If you are 80 kg I guess it would be like removing an 8kg backpack. I think you would notice that immediatly.

My mother would notice immediately ! Scale !

How fast is "suddenly?" @D.J.Klomp is right, 8kg is about 18 pounds. If "suddenly" was a tenth of a second, that would be like 18 pounds being dropped on your feet. That's hurt like the dickens. It would knock people down. Cars would be pressed against their shocks and springs, which would then release. Exciting! But if "suddenly" is over 10 seconds or minutes... the drama would decrease a lot. So, what exactly does "suddenly" mean?

@JoinJBHonCodidact if you're reducing gravity suddenly it would be like you have 8kg lifted off your feet. Still enough to jerk you awake or push people off balance though.

This gravity change.. are you referring to a change in G or a change in g ? Does the planet weight change (g), or the laws of physics (G) ? I ask this because if you change G 10% down, Earth will go to a wider orbit and we could loose the moon (minor points of course)

I am curious: what is the actual worldbuilding context for this question? It seems like a bit of idle curiosity to me. A purely Real World Question that is much more suited to Physics.SE than here. Related: why the magic tag? How do the tag and the question fit together?

@elemtilas physics - or geology - does not do "what would happen if .." questions tagged "magic", especially if there is no plausible cause stated. In theory, a abrupt change in gravity would cause enormous energy release, it could have very unpredictable effects and there would be no underlying "hard-science" explanation for the change at all. Such questions are referred to WB.

@Trioxidane Hah! Total Protonic Reversal! You're right! I misread the statement. That means everybody would pass out (if not die from a gas embolism) - but the cars would still hop from the weight suddenly lost off the springs and shocks. Hah!

@Goodies We don't do "what would happen if..." questions either (current status notwithstanding). Per the help center, "To prevent your question from being flagged and possibly removed, avoid asking subjective questions where you are asking an open-ended, hypothetical question: 'What if ______ happened?'" Having just completed a summary of "the rules", I suddenly realize this question is badly formed and should be closed.

well the planet would explode, people would notice that, briefly

The first moment of the feeling on your body (not considering the effect on the surrounding) would be the same as if you are in an elevator which starts going down at acceleration of approximately 1 m/s² (1/10 of the free fall acceleration).

Even a semi-gradual change, measured in minutes, would have significant atmospheric effects. The atmosphere is compressed by it own weight. reduce that to 90%, and suddenly there is an unbalanced force of 10KPa = 1 TONNE per square meter accelerating the air upwards, until a new equilibrium is established. Expect some very significant and interesting winds to fluff around while the new normal is achieved.

The short answer is: It would feel like an elevator when it starts to descend. That's it. No biggie, it probably wouldn't wake up most people. OTOH, living in a world with 10% less gravity would be interesting (for instance, the safe upper speed of ground vehicles would be 10% less).

Note: to answer earlier question of @JoinJBHonCodidact.. suddenly would definitely not be instantaneously. It can't. Say it affects the planet (g) Light takes time to travel through planet Earth, change of gravity takes place in spacetime, nothing faster than light. Suppose this bizarre magic is some kind of dampening of gradients in space-time, lowering local gradients around Earth, and as a result a lowering of g, this dampening field would cross planet Earth in 10.6 ms, possibly slower, quora.com/…

Correction: this amount would be 42ms, not 10.6 according to the quora link.

@Goodies Your point is well taken, but you can't answer my question. Only the OP can. (And since it's the OP's fictional world, it can actually violate the limitation you proposed. See Real life cannot be an overriding limitation on any question unless specifically requested. The process of making that request is usually by applying the hard-science tag.)

Agree, with magic anything goes.. it's a pity opener did not work out this interesting question.

Yesterday evening I was brainstorming with a physics student about this question.. his remark was about "magic" and the law of energy preservation. You could hand-waive the speed of light, because of magic, but is it possible to hand-waive energy conservation laws too ? When all weight goes down, a lot of kinetic energy of Earth would get lost in a very small time. Where is this energy going ? Wouldn't this cause heat dissipation inside the planet, during the event ?