# Homework Help: Fly in a train

1. Sep 5, 2009

### semc

Fly in a train!!

Hey guys, i was wondering what happens if an object is suspended in mid air inside a moving train? Why doesnt the object just fly back to the back of the train when the train starts moving? I mean it has got no contact with the train so it will not have the velocity of the train right??

2. Sep 5, 2009

### Staff: Mentor

Re: Fly in a train!!

How is it suspended?

3. Sep 5, 2009

### bp_psy

Re: Fly in a train!!

It actually does. More precisely the fly remains stationary with respect to the ground while train accelerates so it appears to be moving back to an observer in the train.It is a consequence of the first law.

Last edited: Sep 5, 2009
4. Sep 6, 2009

### semc

Re: Fly in a train!!

its just like a fly in a train

so the fly will go to the back of the train when it accelerate? So how is it possible to see a fly flying around in a train.

5. Sep 6, 2009

### PhaseShifter

Re: Fly in a train!!

On a trip of any significant length, how much time does the train actually spend accelerating?

6. Sep 6, 2009

### Staff: Mentor

Re: Fly in a train!!

So what supports the fly? The air, of course.

In order for the fly to accelerate, a net force must act on it. (It's got to push against the air.) If the train lurches forward, the fly will appear to be pushed toward the rear of the train unless it compensates. (Which it can easily do. Flys zoom all over the place creating greater accelerations than you are likely to experience on a train.) Once the train stops accelerating, everything is back to normal.

7. Sep 6, 2009

### semc

Re: Fly in a train!!

Sorry i dont quite get it. So what if the air support the fly? It has got the same speed as the train just like us when we are inside a train?

Wow so the fly doesnt have to fly at the same speed as the train to appear stationary?

8. Sep 6, 2009

### Staff: Mentor

Re: Fly in a train!!

Something must exert a force on the fly to accelerate it.
Not sure what you mean. Flys buzz around.
What do you mean by "fly" at the same speed as the train? To appear stationary within the train, it must have the same speed as the train.