what is the equation for the law of inertia

Answer 1

Newton's first law, or the law of inertia says that an object at rest will remain at rest, or if it's in motion, it will remain in motion (at a constant velocity) unless it is acted on by an external force.

So, although we don't typically use an equation to say this, I supposed you could say, when the sum of the external forces is zero, there is no change in velocity or:

When ∑F_ext = 0, then Δv/Δt = 0