FFVII Cloud's Buster Sword Busted in Ludicrous Video

[William T]

So, I opted not to post this in Science Fiction and get my thread locked~



I watched this video and it seemed quite ludicrous to me. The man in the video measured sword speed (which I assumed was angular velocity) with mph.

He then continued to say that linear momentum applies when swinging a sword.

Last but not least, he expands on centrifugal force and gives an equation for centrifugal force, and uses it to debunk this science-fictitious sword. However, in my one community college physics class, I learned that centrifugal force is not a real force. I forget whether it's a combination of two forces, or just one force and a pre-existing velocity on the object.

What are your expert opinions on this video?

 

[Drakkith]

Hi William T!

Your thread was unlocked but I've unlocked it. There was some questions as to whether to allow your post since you're essentially asking about a fictional character swinging a sword. After some discussion, the mentors decided that as long as this thread focuses on the physics-related questions regarding centrifugal force and momentum then it can stay open. It's already obvious that no one could ever wield a sword like that in real life, so there's no need to discuss whether it's possible.

It would help if you could come up with some specific questions base off the video instead of asking us to watch the whole thing. I doubt most people want to sit through a 10 minute video just to find out where it might go wrong.

 

[russ_watters]

Welcome to PF!

So, I watched a decent fraction of the video and here's my impression:

I watched this video and it seemed quite ludicrous to me. The man in the video measured sword speed (which I assumed was angular velocity) with mph.

He then continued to say that linear momentum applies when swinging a sword.

Presumably, you are suggesting that because there is rotation, linear speed and momentum do not apply. That's not entirely true and in addition the movement may be more complex than at first glance.

When swinging a sword, tennis racket, baseball bat or golf club, you combine multiple rotations and linear motions of different parts of your body. You rotate at the waist (hips), shoulders and wrists at the very least. Ultimately, though, the best results often occur if the tool is moving more linearly than rotationally as it hits what you are swinging at. In baseball, they call that "staying in the hitting zone". That comes from sliding your hips forward over your legs and shifting your weight from the back to the front leg. But either way, due to the multiple rotations the path of the tool tends to be straighter than you might think.

And even if it were true that you could model it as a pure rotation, you could still calculate the linear speed/momentum at the point of contact. Again, in baseball, this is how you differentiate between getting "jammed" and hitting further out on the barrel: the bat is moving at a higher linear speed further out, generating more power.

The bottom line, though, is that an object moving in a circle still has a linear speed -- or, rather, each point on the object has its own linear speed.

Last but not least, he expands on centrifugal force and gives an equation for centrifugal force, and uses it to debunk this science-fictitious sword. However, in my one community college physics class, I learned that centrifugal force is not a real force. I forget whether it's a combination of two forces, or just one force and a pre-existing velocity on the object.

We get this question about once a week and it is caused by introductory physics professors using a popular but misleading word choice to describe centrifugal force as "fictitious". It is called that because its source is reference frame dependent, but if you choose the rotating frame it is very much real. But even in the stationary frame, you just have to make sure you call it by the proper name in that frame: centripetal, not centrifugal (in one frame it points away from the center and in the other it points toward the center). There are some subtleties to it, but ultimately it is the same force: it has the same value whichever way you look at it.

If your general question is whether the video seems plausible? Yeah, it does to me. I didn't get to the end, but presumably he's trying to prove that no human could wield that sword usefully. I agree.

 

[William T]

But what or where exactly does the sword go wrong, and how can I illustrate that? I'm not at all disappointed that the sword will never work in real life - however, I am disappointed by the way he explained and erroneously shoved numbers into an equation.

Or is it that because I took an introductory class, I have no understanding of how to visualize what he did in a rotating frame of reference, and it's confusing the heck out of me?

Here are the dimensions he gives of the sword:

Total Length: 6 ft. / 180 cm
Blade Width: 12 in / 30 cm
Weight: 75-80 lbs / 35 kg
Grip Length: 1.5 ft / 45 cm
Blade Length: 4.5 ft / 135 cm
Sword Speed (?): 40 mph / 65 kph / 18 m/s

He then gives this equation for Centrifugal Force (supposed to be Centripetal, of course):

Mass x Velocity²
----------------------------------------
Radius

I would think you'd have to use an equation for torque, but I don't know? And why did he divide Mass and Velocity² by radius instead of distance traveled?

Though it's probably a terrible sword design and no human could wield the sword usefully, I really don't think someone would have to weigh 2750 lbs just to swing a bench press bar with a 30 lb weight on one end and a lot more surface area and volume, and that's just to swing it at all.

 

[russ_watters]

What equations and numbers you use depends on what you want to calculate -- what, exactly, do you want to calculate?

 

[William T]

I realize that when physics enters the realm of the human body, it becomes more difficult compared to mechanical physics.

What is the easiest calculation I could do to show that the video's numerical conclusion is wrong while still showing that the sword, if used in a real-life scenario, would be inefficient?