Homework Help: What force opens a folding knife?

1. Feb 28, 2012

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?

2. Feb 28, 2012

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 in to 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,

3. Feb 28, 2012

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.

4. Feb 28, 2012

PeterO

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.