Train inertial referance frame

[physicsgurl12]

Homework Statement

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.

Homework Equations

true

The Attempt at a Solution

its true but why??

 

[I like Serena]

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

 

[physicsgurl12]

a FRAME REFERANCE THAT DESCRIBES TIME AND SPACE

 

[physicsgurl12]

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

 

[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?

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

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

 

[physicsgurl12]

if the train accelerates it would stay still

 

[I like Serena]

if the train accelerates it would stay still

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?

 

[physicsgurl12]

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

 

[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.

 

[physicsgurl12]

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

 

[I like Serena]

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

 

[physicsgurl12]

so it would just slide back then

 

[I like Serena]

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

What was the question again?

 

[physicsgurl12]

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

 

[I like Serena]

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

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?

 

[physicsgurl12]

yes.because its the opposite of it moving forwards

 

[I like Serena]

Right!

As for the inertial reference frame question:

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

 

[physicsgurl12]

yeah?

 

[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...