diff --git a/_psets/4.md b/_psets/4.md
index 273dc79142e090c612b169cfab497bc446f8ced9..6826e354973687f0a660c1b8d68c466ec5521d8b 100644
--- a/_psets/4.md
+++ b/_psets/4.md
@@ -374,7 +374,11 @@ z component, but we won't end up needing it so we won't give it a name.)
 
 Now calculating the force on the loop is easy. The z component of $$B$$ doesn't matter since it just
 pulls the loop outward. And the radial component produces a force along z. So the force is $$-2 \pi
-r I B_r(z)$$.
+r I B_r(z)$$. So when the scale is balanced
+
+$$
+mg = -2 \pi r I B_r(z)
+$$
 
 But how can we calculate the flux? Consider a z-aligned cylinder of radius $$r$$ and height $$z$$,
 with the center of its base at the origin. The magnetic flux through the bottom is some constant
@@ -422,8 +426,9 @@ the same time?
 Well the voltage is only generated when the coil is moving, but the current is defined as that which
 keeps the coil still. So you can't do both at the same time. You could start the coil moving and
 then measure the current that keeps it moving at a constant velocity, simultaneously measuring the
-resulting voltage. But then the voltage is a combination of the applied voltage and the induced
-voltage, and ditto for the current.
+resulting voltage. But in this scenario some of the current is probably being used to overcome
+friction in the mass balance, rather than just counteract gravity. So you'd have to have a really
+accurate model of that, which isn't practically possible.
 
 At the end of the day the point is to have the effects of the magnetic field cancel out, so it
 doesn't need to be perfect as it did in the previous definition of the ampere. In other words, we