# Inertial Reference problem

1. Sep 16, 2008

### PhoenixRising

1. The problem statement, all variables and given/known data
You are conducting an experiment inside a train car that may move horizontally along rail tracks. A load is hung from the ceiling on a string. The load is not swinging, and the string is observed to make a constant angle of with the horizontal. No other forces are acting on the load. Which of the following statements are correct?

Statements include:
The train is an inertial frame of reference.
The train is not an inertial frame of reference.
The train may be at rest.
The train may be moving at a constant speed in a straight line.
The train may be moving at a constant speed in a circle.
The train must be speeding up.
The train must be slowing down.
The train must be accelerating.

Please help, and if you could, explain why as well. I am doing these as practice for an exam and would really like to lock down the concept.

2. Sep 16, 2008

### edziura

"and the string is observed to make a constant angle of with the horizontal"

The answers depend on whether this angle is 90 degrees (i.e., hanging vertically) or some other angle.

Should the word 'horizontal' be replaced with 'vertical'?

3. Sep 16, 2008

### PhoenixRising

it's a 45 degree angle from the horizontal, sorry :)

4. Sep 16, 2008

### edziura

The train is an inertial frame of reference.
The train is not an inertial frame of reference.

Draw a free body diagram of the load. If F = ma indicates that a is 0, its inertial; if a is not 0, its not inertial. You should find that a not zero, and it is constant.

The train may be at rest.

The load in moving and accelerating at the same rate as the train. Answer the question: Can an object be accelerating if its velocity is zero? Is an object tossed in the air accelerating at the peak of its path?

The train may be moving at a constant speed in a straight line.

Can't be if its accelerating.

The train may be moving at a constant speed in a circle.

In this case, is its acceleration constant?

The train must be speeding up.

Nope.

The train must be slowing down.

Nope

The train must be accelerating.

Yep.

5. Sep 16, 2008

### PhoenixRising

Thank you, hard to wrap my head around it all sometimes.