Train inertial referance frame

1. Oct 10, 2011

physicsgurl12

1. The problem statement, all variables and given/known data

You are in a train traveling on a horizontal track and notice that a iece of luggage starts to slide toward the front of the train. from this observation you can conclude that this train is not an inertial reference frame because it is slowing down.

2. Relevant equations

true

3. The attempt at a solution its true but why??

2. Oct 10, 2011

I like Serena

Uhh... what's the definition of an inertial reference frame?

3. Oct 10, 2011

physicsgurl12

a FRAME REFERANCE THAT DESCRIBES TIME AND SPACE

4. Oct 10, 2011

physicsgurl12

it has no acceleration
and for the crate to move there must be deacceleration

5. Oct 10, 2011

I like Serena

Hmm, let's take a look at wikipedia.
It says (skipping a couple of paragraphs):

"In an inertial frame, Newton's first law (the law of inertia) is satisfied: Any free motion has a constant magnitude and direction"

Does the piece of luggage have a motion with constant magnitude and direction?

Turning this around: if the train deaccelerates, the crate would move.
What if the train accelerates?

6. Oct 10, 2011

physicsgurl12

if the train accelerates it would stay still

7. Oct 10, 2011

I like Serena

Have you ever been standing in a train or bus that accelerates?
If you have, did you have trouble to keep standing?

Or in a car, can you feel it if the car accelerates?

8. Oct 10, 2011

physicsgurl12

um it was a while algo, but is it like when it takes off it acclerates so you kinda jerk forward?

9. Oct 10, 2011

I like Serena

Well, if a train accelerates, you are jerked forward.
And if you don't compensate you'll be flying to the back of the train.

In a car you are pressed against the back of the seat.
And when a car is braked, you'd fly forward except that the safety belts keep you in place.

10. Oct 10, 2011

physicsgurl12

okay so for the luggage to slide forward you must be accelerating

11. Oct 10, 2011

I like Serena

Well... a piece of luggage would not try to compensate...

12. Oct 10, 2011

physicsgurl12

so it would just slide back then

13. Oct 10, 2011

I like Serena

Yes - if the train were accelerating, the piece of luggage would slide back.

What was the question again?

14. Oct 10, 2011

physicsgurl12

why can you conclude it is not an initial referance frame?

15. Oct 10, 2011

I like Serena

Not exactly...

The question starts by stating that the piece of luggage starts moving forward.
... and it ends with the question whether the train is slowing down.

Does it?

16. Oct 10, 2011

physicsgurl12

yes.because its the opposite of it moving forwards

17. Oct 10, 2011

I like Serena

Right!

As for the inertial reference frame question:

Does the piece of luggage have a motion with constant magnitude and direction?

18. Oct 10, 2011

physicsgurl12

yeah?

19. Oct 10, 2011

I like Serena

Well, it's rather important that the piece of luggage "starts" moving.
That means that its speed (or motion) is *not* constant...