in newtons.
2. Inside the loop, work out F, the force at the distance d.
3. Find far, the first distance on the list where the force is less than F1 / 10.