Two small objects each with a net charge of + q exert a force of magnitude f on each other: we replace one of the objects with another whose net charge is + 4q: the original magnitude of the force on the + q charge was f; what is the magnitude of the force on the + q charge now?

The force between 2 charged objects is given by, F = kq1q2/d².

where k ⇒ constant

d ⇒ distant between the 2 objects.

In this case, k and d are constants.

So, let k/d² = x

since q1=q2,

F = xq²

x = F/q²

After replacing q1 with 4q, the new force on + q will be:

F = xq. 4q

= Fq4q/q²

= 4F

The new charge on + q is 4F.

The correct answer is 672