Why Does Hitting Water From a Height Feel Like Hitting Concrete?

Why Does Hitting Water From a Height Feel Like Hitting Concrete?

If you’ve ever heard people say that hitting water from a great height feels like hitting concrete, you’re probably wondering why. Water is, after all, a liquid! But there’s a lot more going on when you fall into water at high speeds than you might expect. Let’s break it down using some physics.

Debunking the Surface Tension Myth

One of the first things we need to address is a common misconception: the idea that surface tension is the main reason water feels so hard when you hit it fast. Sure, surface tension does exist, and it’s what allows small insects to walk on water. But when we’re talking about a human body falling from a height, surface tension isn’t the major factor.

Instead, the problem lies in how water behaves as a substance. Water is dense and, crucially, incompressible. When you hit the water at high speed, it doesn’t get out of your way fast enough, which is why it can feel so solid on impact.

Water Is Like a Crowd—But It Won’t Move for You

A great way to visualize this is to think of water like a crowd of people. Imagine you’re walking through a crowd—people will move aside, and you’ll keep going. But now, imagine sprinting full speed into that same crowd. You’d crash into them, and they wouldn’t have time to move out of your way.

It’s the same with water. When you fall into water from a height, the water molecules don’t move aside fast enough to let you pass smoothly. The faster you’re going, the harder it is for the water to move aside, making the impact far more intense than you might expect.

Incompressibility: The Real Culprit

The main reason water can feel like concrete on impact comes down to its incompressibility. Water doesn’t compress easily, especially at the speeds we’re discussing. Unlike air, which will move aside as you fall through it, water resists being pushed aside at high speeds.

When you hit the water, your body’s momentum doesn’t just disappear. The water resists that motion, and this resistance translates into the force you feel. So, the faster you’re falling, the harder it is for the water to move aside, and the more painful the impact.

Why Does Speed Matter So Much?

One of the most important factors when falling into water is speed. The faster you’re going, the more momentum your body has. And when that momentum suddenly stops—like when you hit the water—the impact feels severe.

This concept is similar to how air resistance works. We know that at high speeds, even air can create substantial resistance, as seen in the famous XKCD comic about the relativistic baseball. Now imagine hitting something denser, like water. The resistance and the force you experience would be even greater.

Is It Really Like Hitting Concrete?

Now, how accurate is the comparison to concrete? While it’s an exaggeration, the analogy gets the point across. Falling into water isn’t exactly the same as hitting a solid surface, but the force is strong enough to cause serious injuries if you’re falling from a significant height.

In fact, experiments like the one on Mythbusters—where pig carcasses were dropped from a helicopter—showed that water can cause significant injuries, including broken bones. However, the pigs that hit the water fared better than those that hit concrete. So while water is softer, it’s still dangerous at high speeds.

Why Do Professional Divers Use Bubble Systems?

Professional high divers, who dive from extreme heights, often have pools with bubble systems. These systems aerate the water by adding air bubbles, which makes the water less dense. This effectively cushions their impact, giving the water more “give.”

By adding air, divers are essentially creating little pockets where the water can be pushed aside more easily, making the water feel less solid. It’s like trying to punch your way through a cloud of cotton candy instead of a brick wall. The bubbles reduce the density of the water, which in turn reduces the impact.

Can Other Liquids Offer Softer Landings?

Some commenters raised the question of non-Newtonian fluids—substances that behave differently under stress. A famous example is Oobleck, a mixture of cornstarch and water that acts like a solid when you hit it with force but behaves like a liquid when you move slowly. Could something like this provide an even softer landing?

It’s a fascinating idea! But for now, we don’t have a high-impact-friendly non-Newtonian fluid designed for human landings. Still, the concept reminds us that the physical properties of liquids can vary greatly depending on the conditions.

What Can You Do to Minimize Injury?

So, if you’re ever in a situation where you’re falling towards water from a height, is there anything you can do? Well, you can’t completely cheat physics, but you can try to work with it. Professional divers aim for a feet-first entry to reduce the surface area hitting the water and minimize the force of impact. Even if you’re not a professional, aiming for a smaller surface area, like your feet, might help lessen the blow.

FAQ

What is incompressibility in water?

Incompressibility means that water does not easily compress or reduce in volume under pressure. This property is why water can resist moving out of the way when you hit it at high speeds, leading to a hard impact.

Why does falling into water from a height feel hard?

Falling into water from a height feels hard because of the speed at which you’re traveling. The water can’t move out of the way quickly enough, so it resists your motion, causing the impact to feel intense.

How do professional divers reduce the impact when diving from great heights?

Professional divers use bubble systems to aerate the water, making it less dense. This creates a cushioning effect, which reduces the impact when they hit the water.

Is hitting water really like hitting concrete?

While hitting water isn’t exactly the same as hitting concrete, the force of the impact can be very strong at high speeds. However, water does cause less damage than a solid surface like concrete.

Can other liquids offer a softer landing than water?

Some non-Newtonian fluids, like Oobleck, behave differently under stress and could theoretically offer a softer landing. However, we don’t have practical applications of such fluids for human impacts at high speeds.

Join Alex Thompson and Maria Davis as they navigate the fascinating world of knowledge. With their combined expertise and passion for learning, they simplify the complex and make every episode a journey worth taking.