Speed of throwing knives

[NotaNuffian]

So I was doing my usual BS and have a thought all of a sudden. What if my MC can throw small things fast enough to impale foes from afar. I had the poor guy throw boulders, tree trunks, fist sized stones, carts and other big stuff. Had the guy even lobbed spears too.

But I am greedy, I want the guy to lob something small and puncture a person too, like the gun he has in his hands.

So I picked a simple throwing knife, tip area assume to be 1cm^2, mass is 20g and human puncture pressure to be 100 PSI or 690 kPa. I got the initial velocity (throwing velocity) be more than mach two for a travel time of two seconds. Can anyone here help me check the math?

 

[MasterHiatus]

No math here, I suck at it but, the speed and force of the knife can change depending on the force put on it. If you're mc can throw big stuff, then you can be assured that it will be plausible.

 

[MagicalDucky]

I think the reason you got such a high initial velocity is that the theoretical measurements for your throwing knife look way off. Assuming that the throwing knife you're thinking of is an average-sized one made of metal, the mass would probably be more along the lines of 250 to 300 grams (according to Google), which is over 10 times your estimate of 20 grams. The area of the tip (I'm assuming this is the tip of the knife where it makes contact first when thrown and actually punctures, which would essentially just be a point) would be way, way smaller than 1cm² if the knife was even remotely sharp. Probably more along the lines of 1μm² for a properly sharpened knife. I'm not sure what formula you used to calculate your initial velocity though, so I would try redoing the math with different measurements and see if that changes the initial velocity you get.

 

[GloriouslyFamous]

The best way to work out such a problem should be to find the force at impact.
Force = Mass x Acceleration
F= 0.02kg x 333.3m/s2 ~= 6.66 N
Pressure = Force / Area
P = 6.66 / 0.0001 = 66,600 Pa = 66.6 Kpa

The problem is the weight of the blade 20 g is too little. (More accurate results should be between 200 - 800 g.) Also, speed of sound changes based on temperature and location and this assuming that the impact happens after 2 seconds.

 

[GloriouslyFamous]

The best way to work out such a problem should be to find the force at impact.
Force = Mass x Acceleration
F= 0.02kg x 333.3m/s2 ~= 6.66 N
Pressure = Force / Area
P = 6.66 / 0.0001 = 66,600 Pa = 66.6 Kpa

The problem is the weight of the blade 20 g is too little. (More accurate results should be between 200 - 800 g.) Also, speed of sound changes based on temperature and location and this assuming that the impact happens after 2 seconds.

Also, as MagicalDucky pointed out, the tip should be much smaller.

 

[Jemini]

You also gotta consider your throwing technique in this. The carnie method (timing the spin) can launch the blade at a far higher initial velocity, but it can only hit accurately at an exact pre-determined range. The REAL method, reducing or eliminating the spin with a specialized throwing technique, sends the blade off at a far slower speed and is good for about 50 foot distance max if you are an absolute master at it. (I believe 40 feet is the record consistently accurate hits, a 50 foot distance is hitable but there will be a sharp drop-off of accuracy involved.)

That's in the real world. Some kind of super strength can send a throwing knife farther than 50 feet at a faster speed, but only if the person with super strength has trained with their extreme strength to get the technique down.

 

[NotaNuffian]

You also gotta consider your throwing technique in this. The carnie method (timing the spin) can launch the blade at a far higher initial velocity, but it can only hit accurately at an exact pre-determined range. The REAL method, reducing or eliminating the spin with a specialized throwing technique, sends the blade off at a far slower speed and is good for about 50 foot distance max if you are an absolute master at it. (I believe 40 feet is the record consistently accurate hits, a 50 foot distance is hitable but there will be a sharp drop-off of accuracy involved.)

That's in the real world. Some kind of super strength can send a throwing knife farther than 50 feet at a faster speed, but only if the person with super strength has trained with their extreme strength to get the technique down.

Yeah, technique... I think I forget to mention that my MC is a moron, as stated by the entire cast including his mentor, so if I bother with the scene, will be just him grabbing the blade like a baseball and throws it.

 

[Jemini]

Yeah, technique... I think I forget to mention that my MC is a moron, as stated by the entire cast including his mentor, so if I bother with the scene, will be just him grabbing the blade like a baseball and throws it.

Well, that is most likely only going to land in such a way the blade would pierce the target around 5% of the time or less if that's the case. 30% of the time it will be hilt first, 50% of the time it will hit sideways and do the least damage of any position it's capable of landing, and 15% of the time it will hit with the blade sorta forward, but only enough for a glancing cut and not actually sinking the blade into the target.

 

[ChronicleCrawler]

The best way to work out such a problem should be to find the force at impact.
Force = Mass x Acceleration
F= 0.02kg x 333.3m/s2 ~= 6.66 N
Pressure = Force / Area
P = 6.66 / 0.0001 = 66,600 Pa = 66.6 Kpa

The problem is the weight of the blade 20 g is too little. (More accurate results should be between 200 - 800 g.) Also, speed of sound changes based on temperature and location and this assuming that the impact happens after 2 seconds.

Too detailed! Even so, I like the logic within. Okay kids, let's do some physics. Mehehe.

 

[MasterHiatus]

Too detailed! Even so, I like the logic within. Okay kids, let's do some physics. Mehehe.

Me, who's been tortured to study science: No, please make it stop!

 

[NotaNuffian]

Well, that is most likely only going to land in such a way the blade would pierce the target around 5% of the time or less if that's the case. 30% of the time it will be hilt first, 50% of the time it will hit sideways and do the least damage of any position it's capable of landing, and 15% of the time it will hit with the blade sorta forward, but only enough for a glancing cut and not actually sinking the blade into the target.

Ok, now I feel like writing about the hilt went in and then the MC grabs the poor fool and use him to stab others