Maybe you’ve seen the trailer for the upcoming Kraven the Hunter movie.

Maybe you have thoughts. 

While comic fans are dissecting costumes, accents, origin, and more, one thing stuck out to me: the crossbow scene right around the 2:30 mark. The character comes in looking for Mr. Taglin. Kraven says, “You’re standing in him,” and fires the crossbow. The bolt slams him into the back wall. Here, check it out: 

Physics teachers can see a conservation of momentum problem coming a mile away, and that’s what this is. 

Momentum is a property that all moving objects have. It’s connected to Newton’s Second and Third Laws of Motion and helps explain gun recoil, collisions between moving objects, Double Asteroid Redirect Tests, and all sorts of other interactions between two objects where at least one is moving. Momentum is also conserved, meaning that the total “amount” of momentum before the interaction must be equal to the amount after the interaction. There are some complications, but that’s the basics. 

Mathematically, momentum is the product of an object’s mass and its velocity, or: 

Momentum = (mass)(velocity), which means its units are the uncomfortable, “kg ⋅ m/s.” Since velocity is a vector (it has a magnitude and a direction), momentum is likewise a vector. 

When classifying interactions, physicists lump them into two broad categories: elastic and inelastic — and explosion, a backward inelastic collision. Side note – for a quick explosion example, check out our article on “Helping Hand” from the first season of Netflix’s Love, Death, and Robots. 

Simply put, an elastic collision is a “hit and bounce” two objects interact, and two objects move in separate directions afterward. Inelastic collisions are “hit and stick” — the two objects connect, functionally becoming one object. 

The First Conservation

The scene in the Kraven trailer shows a classic inelastic collision: the crossbow bolt hits and sticks in the unfortunate visitor. The man and the bolt are now one object, moving with a new velocity. Before the collision, the man had no momentum (he was stationary), while the bolt had it all for this particular system (the guy and the bolt). 

Mathematically, an inelastic collision can be modeled like this: 

m_av_a + m_bv_b = (m_a + m_b)v_f

The velocity of the connected objects (guy + bolt) is identified as “v_f” because it is a new velocity for the guy/bolt combo (one thing in the eyes of physics).

So what I’m interested in is just how fast a crossbow bolt has to fly to throw an average-sized man about two meters backward into a wall. My guess is comic book universe fast, but we can figure it out with numbers. 

I’ll be making a few assumptions here: 

• The bolt weighs 400 grains and looks to have a broadhead tip — 125 grains (1 gram = 15.4324 troy grains). Since this is such close range, I’m not concerned about the specific type of broadhead tip or its penetrating power. It’s close enough to get the job done.
• The distance between Kraven’s crossbow and the poor guy is 2.5 meters, and it’s 2 meters from the guy to the wall. Mr. Taglin had a big office.
• An average bolt fired from an average crossbow wouldn’t see any significant vertical drop in the 2.5 meters. I won’t be worried about that. 
• The target is of average mass, so 75 kg. 
• The bolt impact does not lift him. I’m assuming he hits the wall directly behind him. YMMV, and I can see the case for saying he went up and back in the classic Hollywood “hit by a projectile” style. But after watching the video a lot, it’s not working for me. His feet do come off the ground, though.   
• Finally, and most importantly (and … gross) — the bolt lodged in the guy’s thoracic vertebrae. It’s wildly unlikely, given the speed the bolt must have been traveling at, and in the real world, would’ve blasted through his back, but this isn’t the real world. Given that the guy was pushed back, the bolt must have been pushing against something, and his vertebrae are what’s there. That is unless he was wearing body armor on his back, in which case, ironic death.

Let’s figure out how fast that bolt moved when it left the crossbow. Again, this is a simple, inelastic collision problem. Our variables look like this: 

• Mass_man = 75 kg
• Mass_bolt = 525 grains -> 525/15.4324 = 34.0 grams/1000 g per kilogram -> 0.0340 kg
• Velocity_man = 0.0 meters/second
• Velocity_man+bolt = (by my count, it took him 0.35 seconds to move the 2 meters to the back wall, so the velocity of the man+bolt = 2 meters/0.35 seconds) = 5.7 meters/second.

Setting things up: 

m_manv_man + m_boltv_bolt = (m_man + m_bolt)v_f

(75 kg)(0 m/s) + (0.0340 kg)(v_bolt) = (75.0340 kg)(5.7 m/s)

0 kg · m/s + (0.0340 kg)(v_bolt) = 427.69 kg · m/s

v_bolt= 12579 m/s

That’s pretty fast. About 37x the speed of sound. 28,138 miles per hour. Supposing it had traveled that fast, the bolt would have covered the distance in 0.2 milliseconds. The average human reaction time is 250 milliseconds — a quarter of a second. And it would have been uncapturable by regular movie film – the trailer is 24 frames per second — or a frame every 0.04 seconds or 40 milliseconds. 

Top modern crossbows are just starting to reach 500 feet per second (FPS), about 152.4 m/s (341 mph). Could Kraven have used a heavier bolt and gotten the same effect with a more reasonable initial speed coming out of the crossbow? Sure – but I chose the 400-grain bolt because it’s a standard weight for crossbow bolt calculations. But as with any calculation, go back in with your numbers and try it if anything looks suspicious. But heads up — using a 600-grain bolt with a 125-grain broadhead only slows the initial velocity to 9128 m/s (29948 FPS).  

But yet, here we are. In a comic book universe with the comic book universe physics. It’s a cool effect, right? 

The Other One

While the pinning of the guy was one example of conservation of momentum in the scene, there was another — although it was subtle. Like “not there” subtle.

Before Kraven fired the bolt, that system’s momentum (Kraven, the crossbow, the bolt, the chair) was zero. The bolt had momentum when Kraven fired it (at our assumed ludicrous velocity of 12,579 m/s). But momentum has to be conserved. That means that the momentum of the remainders of the system (Kraven, the chair, and the crossbow) have to have momentum equal to that of the crossbow, but the velocity is in the opposite direction. 

This is an explosion type of “collision” — the whole system’s momentum of zero must be matched by the momenta of the pieces of the system (the bolt) as they travel away from the “new” system (Kraven, crossbow and chair). 

So our numbers: 

• Kraven’s mass: he’s beefy, so 91 kg
• The Ravin 500E (example of a modern, fast crossbow): 3.8 kg
• The bolt: 0.0340 kg
• Herman Miller Embody chair (it’s a fancy office): 23 kg
• The bolt’s velocity: 12579 m/s (above)
• Initial velocity of the system: 0 m/s
• Masssystem = 117.834 kg

Mathematically, an explosion is the reverse of an inelastic collision and looks like this: 

m_systemv_system + = m_boltv_bolt + m_newsystemv_newsystem

(117.834 kg)(0 m/s) = (0.0340 kg)(12579 m/s) + (117.8 kg)(v_newsystem)

0 = 427.686 + 117.8v_newsystem

v_newsystem = -3.63 m/s

The negative sign is essential — that’s what tells us that the velocity of the “new” system is in the opposite direction of the bolt headed toward the guy across the office. 3.63 m/s is about 8.12 miles per hour. As the trailer shows, Kraven doesn’t budge. Why? Friction. There’s friction between Kraven’s boots and the desktop and between the chair’s wheels and the floor, apparently enough to prevent movement. Kraven does not skip leg day.

As always, our physics doesn’t match a superhero universe. 

But this trailer – it just keeps on giving. Find your own wild science moment, apply science to it, and see how it checks out.