When something gets "stuck", whats going on?

CuriousBanker

Hi,

So I see things in real life, and wonder what is going on physically. I question things that even seem intuitive. I know most of my questions are stupid, but I don't know much about physics. I just bought "conceptual physics" by hewitt on ebay for $7 shipping included. I can't read it yet though because I am studying for my CFA which takes up most of my time.

Anyway, I was at a renaissance fair today, and there was a knife thrower. The tip of the knife strikes the wooden wall, and the knife gets stuck in the wall. But how? I know if the entire knife went in the wall, obviously gravity is pulling it downward and it is resting in the wall. But if only the tip of the knife penetrates the wall, and the other 95% is suspended in the air, how does it not just fall to the floor? Why wouldn't the force of gravity pull the exposed part to the floor? Is there force of gravity between the sides of the knife and the walls touching it overcoming the gravity of the earth since they are at such a close distance?

jedishrfu

have you ever stabbed a knife into a watermelon or something similar and then pulled it out. a suction is created that must be overcome to finally remove it. In a similar fashion, the tip is stuck in the wood and the frictional force between the metal and the wood is enough to hold it in place against the force of gravity trying to pull it down. When the knife penetrates the wood the wood is pusshed to the side and then springs back to make a good seal on the blade.

This frictional force can be quite strong. Many times, you'll see knife throwers jiggle the knife back and forth to loosen the frictional grip ( makes the knife cut a little bigger. and finally remove it.

CuriousBanker

Oh ok. i was wondering what the force was...so it's friction?

I guess I should really just pick up that physics textbook...

davenn

walls are usually vertical so gravity, I imagine isnt playing much part in the knife staying in the wall. rather gravity would be trying to get the knife down to the lowest place ... namely the floor.

the thing keeping the knife in the wall is the adhesion and frictional forces between the knife blade and the wall material. Imagine the wall being made out of different materials
and think about how the strength and density of those different materials would affect the friction between the 2 surfaces. High friction ... the knife stays in there, low friction the knife falls out ... gravity overcomes the frictional forces

just my thoughts :)

one of our guru's may light to put a few physics formula into the situation :)

Dave

Lsos

Gravity is a very weak force that exists between masses. It's so weak that it's usually only a concern when we're dealing with astronomical-sized objects (such as the earth attracting the knife). However, the gravitational force between the knife and the wall is quite insignificant. I would guess that even a single bacteria would be strong enough to overcome it...

mikelepore

I would add these little details:

It is static friction, the kind of friction that exists when two surfaces are not sliding past one another.

It is a different mechanism from kinetic friction, the kind of friction that you have when sliding is taking place.

Static friction is produced by the overlap of the electron clouds in the two surfaces, much weaker than covalent bonding but related to it in origin.

The force that the wall exerts on the knife is called normal force because the force is perpendicular to the surfaces, and normal means perpendicular.

The two surfaces in contact are characterized by a number called the coefficient of static friction, which is determined by what materials the wall and the knife are made of, as well as smoothness or roughness.

The magnitude of the static friction is calculated by multiplying the coefficient of static friction by the normal force.