A fly in a rocketship.................

Let’s say that there is going to a manned rocket launch somewhere in the world. As the cosmonauts enter the ship, a fruit fly slips in. The fly is a strong one, and it buzzes all around the inside of the rocket without sitting down to rest. The rocket takes off like a bullet, what happens to the fly? Does it get smacked into the back of the cockpit?


The same as as a fly inside a car that accelerates rapidly. Or a bus, plane, train, etc.

This has been discussed many times before. Not only does the vehicle move in a particular direction, but so does everything else in the vehicle. Owing to the size of the fly, the chances of it “slamming” into the back of the cockpit are almost non-existent. Remember all the air in the cockpit is accelerating forward as well.

OTOH, a very heavy object will have greater inertia than the fly - an object at rest tends to say at rest - and could very well “slam” into the back of the cockpit.

Without being illegal (A professional driver on a close course thingie disclaimer.) sit in a very high performance vehicle, holding a marblel in one hand and a 16-pound bowling ball in the other, both at arm’s length directly in front of you. Now have the drive pop the clutch and floor it from a standing start to 100 mph in five seconds.

Which arm and which part of your body is going to hurt from that acceleration?


Not much, probably.

Here’s something to try - take a helium balloon for a ride in your car. Get some speed and hit the brakes - the balloon smacks into… the back windshield? The air in your car sloshed forward, the floating balloon went backwards.

I think the fly can be assumed to be floating in a medium, and therefore won’t be affected signifigantly.

Acceleration is the same as gravity; if the rocket accelerates at, say 20G, then the fly will have the same trouble remaining aloft as it would if it were suddenly transported to a large planet with a gravity equivalent to twenty times that of Earth - i.e. it will probably smack into the back wall of the cabin.

  • I speak from experience here; having caught insects such as wasps using the glass-and-postcard method, then found that I had a problem with releasing them while they were angry, I found that by shaking the glass (while being careful to hold the card in place), the insect is bashed against the inside of the container, concussing it - the only thing I was doing by shaking the container was applying acceleration.

While the rocket is speeding up, the fly will be ‘pushed’ towards the floor, just as if he was in a rocket sitting on Jupiter or somewhere else with stronger gravity (in fact, as Einstein showed, if it can’t look outside, there’s no way for the fly to tell whether it’s on Jupiter or in an accelerating rocket). The fly may or may not be able to stay airborne, depending on how strong the ‘force’ is (i.e. how fast the rocket ship is accelerating).
Once the rocket reaches speed and stays there, the fly, like everything else, is weightless (OK, I’m ignoring Earth gravity here, for now). It will probably be very confused trying to fly in zero G, of course.

Duckster – a bowling ball falling will obviously hurt more if it hits you than a marble, but they will fall at the same acceleration & speed. They will both hit the floor of the rocket at the same speed. If you think the heavier one ‘slams’ while the marble just ‘clinks’ that’s fine, but it’s a judgement call.
This ignores air resistance of course, which depends on the shape of the object as much as the size. But it’s going to be basically negligible for both a marble and bowling ball falling a few feet. (though not the fly of course)