How is a Manu Bomb So Powerful?

I show you how a Manu, or water bomb creates a water jet (Worthington jet) that is so powerful it can send a ball shooting into the sky.

[leafmanwithafryinpan]

"Did you need the electromagnet to drop the metal ball?"
ActionLab: "Yes"

[dr.jackauty]

There is nothing I love more than the Manu getting the international attention it deserves.

[fabianstriebeck]

gotta love a person who can explain science so well, thank you. you make it very easy to understand.

[DPDK]

When I was growing up we had two things we did from the high board (3 meter). One was a "cannon ball" which was the same principle as the steel ball. The chubby kids were really good with a cannonball. It also was a way for kids who didn't know how to control their position in mid air to really hurt their backs. The other was a "can opener". This, was a far more effective splash for people that actually knew how to do it. It is basically the same as the "manu". The trick is to lean back as you enter the water. Instead of grabbing your knee at the height of your jump, you wait until just before you reach the water. By bringing one knee up towards your torso your body begins to rotate backwards. Your extended foot pierces the water so that as you flatten your back, the water doesn't slap it as it would in a belly flop. If timed just right you could soak the lifeguard sitting 30 feet away and 10 feet in the air. The most addicting thing about doing this is the intense concussion of water you feel as the bubble collapses around you. A good can-opener rattles your head. You know you did it right if your body abruptly and completely stops moving down through the water just a couple of feet below the surface. You actually hear the concussion like an underwater bomb. "sssSSS-BOOM"

[ashvw]

I never knew that the Manu was a thing. I've been doing them for about 35 years, made it up when I was a teenager. I've always amazed all my friends and family at how big of a splash I could make.

[jamesbarisitz]

We did these at the public pool. There's two versions. The can opener, which is a cannonball holding one knee only and keeping one leg straight out in front. You have to hit the surface tilted back at about forty degrees. The second was the chair. After springing off the board you a assumed the back lean like the can opener but froze in a seated position with your arms tucked into your sides with your palms covering your eyes. Getting the angle right doesn't take long. You know you did it right when you hear the tremendous thump of the water cavity collapsing underwater.

[samhodge]

You've just solved an old childhood mystery. My brothers and I would have "highest splash" contests, and sometimes we'd generate HUGE splashes seemingly at random. None of us were able to narrow down exactly what we were doing to make them.

[jaxkovak]

I worked as a lifeguard in a pool for quite a few years and we were always messing about doing "Bombs" from various diving boards to see who could make the largest splash. Its interesting to see the actual physics behind it.

[eugenelebedev]

Interesting. At the pool we would always do a "cannon ball" and make a big splash. But the biggest splash was made by a "can opener". I would jump in feet first, tuck one knee in, and on impact lean back to hear a loud "boom". That's when I knew the splash was really high. Knowing how to control it allowed you to direct it and... make the lifeguard wet.

[mattp]

One thing I love about Action Lab is you never know what new topic it’s going to cover. It’s almost always something fascinating that I didn’t know was a thing. And, of course, explained so the physics can be grasped by a non-physicist.

[discoisdead]

I use to Manu bomb as a kid all the time. My technic was to jump in like you're sitting on a chair, slightly laid back, then when submerged in the water flatten out such that you make a bigger hole for the resulting upward rush for the splash. If you do it correctly, the splash should sound like a loud thud from the shock wave (especially in a pool). This bomb technic is a very Kiwi thing, especially among Maori children.

[joho]

I just realized during your ball drop experiment that this is a great analogy for what happens during a lightning strike. The initial bolt rips the air apart and creates a cavity in the atmosphere, and when it slams back shut, the "jet" of air it creates is perceived as thunder.

I wonder if a literal jet of air is created in the upper atmosphere and if this could have anything to do with sprites, jets and elves (the atmospheric anomalies, not the creatures).

[Knownasnemoo]

The Slow Mo Guys did an amazing experiment inside a wave simulator that has complete round basin that can generate waves from all sides, and they do this, not by dropping in a round object, but to crash precise waves into the center, giving the same effect of a very high jet.

[Washyourbum]

A bomb that goes a bit higher than a manu is an easy one called the coffin, you jump in feet first but as soon as your feet touch the water you lean back as fast as you can

[russchadwell]

I'm amazed he hit the falling ball with the up-shooting water.

[timothygooding]

2:37 funny enough a HEAT round from a tank or RPG works somewhat similar to this, just instead of water collapsing back together its an explosive forcing a copper lining into one point. Both things concentrate all the energy into a single point to create an extremely fast jet simply due to the geometry

[michaelharrison]

I can remember as a kid growing up my family would visit the Okere Falls near Rotoiti (New Zealand) and there is a place here that all the locals used to jump of a high cliff into the river below where the water was very deep. I took a lot to build the courage to jump this height and at this height you made sure you kept your body straight and upright and hit the water feet first. I can remember one day seeing a group of local Maori teenagers who decided to manu from this jump height.

[johnloosemore]

This seems closely related to how a shaped charge explosive can penetrate crazy thicknesses of steel plate. It always amazes me how a relatively small diameter hole can keep going and going through a couple feet (or more) of steel, without even "mushrooming" out and dissipating.

[shelfridges]

This also perfectly explains the toilet splashback bidet effect.

Nice work!

[blkcc]

As a kid (now 56) my friends and I would do what is called a "jackknife" from the high dive and could aim our at the lifeguard on duty! Same principle, hit the water with your body at about a 45* angle with one leg bent up towards your abdomen held by your hands and when your foot touches the water you quickly lean back to create the same motion and effect as mentioned in this video. Good explanation of what my 12y/o self did so many years ago. Even if the science behind it didn't matter to me before, it brought back a lot of forgotten memories of being young and having some innocent fun. Good video!