Answer:
1. Largest force: C; smallest force: B; 2. ratio = 9:1
Explanation:
The formula for the force exerted between two charges is

where K is the Coulomb constant.
q₁ and q₂ are also identical and constant, so Kq₁q₂ is also constant.
For simplicity, let's combine Kq₁q₂ into a single constant, k.
Then, we can write

1. Net force on each particle
Let's
- Call the distance between adjacent charges d.
- Remember that like charges repel and unlike charges attract.
Define forces exerted to the right as positive and those to the left as negative.
(a) Force on A

(b) Force on B

(C) Force on C

(d) Force on D

(e) Relative net forces
In comparing net forces, we are interested in their magnitude, not their direction (sign), so we use their absolute values.

2. Ratio of largest force to smallest

We can't tell without more information. We know it will be higher than 40 and lower than 75, but we don't know exactly where it will settle. In order to work that out, we would need to know the volumes of the water and the cube, and WHAT metal the cube is made of.
Gravity adds 9.8 m/s to the downward speed of a falling object every second. (On Earth.)
In 7 seconds, gravity adds (7 x 9.8 m/s) = <em>68.6 m/s</em> to the downward speed of a falling object.
(IF air resistance has no effect on the object.)