This isn't it. The shape of the handle is orthogonal to whether a scissor is left or right handed. The real difference is in which blade half is on the bottom.
> Right, because which blade half is on the bottom affects which direction you need to apply force so the blades meet tightly.
That would violate Newton's Third Law of motion, so no.
You don't have to apply force in any different direction if you put scissors with symmetric handles in the wrong hand. You will, however, end up tilting the scissors to the side so that you can see what you're doing. But that's not the same as needing to apply force any differently. It's a required sight issue not a required force issue. Anything else only comes from handle shape.
If the scissors don't have ergonomically sloped handles, an ambidextrous person can comfortably cut with either hand if they don't look at what they're doing.
Have you watched little kids learn to use scissors? You really do need to apply force to bring the blades together or they won't cut well. You likely do this unconsciously. I didn't think about it until I was teaching my children how to cut with scissors and noticed that they would generally end up folding the paper were I could cut it (toddler scissors aren't very sharp, so it's even more important to push the blades together from the side).
It depends on the scissors. With the pair I'm holding right now, even if I'm actively pushing so the blades would move apart, they stay tight enough to cut fine.
Notably, the blades are curved into each other, which is capable of compensating for a moderate amount of hinge slack and narrowing the cutting to a single point. And the hinge has very little slack in the first place because it has a big flat mating surface.
For precision made scissors with tight tolerances at the pivot joint and sharp blades (premium brands like Kai, Gingher, etc), it will cut with either hand.
But for scissors with a loose rivet (like cheaper scissors very common in kids' schools), the gp (ianferrel) you replied to is correct: the blades will not close tightly if the wrong hand is used to squeeze. The loose rivet scissors in the wrong hand will fold the paper instead of cutting it because the natural finger-closing motion will spread the blades apart creating a tiny gap instead of making them touch.
Here's a 1-minute video that tries to visually explain what a left-handed person struggling with a right-handed scissor experiences: https://www.youtube.com/watch?v=wyGCvSCnkWk
Scissor ergonomics aligns the handles so applying the finger-to-base-of-thumb grip to the handles (with the upper handle pressed against the outside of your thumb’s knuckle) pulls the blades together. In order to do that, scissors have to be designed to fit either a left or right hand.
To use scissors wrong handed you have to catch the outside of your finger knuckles against the lower handle and pinch the upper handle down with your thumb.
Yes! You do!
So scissors hinge on a pivot point, and they hinge in two dimensions. They mostly move in the dimension that they cut in, but they also move a little in the perpendicular dimension. And in order to bring the cutting edges closer, you move the handle ends farther away. If you're using the correct-handed scissors, doing so is easy and natural because your hand grip curls one way easily. If you then switch the same scissors to the other hand, it's hard and uncomfortable because your other hand is a mirror image and your natural hand grip curls to bring the handles closer and the cutting edges farther away.
The fact that you keep mentioning that this violates some law of physics is bonkers.
What makes you so sure? The ergonomics of the thumb, fingers, and hand are not symmetric. The left-handed people who struggled with right-handed scissors as kids suggest otherwise.
When holding scissors, the bottom blade is held fairly steady. The top blade tends to get pushed away from the rest of the hand by the action of the thumb. If the top blade is on the far side of the scissors (away from the hand), the far blade edge will get pushed towards the near blade edge when the thumb handle is pushed away (because the blade is on the other side of the hinge from the handle). If the top blade is on the near side of the scissors (the hand side), by pushing the thumb handle away from you, you are separating the two blades rather than pushing them together.
So on right handed scissors, with the thumb handle facing up, the thumb blade is on the left side of the hinge. That way the natural "push away" force from the thumb will keep the blades close together when held in the right hand. On left handed scissors, the thumb blade is on the right side of the hinge, for the correct push-together force when held in the left hand.