I encountered a Physics Olympiad problem:
A ball bearing rests on a ramp fixed to the top of a car which is accelerating horizontally. The position of the ball bearing relative to the ramp is used as a measure of the acceleration of the car. Show that if the acceleration is to be proportional to the horizontal distance moved by the ball (measured relative to the ramp), then the ramp must be curved upwards in the shape of a parabola.
Attempt at a solution:
Let $A$=magnitude of acceleration of the car w.r.t a stationary observer, $a$=magnitude of acceleration of of the ball bearing w.r.t the stationary observer and $a'$=magnitude of acceleration of the ball bearing as observed in the accelerating car frame, then $$A=\mathcal{k}x' \tag{1}$$
where $x'$ is the horizontal position of the ball bearing as measured in the accelerating frame, with $x'=0$ at the bottom end of the ramp.
Since the ramp is a parabola, the position function of the ball bearing in the accelerating frame, should take the form $$y'=\alpha (x')^2 \tag{2}$$ where $\alpha$ is some constant and $y'$ is the vertical position as measured in the accelerating frame.
Using the definition of the fictitious force/acceleration $$\mathbf{a'=a-A} \tag{3}$$ The LHS of $(1)$ reads $$a+a'=kx'\tag{4}$$ (positive sense towards the direction of $\mathbf{A}$). But this doesn't seem to provide any useful information since I cannot define $a'$.
Should I try something like $\frac{dy'}{dx'}=\frac{\dot{y}}{\dot{x}}$? I am lost because I do not know whether I should analyse the ball bearing in its equilibrium (i.e. $A=constant, x'=constant$) to obtain some expressions for $y$ or it as a function $A(t)=kx'(t)$?