Q.Attach a suitable mass to spring balance so that it stretches by about half its capacity. Now whirl the spring balance so that the mass performs a horizontal motion. You will notice that the balance now reads more mass for the same mass. Can you explain this?

While whirling, the spring must supply the centripetal force IN ADDITION to what it read at rest. …

At rest the spring stretches just enough for its tension to balance the weight it reads. Once the mass is whirled in a (nearly) horizontal circle, the mass is continuously accelerating towards the centre, so the spring must pull it inward with an additional force $mr\omega^2$ beyond the static stretch. A spring balance displays whatever tension it carries — so it shows a LARGER apparent mass. Viewed from the rotating frame instead: the mass is at rest there, but the outward centrifugal pseudo force $+mr\omega^2$ acts on it, and the spring stretches until its tension balances this extra outward pull — the same larger reading, explained in either frame. …

Quantify it: if the balance reads $m_{app}$ while whirling at angular speed ω with the spring nearly horizontal at radius r, then $m_{app}g \approx \sqrt{(mg)^2+(mr\omega^2)^2} …

Saying the mass 'becomes heavier' — the gravitational mass is unchanged; only the spring TENSION (what the instrument displays) has grown, because it now does …