USA Bus Lines

A fly is buzzing around in a school bus on the freeway. Suddenly the bus stops...what happens to the fly?

I know, this old gem, but I really want to know what happens, and, WHY? My answer (most likely wrong): The fly is part of the schoolbus system, so it is able to fly around in the bus without breaking the world record for fly flying speed. However, since it shares the bus's momentum/energy, when the bus slams to a stop, the fly is propelled forward (not having the force of friction to stop it) and goes splat. Best answer to the best explanation! [celebrating off camera] - Thanks bklynleon, definitely nice to know some of that Physics is paying off... So, even if a smaller body is flying around in a larger, air-filled body, the smaller body is a part of the larger body's system and has its momentum and velocity? I guess this makes sense - I mean you can toss a ball up and down in a moving car, just seems strange... Midatlantian, I bow before your genius...

Public Comments

1. Your hypothesis is correct. That's exactly what would happen. Think about it if you were on a train and it stops. Naturally, you feel the force pushing you to the front of the train. If you don't hold on to something, you'll fall. Now if you were in the air like the fly and the train stops... you're not holding on to anything so you're going to fly into that windshield and go splat.
2. well because of inertia, all objects move forward. thats why in a car, we wear seat belts to counter act our bodies moving forward and hitting the dashboard or whatever is in front of you. as for the fly, well it's movement does not get counteracted and that is what causes it to propel forward. I hope i made sense :]
3. Yep you're right, the fly splats. This is a good example of Newton's First Law: the one that says a body in motion stays in motion until acted on by an outside force. For your fly, who has a constant velocity and direction inside the bus, that outside force is the inside of the windshield. It's the same reason you end up with soda on your front dash when you slam on the brakes.
4. If a body is in motion, it tends to stay in motion, if a body at rest tends to stay at rest. Newtons First law of Inertia.
5. Yes, you are correct, BUT! The fly is a very small mass, flying in air. And the dynamics of the air in the bus are more complicated that you have assumed. When the bus stops suddenly, the air in the bus, which is also moving with the bus, ALSO shifts forwards due to its inertia. But in doing so, the air pressure in the front of the bus increases very quickly, and that actually creates a pressure wave that then pushes the air back toward the rear of the bus. Remember that when the air shifts forwards, the pressure in the front of the bus rises, but at the back it falls. So in the next phase, there is more pressure in the front and less in the rear, and that leads to the acceleration of the air towards the back of the bus. If all of the windows of the bus are opened, this effect is still there, but much less.It takes time for the air in the front to move left or right to the 'exits' and the effect of the pressure is very fast. So this happens quite quickly, and the hapless fly is also caught up in this, even as it tries to regain control of its direction by flapping its wings. So what really happens is that the fly will initially lurch forwards, then, if it was not very close to some vertical obstacle - like a windshield - it would soon stop moving forward and might even rebound backwards a little. Edit: This idea of a 'frame of reference' is a very important part of physics. In every frame of reference - on a moving bus, or a moving planet or a moving galaxy, it does not matter how fast you are moving relative to some other ... bus or planet or galaxy. All the rules of physics apply to you, whatever frame you are in. You throw a ball into the air, and it falls to earth at about 10 meters per second per second, even as the galaxy is moving at some huge speed across the universe.