A couple of weeks ago I was travelling in a car (120 km/h approximately) and I saw a fly flying in front of me (inside the car, near my nose, windows closed). I wonder how was that possible.

Does it mean is really flying at 120 km/h or the fly is being affected by some kind of gravity/force?


Basically from the frame of observation as your car:

The fly was inside your car, so its speed with respect to the car is zero. Its just as much inside the car as you are. Both are travelling at 120 with respect to any observer on the road. ut with respect to anyone inside car you both are just sitting inside the car.

So the speed of fly with respect to you is $v=0\,\frac{m}{s}$, with respect to some observer on the road is $120\,$km/h.

Its no more than a tissue paper you might keep near the steering wheel, in front of you.

  • 3
    $\begingroup$ What if the windows were opened? $\endgroup$ – Jake Oct 2 '14 at 4:10

For the same reason that when you are in a train that moves at 120 km/h, you can walk down the aisle without running at 120 km/h yourself.

In your case, you are walking on a floor that is moving at 120 km/h alredy. In the case of a fly, it is flying in air that is moving at 120 km/h.

  • $\begingroup$ Thanks, Florian. I get it now, the fly is moving along with the car and all of what's inside including the air surrounding us. Good explanation. $\endgroup$ – César Oct 25 '18 at 16:54

Well, probably one of the first person to note and describe that was Galileo. As he beautifully describes in his Dialogue Concerning the Two Chief World Systems:

Shut yourself up with some friend in the main cabin below decks on some large ship, and have with you there some flies, butterflies, and other small flying animals.

With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. When you have observed all these things carefully (though doubtless when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still.

Finally the butterflies and flies will continue their flights indifferently toward every side, nor will it ever happen that they are concentrated toward the stern, as if tired out from keeping up with the course of the ship, from which they will have been separated during long intervals by keeping themselves in the air.

This observation lead Galileo to enunciate that the laws of mechanics are invariant under a change of inertial frames. Nowadays we believe that not only the laws of mechanics but also the laws of physics shall be invariant under the change of any frame of reference. This is a fundamental principle of physics, called the Relativity Principle.

Restricting ourselves to mechanics, it means that there is no mechanical experiment capable of detecting the absolute motion of an inertial frame. This is actually contained in the first Newton's law (which is due to Galileo). When the fly rests on the car it has $120\, km/h$ forwards with respect to the ground. When it jumps upwards, the first law asserts that it must keep this $120\, km/h$ forwards since there is no force pushing or pulling it in the horizontal direction. It can fly forward at $5\, m/s$ respective to the car exactly in the same way it would fly respective to the ground if it was outside the car.

  • $\begingroup$ Thanks, that was a great exaplanation. I guess the same will happen if humans travelling on a bus jump and suspend in the air for a second, it's not tham I'm jumping and moving at 120 Km / h at the same time, right? $\endgroup$ – César Feb 23 '17 at 20:43

I'm not exactly an expert, just a highschool physics teacher. But I think that there's more to do with air pressure than it seems. If you think of a styrofoam ball in the car, if you attach it to the roof of the car using a string it will accelerate in the opposite direction of the acceleration of the car. If it is immersed on a recipient filled with water and attached to the bottom of this recipient instead, the ball will move forward when the car accelerates forward. If you think the fly is like a still object that does not fall, is like it has the same density of the medium it is in. So, like the ideal perfectly homogeneous air in the car it won't have the tendency to go forward or backwards. In reality though, we know the air inside the car creates a higher pressure in the back when accelerated forward. So the fly probably have to adjust it's flight direction whenever the car accelerates (because a dead fly does not float in the air, so it's more dense than the medium), but it's adjustment are a fraction of the acceleration of the car, since it will be helped by the buoyancy of the air inside the car. This is my guess


Firstly it was a good observation. Second thing is that flies can fly maximum at a speed of 6.7 km/hr. The question arises when car is in motion, the bodies connected in car physically (like you were seating on seat) is normal so you will have a speed equal to the speed of car but the object not connected to car physically (like insect which was flying) should be treated with relative speed.

The reason is car is an isolated system, when the car is at rest or in motion there is little or negligible kinetic force between the particles present in the car, which results providing a moving car an environment of room( where insects fly’s at normal speed). No matter your car is moving at 120km/hr or 300 km/hr, the environment of your isolated car will be similar to a room’s environment for fly.

If you want to realise the actual speed of car to insect, just open the window and let that insect have fun 😁

  • $\begingroup$ Hahaha, excellent and detailed answer. Thanks! $\endgroup$ – César Mar 11 '19 at 1:09

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