2.8 Friction and Air Resistance

Critical Questions:

  • What is friction, and why does it happen?
  • If you drop a penny from a tall building, could it kill someone below?

Before reading this website, you might not have thought of friction as a force. In common language, the word is used to refer to almost anything that happens when two things come in contact, like when you start a fire by rubbing two sticks together or when two people get in an argument. Just like with all of our physics terms, however, we are going to give this one a much more specific definition.

Polar bear on ice
I think we’ve all been there.

Friction is the force that results when two objects rub together. We’ve already seen the example of a book moving across a table, but I’ve also briefly mentioned the more interesting example of the frictional force between car tires and the road, which actually moves the car forward.

So what causes friction? I’ve hinted at that, too. In order to solve this problem, we have to go down to the microscopic level. If you’re sitting at a table, run your hand over the surface. It probably feels pretty smooth, doesn’t it? But if you had a powerful enough microscope, you could see that the seemingly solid tabletop is actually made up of billions of smaller particles – molecules, atoms, and subatomic particles.

And if you held your microscope at the edge of the table and looked at that surface from the side, you would not see a nice, smooth surface. You would actually be looking at a scene reminiscent of the Swiss Alps: jagged mountains of molecules, deep chasms of intermolecular gaps, and everywhere in between rough and bumpy with little atoms. Depending on the material, these bumps might be bigger or smaller. The bumps on sandpaper are sometimes big enough to see with the naked eye, for example, while you could look closer and closer at the edge of a smooth crystal and never see a bump until you got close enough to make out individual molecules.

What this means is that when even the smoothest surfaces rub together, the bumps of one run into the bumps of the other. This is friction. Rougher surfaces have more and bigger bumps, which means more of these collisions will occur, which means more friction.

Close-up view of Friction
Two surfaces experiencing friction. The blue arrows represent the vertical repulsive forces between the two surfaces, while the red ones represent horizontal repulsion – i.e., friction.

We can be even more specific than that, though. Because we’re down at the atomic level, we can say that friction is just the combined electromagnetic repulsion of a lot of electrons. Remember when I explained how your hand can push a door without ever coming into “contact” with it due to the fact that both surfaces are lined with electrons, which repel each other? (It was in Section 2.2, if you want to go back and check.) Now imagine two bumpy surfaces on top of each other. It’s true that they’ll be repelled away from each other, but those forces are cancelled by whatever is keeping them together (like gravity, in the case of a book on a table). However, if two bumps come up next to each other, they’ll repel each other sideways. It is this sideways repulsion, added up over all of the millions of bump collisions, that is the force of friction.

What’s more, you can increase the amount of friction by pushing the two surfaces closer together – that is, by working against the vertical repulsion. When you do that, more bumps have a chance to collide, resulting in more sideways repulsion, which means more friction. That’s why a heavier couch is harder to slide across the floor and why sandpaper works better the harder you push it down against the wood.

I should stop here and say that so far I’ve only been talking about one particular kind of friction, which is called kinetic friction. ‘Kinetic’ comes from the Greek word for movement, which makes it appropriate for describing two objects rubbing against each other. The other kind of friction is static friction. Many people are more familiar with the word ‘static’ referring to the black and white fuzz on a tv set, like the one which starts talking to the little girl in Poltergeist and eventually, spoiler alert, reaches out and pulls her into an alternate dimension. In fact, the original definition of ‘static’ refers to objects that aren’t moving.

Take a heavy, solid object nearby, put it on a flat surface, and try pushing it very gently. You should find that you can push harder and harder before you actually accelerate the object forward. While the object is still stationary, we can say with confidence that it is not accelerating, and that therefore the net force on it must be zero, and therefore there must be some other force that is cancelling out your push.

This force is static friction. It is caused by exactly the same thing as its kinetic cousin: bumps of various sizes running into each other. One of its properties is that it will only ever be as strong as it needs to – that is, just strong enough to counteract any applied forces. But one other interesting thing to note is that if you’ve picked a heavy enough object to be able to feel the difference, you should have found that it was harder to get it moving than it was to push it once it had some motion.

The reason for this has to do with something I haven’t yet mentioned: although the electrons in the atoms of one surface do repel the electrons on another nearby surface, the positively-charged nuclei they are orbiting also tend to attract electrons from that other surface towards themselves. Usually this attractive force is so much weaker than the repulsive ones around it that it has no effect. But if you place two surfaces together and keep them still for a short time, a few of the electrons on one have the chance to slip in between the electrons on the other and become attracted to the second surface’s positively-charged nuclei.

And I have to say right now that this explanation is simplified to the point of gross inaccuracy, but in order to give a more complete description, we’ll need to know more about quantum physics, which is coming later. In the meantime, suffice it to say that two stationary surfaces in contact will act like teenagers left alone for too long and develop a little chemistry. That is, the surfaces will start forming a small number of weak chemical bonds between them. When someone comes along and applies a force to one of the two objects, these chemical bonds will make it even harder for the objects to pull away from each other. Once they are separated and in motion, however, there will not be enough time or close contact to create more chemical bonds, and this is why the force of kinetic friction is weaker than the maximum force of static friction.

One key feature of kinetic friction is that it produces heat. The reason for this is simple to understand once you know a bit more about energy. I’ll discuss the consequences of this at length in the chapter on heat and energy, but for now we can say that this heat represents a loss of energy. This energy loss is especially troublesome in the case of machinery. Friction makes everything less efficient, from refrigerators to car engines. In such cases, we use lubricants like grease or oil to fill in the areas between the bumps and thus reduce friction, but we can never get rid of it entirely.

But before you go wishing that friction didn’t exist so that you could spend less money on gas for your car, there are two important points I should mention. The first is that because friction is really only a manifestation of the fundamental electromagnetic force, a world without friction would mean a world where we didn’t exist: as I’ve already said, electromagnetism isn’t responsible for just lightning and battery power, it also acts as the glue holding together the atoms and molecules that make up all matter, including the human body. The second point is that even if you could exist in a world where all surfaces were frictionless, you would find that walking would be impossible, since your shoes wouldn’t grip on the floor. And forget about saving on gas – even if you floored your car’s engine, the wheels would just spin in place.

Air resistance is another form of friction, but it has some interesting properties. When you learn about air resistance in school, the first thing you learn is that it only depends on the size and shape of the object. An eighteen-wheeler experiences more air resistance than a Porsche, for example, in part because it presents a larger surface area for air to run into. This much is obvious. What we found out when we started experimenting with wind tunnels and curvy surfaces, however, is that the motion of fluids is much stranger than we’d initially suspected, and the little whirls and eddies that form when air moves around an object play surprisingly important roles in determining that object’s aerodynamic properties. So the shape of the entire object, and not just the front part, determines how easily it will move through the air.

But if you continued on in physics, you would later have learned that the amount of air resistance an object experiences depends not only on its shape, but also on how fast it is moving.[2. And also how fast it’s accelerating, and how quickly its acceleration is changing, and so on. If you have a math background, this has to do with the expansion of an infinite series.] In other words, as a car speeds up, it experiences more ‘drag’, or air resistance, pulling it backwards. This phenomenon causes a limit on the speed of a falling object, called the ‘terminal velocity’.

Terminal Velocity
Starring Charlie Sheen.

Let’s Think Like A Physicist and carefully figure out why this happens. I’ve already said that if an object is moving due to a constant force, and there are no other forces acting on it, it will continue to accelerate forever. But now imagine that a constant force is applied to an object in the presence of air resistance. At first the object will have a high acceleration, but as it speeds up, it will experience more drag. This decreases the net force acting on the object, which decreases its acceleration. (Its speed is still increasing, but increasing less quickly than before.)

As the object speeds up, the drag force becomes stronger and stronger, and the net force on it gets smaller and smaller. Eventually, at some certain speed, the drag force is exactly equal to the applied constant force, the net force is equal to zero, and the object stops accelerating (i.e. it is now moving forward at a constant speed). This speed is called the ‘terminal velocity’. It is different in every situation, depending on the size and shape of the moving object as well as other factors such as air pressure.

This scenario describes every falling object; the constant force here is gravity[3. Actually, as will be discussed later, the force of gravity is not really constant for a falling object but gets stronger as the object nears the ground. However, that change is not a significant one, and so we usually approximate gravity as a constant force anywhere on or near the surface of the Earth.].

When a human being is falling through the sky, their terminal velocity is about 194 km/h. Experienced skydivers wearing suits made of low-friction materials and fancy helmets can reach speeds of over 322 km/h by diving head-first with their arms held tightly at their sides. Once a parachute is opened, the drag force is significantly increased, causing an upwards acceleration (slowing down) and resulting in a final speed of about 29 km/h.

A falling penny, meanwhile, won’t reach a very high terminal velocity because it has a large surface area relative to its overall size, meaning it wouldn’t kill somebody if it were dropped from the Empire State Building (contrary to popular belief). That being said, I don’t recommend trying it unless you really, really dislike the person you plan to hit.

Big Ideas:

  • Friction is caused by collisions between bumps of varying sizes on two surfaces in contact. The bumps can’t get past each other because of electromagnetic repulsions between electrons orbiting the atoms on both surfaces.
  • Static friction occurs when a force is applied to a stationary object that is in contact with another surface, and kinetic friction occurs when two surfaces rub together. Static friction can be stronger than kinetic because weak chemical bonds occur between atoms of stationary surfaces.
  • The direction of the frictional force is always opposite to the direction of motion or attempted motion.
  • Air resistance increases with increasing speed. A falling object’s terminal velocity is the speed at which the drag force is equal to the force of gravity acting on the object.

Next: 2.9 – Circular Motion and Orbits

Previous: 2.7 – Newton’s Third Law