When a body accelerates, a pseudo force acts on it. Pseudo force acts opposite to the accelerated body. However, we use pseudo force only when we take the frame of reference as the moving body itself.
For example, if you are in a fast moving vehicle, you feel like you are getting pulled in the backward direction, even though the only force that seems to be acting on you is gravitational force.
This is why it goes upwards.
In the problem, as there is a force on the inclined plane towards the positive x axis, the pseudo force acts in the negative x axis.
(reference as ground):
they wanted you to visualise it was like, if the individual acceleration of inclined plane is more than individual acceleration of the block, it would go in the leftwards direction, as the wedge is faster than the block.
(reference point as moving inclined plane):
if pseudo force is Fp and if F is the horizontal force of inclined plane, and F2 is the net horizontal acceleration on inclined plane for the block,
F2 = F - Fp = Ma - ma
This is what i visualise:
If incline of inclined plane is Θ, then we can write equation of motion along incline of inclined plane as:
FpcosΘ - FgsinΘ = mA
where Fp = ma
Fg = mg
A is acceleration of block with mass m along the slant of inclined plane
a is the acceleration of inclined plane
this makes the equation:
m(acosΘ-gsinΘ) = mA
Therefore, you can see, that if acosΘ > gsinΘ, A will be positive
if acosΘ < gsinΘ, A will be negative.
When A is positive, it moves up the plane, when A is negative it moves down the plane.
Please let me know if you have any queries related to my answer, as this is my first answer and i am not sure whether i have formatted my answer correctly.
