The Physics of RWBY- Crescent Rose

The Physics of RWBY- Crescent Rose

Crescent Rose

 

Ruby Rose, the protagonist of RWBY, is Huntress in training using her beloved Crescent Rose when fighting against the Grim found all over Remnant.  Crescent Rose, like many of the weapons found on the world of Remnant, is both a melee weapon and a firearm, in particular a giant scythe that is also a sniper rifle.

Crescent Rose

 

Like Nora Valkyrie, Ruby also uses her weapon to move around the battlefield, which allows me to determine the recoil force of Crescent Rose.  In this case I am using the scene when she is jumping off the ship at the end of Season 3 Episode 12.  It was a bit tricky as she is falling, which made finding all of the necessary data possible, however I did manage it.  Also, while I didn’t run the numbers by a physicist, I did run the basic idea past a physics teacher and he approved of the way I am going about determining the force of Crescent Rose’s recoil.

 

Initial Speed

Since Ruby is falling when she fires Crescent Rose, I first need her speed of descent.  This is actually pretty easy since it’s 5 seconds from when RWBY jumps off the side of the ship to when she fires Crescent Rose.  I am going with a horizontal jump instead of a vertical jump up or down off the plane.  If she made a vertical leap, then her descent does not begin until she reaches the apex of the jump, which is impossible to tell from the episode.  If Ruby made a jump propelling herself downward, there would be extra velocity, which again is impossible to tell from the episode.  In this case I am going to use the second velocity equation from my previous RWBY post; additionally, the acceleration I will be using is the acceleration due to gravity or 9.8m/s^2 since Remnant is assumed to an Earth-like planet.

Acceleration = (Final velocity – initial velocity) / time

-9.8m/s^2 = (Final velocity – 0)/ 5s

-9.8m/s^2 = Final velocity/ 5s

-9.8m/s^2 * 5s = Final velocity

-49m/s = Final velocity

FYI- The velocity is negative because she is falling

 

Mass

The next piece of information needed to solve our little physics proble is the combined mass of Ruby Rose and Crescent Rose.  The mass of Crescent Rose was again fairly easy to determine since it is based on a M82 sniper rifle, which has a mass of 14kg.  However, Crescent Rose is also a massive scythe so I think it’s safe to round the total mass up to 20kg.  According to the RWBY wiki, Ruby’s height is 1.57m (5’2”); using this and the fact that she is a 15-year-old girl training to be a Huntress, I can estimate her mass to be 59kg, which is the upper range for a healthy girl.  That being said, I received some comments about making Nora too light in my previous RWBY post, so let’s assume that Ruby has more muscle mass due to her training, putting her in the overweight category with a mass of 64kg.  Add in some clothing and this brings Ruby’s and Crescent Rose’s total mass 85kg.

 

Time

I also need to know how long the explosion from a gunshot lasts, and I am going to use the 300 microseconds number from the previous RWBY post.

 

Impulse

Initially I thought this would be all I needed to figure out how powerful Crescent Rose is, but I hit a little snag in that I forgot to account for the momentum Ruby would have after falling for 5 seconds.  Thus, I had to do a little research and refresh my knowledge of physics, specifically impulse.  Impulse is important when looking into how objects change direction; specifically it means:

 

An impulse is equal to the net force on the object times the time period over which this force is applied.

 

The formula for impulse is:

Force * (Final time – initial time) = mass * (Final velocity – Initial Velocity)

Force * (0.0003s – 0) = 85kg * (Final velocity – 49m/s)

 

And now you can see the problem: I don’t have the velocity Ruby is moving at after she fires Crescent Rose.

 

Final Velocity

While difficult, it is possible to determine the speed Ruby moves at after firing Crescent Rose.  During her falling sequence in the episode there is a moment when she travels across the ship in the background after firing Crescent Rose, which gives us a measuring stick to determine her velocity after firing.

Crescent Rose

 

If we use the light blue lined section and Ruby as a measuring stick, we can determine her velocity.  Ruby travels in a straight line from top to bottom in the direction she is pointed across the segment.  The tricky part is figuring out how long Crescent Rose is, which seems to fluctuate.

Ruby Rose

 

This was the best picture I could find and here it seems Crescent Rose is 84.5in long, which translates to 2.2m.  If you check the picture above, it appears like the segment is 2 Crescent Roses long, which means the distance she traveled is 4.4m.  The time it takes her to travel this distance is 1s.

Velocity = distance/time

Velocity = 4.4 meters / 1 second

Velocity = 4.4m/s

 

Back to Impulse

Now I finally have all of the pieces of the puzzle, so I can finally figure out how strong Crescent Rose is.  Just to recap, Ruby is falling and fires Crescent Rose, and if I know how Ruby is moving before and after she fires, I can figure out how strong her weapon is.

Impulse Formula

Force * (Final time – initial time) = mass * (Final velocity – Initial Velocity)

Force * (0.0003s – 0) = 85kg * (4.4m/s – 49m/s)

Force * 0.0003s = 85kg * (4.4m/s – -49m/s)

Force * 0.0003s = 85kg * 53.4m/s

Force * 0.0003s = 4,539 (kgm)/s

Force = 15,130,000N

 

Well, if you thought Magnhild was crazy, consider that Crescent Rose is 5.8 times stronger with a recoil force of 15,130,000N.  This means Crescent Rose is over 8 times the power of the space shuttles main engine at lift off.

 

Conclusion

I hope you enjoyed another foray into the Physics of RWBY and I think it’s time we rethink just who the heavy hitter of the Remnant is.  With a recoil force over 15 million newtons Ruby must be insanely strong in order to use Crescent Rose, otherwise she would be turned into paste.  This also explains why she routinely uses the scythe section of Crescent Rose as a stabilizer when firing.

Weiss

 

Leave any comment below and stay tuned as I think I found a scene I can use to determine the recoil force of Yang’s Ember Celica.