A fly (or any bug) inside a car going 100 km/h stays level because the air sealed in the cabin moves with the car. From the fly’s perspective, it’s flying through still air, which is Galilean relativity in action. Motion only matters relative to a reference frame, so a sealed cabin is functionally at rest. Open a window and that bubble breaks.
Have you ever been driven mad by a bug (usually a fly) that somehow entered your car and flew around randomly, buzzing in your ears, and landing just out of your smacking reach on the dashboard? Not only is a bug in the car hugely irritable, it can also be very dangerous, as a bug can easily distract a driver and pull their attention away from the road.
The real question here is, how can such a small bug match the speed of a car that is moving at a speed of 100 km/h? Does it travel as fast as the car moves just to keep up and irritate you?
The Case Of A Moving Car
We’re all pretty well-versed in the idea of motion, correct? If an object is moving, then we say that it’s in motion. There is nothing simpler than judging whether a body is in motion or not (i.e. at rest). For example, when you are sitting in a car and see bushes and trees whipping by, then you know that you, or rather the car that is carrying you, is in motion. However, what if you aren’t actually moving? Instead, what if the trees are traveling in the opposite direction, giving you the convincing impression that you are traveling forward?
Obviously, it is the car that is moving, and not the trees, but that’s not the point I wanted to make. If you didn’t already know, I wanted to introduce the idea of relative motion.
Relative Motion
You are sitting in a car that is moving, so from the perspective of the objects outside that are at rest, you are clearly moving. However, what if a car appears alongside yours that is traveling at precisely the same speed as your car? Would you be able to determine if that car was in motion? If we eliminate all external visual and auditory cues, you wouldn’t be able to tell if the other car was in motion. In fact, you would likely think that both cars were at rest.
You ‘know’ that both the cars are moving, but it simply appears that the cars are at rest. This happens because both cars’ velocities are the same, and therefore, there is no relative motion.
What Does Relative Motion Have To Do With The Bug In My Car?
Simple answer: it has everything to do with the bug in your car.
When your car is completely sealed from the outside air, i.e. there are no windows or doors open (sunroof included), there is a specific amount of air that is enveloped within the walls of the car. This ‘packet’ of air is moving at the same speed as the car, no matter how fast or slow you’re driving. So, from the packet of air perspective, there is no relative motion (just like staring at the car next to you going to same speed!). Now, where is that pesky bug? You guessed it, right in that packet of air.
All that matters to the bug is the immediate air, which has nothing to do with the speed at which you are driving. It also doesn’t enjoy the scenery nearly as much as you do when you take a detour through the countryside. In order to stay airborne, all the bug has to do is flap its wings in that enclosed packet of air, without any abnormal strain or speed. If it had to travel at the same speed as your car, it would no longer be an ordinary bug, and the laws of relative motion would be shattered.
Don’t tell me you thought that bugs in airplanes traveled at 800 km/h just to irritate you…
What If You Open A Window?
In that case, things will change very quickly, and it will be bad news for the fly inside your car. The moment the window cracks open, that calm packet of air is no longer enclosed; outside air comes ripping past the gap at the car’s actual speed, sets up turbulent eddies in the cabin and effectively pulls the inside air out with it. The fly suddenly has to deal with the full relative velocity between the outside air and the car (100 km/h of headwind, in your example). That’s nowhere close to how fast it can flap, so it gets pinned to a side of the car, or, more often, sucked clean out of the window.
Just remember, if a bug is irritating you while you’re driving through a particularly stunning area, don’t bother increasing the speed of your car to push him to the back window… thanks to the laws of relative motion, it won’t help. Just crack a window and hope for the best!
How Fast Is The Fly Actually Going?
Here is the part that trips people up: from your seat, the fly is dawdling along at the gentle pace any housefly manages, perhaps a few km/h as it lazily circles the dashboard. But to someone standing on the roadside watching your car shoot past, that same fly is screaming along at roughly 100 km/h (62 mph), give or take its own little wing-flapping speed.
Both answers are correct at the same time, and that is the whole point of relativity. Speed is never an absolute property of an object; it only means something once you name the reference frame you are measuring it against. In classical (Galilean) physics, the rule for switching frames is simple addition: the fly’s speed relative to the road equals its speed relative to the car plus the car’s speed relative to the road. Inside a cabin cruising at a steady 100 km/h, the fly only has to supply its own modest flapping speed; the cabin generously hands it the other 100 km/h for free.
This is exactly why nobody is flung against the back seat when a plane levels off at roughly 800 km/h (about 500 mph). You, your coffee, and any stowaway insect are all already moving at that speed relative to the ground, so inside the cabin everything behaves as though it were sitting still. The same logic, scaled all the way up, is the starting line for Einstein’s special theory of relativity.
What Happens When You Brake, Speed Up, Or Turn?
Everything above quietly assumes one thing: that the car is moving at a steady speed in a straight line. That kind of frame, one that is not accelerating, is called an inertial frame, and it is the only place where the tidy idea of the air packet just sitting still holds up. The moment you stamp on the brakes, floor the accelerator, or swing around a bend, the cabin becomes a non-inertial frame, and the bug’s easy ride gets bumpy.
Think about what you feel in those moments. Brake hard and you lurch forward against your seatbelt; take a corner quickly and you get pressed toward the outside of the turn. Physicists call these apparent shoves fictitious forces, because there is no real push acting on you. You are simply trying to keep going in a straight line, as Newton said you would, while the car changes its motion underneath you. Inside an accelerating frame, objects seem to drift around with no obvious cause, which is the tell-tale sign of a non-inertial frame.
The fly experiences the same thing, only it is not strapped in. When you brake suddenly, the cabin air (and the fly riding in it) keeps moving forward, so the bug gets carried toward the windshield until it can flap its way back to equilibrium. A sharp turn nudges it toward the outer window, the airborne cousin of the centrifugal effect you feel in your own seat. None of this is dangerous for the fly; it just has to do a little extra flying to settle back into its calm packet of air once the car returns to a constant speed.
