# Homework Help: Discussions on answer of a problem

1. Sep 9, 2007

### neelakash

I want to discuss the answers (what I get and what is given) of the following problem:

A particle slides down a smooth inclined plane of elevation θ,fixed in an elevator going up with an acceleration a.The base of the incline has a length L.Find the time taken by the particle to reach the bottom.

Working from an inertial frame I got the acceleration down the incline as (g-a) sinθ whereas working from elevator frame(non-inertial) one gets the acceleration down the incline as (g+a) sinθ.Why is there a difference?
Normally,when we see an object from outside an elevator and from the elevator,we se that the normal reaction force to be the same.So,what is making the difference?

2. Sep 9, 2007

### cesiumfrog

You do know which equation doesn't contain a mistake, right?

3. Sep 9, 2007

### neelakash

I think that there is some mistake that I am making.Otherwise the forces should be the same...
when I get it I will post that.

4. Sep 10, 2007

### cesiumfrog

The second one is correct. Since the elevator is accelerating upward, the acceleration adds to gravity.

5. Sep 10, 2007

### neelakash

The first answer is with respect to the ground.Show me that it is incorrect!

6. Sep 10, 2007

### cesiumfrog

Because it is obviously unphysical for the solution to pass through zero as the upward acceleration increases.. The particle should slide faster, not turn around and go up the slope.

7. Sep 10, 2007