# Fictitious forces

mystmyst
We have a slope, with mass M, at x degrees. Mass m is on the slope. The slope moves right at constant acceleration $$a_0$$. There is no friction between M and m, and no friction between M and ground.

Determine the Normal Force between M and m.

[PLAIN]http://img638.imageshack.us/img638/620/imaginaryforces2.png [Broken]

## The Attempt at a Solution

I'm a bit confused with this because the slope is moving.
I would just think the answer would be N = mgcosx.
But since the slope is moving, I have to take into consideration $$a_0$$ but I don't know how to.

Thanks!

Last edited by a moderator:

## Answers and Replies

cupid.callin
Hint: Use Pseudo force

Edit: For using Pseudo(fictitious) force, you have acc. of incline

These forces come into action when you need to apply newton's laws in non inertial frame(i.e. a is not 0 of frame)

In this case it will be -mao, where - sign indiate that its direction is opposite to ao

Last edited:
Homework Helper
hi mystmyst! I'm a bit confused with this because the slope is moving.
I would just think the answer would be N = mgcosx.
But since the slope is moving, I have to take into consideration $$a_0$$ but I don't know how to.

well, you know from the question that a fictitious force is involved …

what is that fictitious force in this case?

draw it on the free body diagram, and proceed as normal …

fictitious forces go on the fbd just like ordinary forces … that's the point! mystmyst
hi mystmyst! well, you know from the question that a fictitious force is involved …

what is that fictitious force in this case?

draw it on the free body diagram, and proceed as normal …

fictitious forces go on the fbd just like ordinary forces … that's the point! The fictitious force is $$a_0$$ but I don't know how to draw it on a fbd. Am I supposed to split it into x and y coordinates (I'm using the slope as the x-axis...)

cupid.callin
as the acceleration of incline is along +X ... so this fictitious force will be along -X

as i told you ...

In this case it will be -mao, where - sign indiate that its direction is opposite to ao

cupid.callin
write all other forces like gravity, normal rxn.. and also include this force in your FBD

Homework Helper
The fictitious force is $$a_0$$ but I don't know how to draw it on a fbd. Am I supposed to split it into x and y coordinates (I'm using the slope as the x-axis...)

a0 is horizontal, so you draw it horizontal

if you're using coordinates (you could use trig instead), then yes you'll have to split it into components along the axes you've chosen cupid.callin
Taking X along incline will be easier!!!

cupid.callin, he is taking X along the incline OH!! sorry 