Cling to Move: Friction and Motion

More than two thousand years ago, Aristotle wrongly predicted that heavier objects fall towards the ground faster than lighter ones do. Changing this perspective was part of the revolution brought by Galileo. But why did it take so long to realize he was wrong? Was it just the weight of his dogma?

The reality is that Aristotle, relying solely on his experience and intuition, was somewhat right. He had to come up with a way to account for the simple observation that feathers, leaves, and ashes all fall more slowly than a cannonball. What he got wrong was that lighter objects are not inherently slower; they just interact more with something in between their trajectory: air.

We, too, are baffled when learning for the first time that a hammer and a feather fall together in a vacuum. Everything revolves around simple yet subtle observation. We live inside a medium, and we are forcibly interacting with it, causing friction and resistance.

But what if friction didn’t exist?

To Swim in a Vacuum, to Walk on Ice

Could we swim in a vacuum? Admitting we could somehow stay afloat in an empty pool, how is it possible to move forward without pushing on water? In its simplest form, motion is made possible by the result of muscles and levers acting and pulling against the external environment. In the end, it is just what Newton’s third law says: “the mutual actions of two bodies upon each other are always equal, and directed to contrary parts”. If we want motion, we need a force, and the same force will be applied in the opposite direction onto some medium. In this case, our body must push on something, and that something must resist in order to propel the movement. As a swimmer presses backward against the water, it responds by pushing forward. A bird beats its wings, displacing air, and the air offers lift. Even walking relies on the friction between the foot and the ground. Without that subtle grip between soles and earth, we would simply slide in place. In this sense, the drag we feel when we swim, the air sliced during a flight, these are not just obstacles; they are the very mediums through which motion becomes possible. It defines direction, enables control, and gives substance to effort; otherwise, it would just be like walking on ice.

Friction in Daily Life

Once you start looking for it, it is clear that some form of friction is present in nearly every aspect of daily life.

Put aside air resistance; friction also arises whenever two surfaces interact. This provides what we feel as “grip”, which, as we saw, enables walking and makes actions like braking possible. Screws and bolts also stay in place thanks to the same principle of mechanical locking: the very forces generated between bodies at contact allow them to stay in place.

Heat is another byproduct of friction we experience all the time. In winter, we rub our hands together to warm them up, and matchsticks ignite when struck: the friction between the match head and the rough striking surface rapidly heats the chemicals until combustion begins. The brakes we discussed earlier (whether discs or V-brakes) get hot after use; the friction between brake pads and wheels transforms the vehicle’s motion into heat, which must be dissipated to prevent failure.

Yet another property of friction is its ability to produce sound.

Consider the haunting tone of a crystal glass when a wet finger circles its rim. Here, friction between skin and glass creates minute vibrations that resonate through the bowl, producing sound. This principle extends to more complex systems, such as bowed string instruments. The vibration of the string, which ultimately produces musical sound, is entirely driven by friction between the bow and the string. As the bow passes over the string, the horsehair briefly adheres to it, transferring motion to the instrument string. When this grip breaks, the string snaps back and begins to vibrate. This stick–slip cycle repeats rapidly, generating a continuous tone. The instrument's body captures and amplifies this vibration into audible sound. Friction mediates the transfer of motion from one moving body to another. Continuous gripping and un-gripping cycles make objects oscillate and produce resonances that eventually turn into sound.

Some Definitions

In general, friction is the force that resists the relative motion between two systems in contact. Its nature varies depending on the medium—solid, liquid, or gas.

1. Friction in Solids

In solids, friction arises from microscopic irregularities on surfaces. When two solid bodies come into contact, their asperities interlock. Movement requires overcoming this interlocking, which leads to static friction (resistance to the start of motion) and kinetic friction (resistance during motion). The magnitude depends on surface roughness, the contact area, and the normal force between the bodies.

2. Friction in Liquids (Viscous Resistance)

In liquids, friction is better described as viscous drag. The way to model this is to approximate a portion of liquid as made of parallel solid layers perpendicular to the direction of motion. Drag is caused by internal shear between these adjacent layers of fluid or their solid boundaries. Molecules in a liquid slide past one another, and their cohesive forces create resistance to motion. The greater the bonds between molecules, the more viscous the liquid, and the greater this resistance: honey resists motion more than water.

3. Friction in Gases (Aerodynamic Drag)

In gases, friction takes the form of drag, caused by the collision of gas molecules with each other and with the surface of a moving object. Unlike in liquids, gas molecules are farther apart, and their motion is more chaotic. Drag increases with speed and surface area, becoming especially significant at high velocities, such as those found in aviation or high-speed vehicles.

Having an idea of what happens at the microscale helps us reconcile our daily experiences of friction. Mechanical locking is a mere geometrical and chemical fact, and sound and heat arise by the transfer of macroscopic motion to vibration at various microscopic scales. We can see how friction and drag arise naturally due to the very structure of matter, and we can also understand how complex it would be to tweak them the slightest. There is a field however, in which engineers and scientists have developed technologies and tricks to make the otherwise impossible happen: sports.

Reducing Friction

While friction is essential for generating grip and traction, excessive frictional resistance can hinder athletic performance.

In activities such as running, jumping, or throwing, athletes may experience reduced speed, agility, or power output when frictional forces interfere with free and efficient movement. This can negatively affect overall performance and competitiveness. For this reason, we observe the formation of large aggregates of athletes in endurance and long-distance athletic events: a rider who pedals protected inside the peloton can save up to 30% of energy compared to their colleagues who are pushing to the front.

For the "Breaking4" event, Nike developed a sophisticated pacing strategy for this exact reason. A tight formation of 12 athletes in a diamond pattern (front, side, and rear) aimed to create a low air pressure pocket around Faith Kipyegon, helping reduce draft and keep her focused on the performance. This effort was met with a specially engineered suit to manage the wind around the athlete. Emulating what Speedo did for swimsuits, including micro-scales to mimic sharks' skin to reduce resistance underwater and improve speed, Nike came up with Aeronodes on its tracksuit to mitigate air forces. These nodes are half-spheres that vary in size strategically placed on the suit to keep the wind “attached” to Faith and avoid air vortices. Something similar happens in golf balls: the tiny holes (dimples) generate turbulence, which creates an air “skin” around it, increasing its effective size and diminishing drag. Moreover, adding spin can lead to lift due to the Magnus effect: not only does one have less air resistance, but the ball flies higher!

Increasing Friction

Poorly designed sports equipment that fails to account for frictional interactions can have serious consequences, compromising not only power and efficiency but also causing instability, slips, and injuries. Footwear with inadequate traction or flawed sole design can reduce grip and stability, increasing the risk of slips and falls. Similarly, playing surfaces with inconsistent frictional properties may lead to unpredictable performance and limit athletes' ability to perform at their best. Engineering friction in sport, then, is not just about maximizing grip and reducing drag: it’s about tailoring them to the specific demands of each discipline.

Track spikes are a perfect example. Their metal points bite into the ground, maximizing traction and minimizing energy lost to slipping. Every step becomes more efficient, more explosive. In a different context, climbing shoes rely on a very soft, sticky rubber that, on a microscopic scale, conforms to rock surfaces, amplifying the contact area and increasing contact interlocking forces. Even the soles' tread patterns of hiking boots or car tires are crafted in a specific way to maximize grip. The deep grooves and complex tessellations serve to channel away mud, snow, or water, ensuring that enough rubber remains in contact with the surface to prevent slipping.

To Each Their Own

But sometimes it’s not just a matter of “more” or “less”. Often, the balance between the frictions of different materials is so unique that it becomes an essential part of how a certain sport is played, and part of what a certain sport is.

Take tennis, for instance. Grass courts are faster and more slippery than clay ones, which favor longer rallies and higher bounces. It all boils down to the friction between the ball and the ground. How much speed and rotation are lost due to it when hitting the ground? In 2007, Roger Federer and Rafael Nadal were undisputed champions on their respective surfaces of choice, prompting the organization of a match on a half-grass, half-clay court. In this setting, the players’ strengths and weaknesses were balanced, and the game tactics, tempo, and styles were adapted to the new scenario.

There is a limit to how much we can change friction’s behaviour. What’s left is the domain in which we can explore motion and our interactions with the outside world. Adapting to such rules is not a defeat: it is an everyday game we must play with nature.