What force opens a folding knife?

[bklynrob]

If I flick open a folding knife with a flick of my wrist, what force is that?

Is it centrifugal force, centripetal force, or some other force?

 

[AdrianMay]

The way you phrased it, it could be a whole lot of forces. Let's stay in 2D.

Suppose you pull linearly towards your knuckles and keep going at constant velocity. The blade's C of G will want to stay put, causing a moment about the pivot, which can open it.

Alternatively, you could spin your little finger knucklewards keeping the pivot in one place. That can also open it. Or give a push the other way to get the blade moving and then reverse the handle.

If you were nifty with your kit, I reckon you'd also take advantage of the ballet dancer's trick of pulling their arms into increase angular velocity. I think that would mean moving the pivot towards a place just behind the backbone of the blade once it was already moving.

But by this time you'd already have a bottle in your face, which is what I would have used in the first place.

Happy drinking,
Adrian.

 

[bklynrob]

Adrian, thanks. I'm sure I worded my question poorly. Here's another try: there are certain folding knives (knives with blades that fold into the handle via a pivot on one end) which have a blade that is loose enough that, when you hold the handle and flick your wrist, the blade will snap out and lock into place.

Some call these "gravity knives" but I'm certain the term is wrong. Gravity plays no role in the blade opening, right? Some laws say these knives open with centrifugal force, but I'm not sure that's right either. It seems to me, as a layman, that the force of flicking your wrist coupled with inertia or centripetal force is what causes the blade to snap into place.

 

[PeterO]

If I flick open a folding knife with a flick of my wrist, what force is that?

Is it centrifugal force, centripetal force, or some other force?

Due to there being no such force, you can be certain that it is not centrifugal force - though many a non-physics person will claim it is (even a knife manufacture/advertiser may be tempted).

I think it is most likely the inertia of the blade that results in it to opening. You force the handle to accelerate very quickly, and since there is insufficient force transmitted to the blade, it is "left behind" until it snaps into the "open" position,at which point the handle supplies sufficient force to let the blade then keep up with the handle.

 

[asteriatic]

The force that opens a folding knife with a flick of the wrist is a combination of both centrifugal and centripetal forces. When the knife is flicked, the force of the flick creates a rotational motion, which is known as centrifugal force. At the same time, the hinge of the knife provides a centripetal force, pulling the blade outwards and causing it to open. Without the hinge, the centrifugal force alone would not be enough to open the knife. Therefore, it is the combination of both forces that allows the knife to open with a flick of the wrist.