Step 1: Velocity of particle A.
Particle A moves along x-axis with constant velocity:
\[
\vec{v}_A = (1,0)
\]
Step 2: Position of particle B.
Given:
\[
y = t^3
\]
So velocity of B is:
\[
v_B = \frac{dy}{dt} = 3t^2
\]
Step 3: Velocity of particle B at \(t=1\).
At \(t=1\):
\[
\vec{v}_B = (0,3)
\]
Step 4: Relative velocity of A with respect to B.
\[
\vec{v}_{AB} = \vec{v}_A - \vec{v}_B = (1,0) - (0,3) = (1,-3)
\]
Step 5: Magnitude of relative velocity.
\[
|\vec{v}_{AB}| = \sqrt{1^2 + (-3)^2} = \sqrt{1+9} = \sqrt{10}
\]
Step 6: Final conclusion.
Therefore, the magnitude of relative velocity is \( \sqrt{10} \, m\,s^{-1} \).
Final Answer:
\[
\boxed{\sqrt{10} \, m\,s^{-1}}
\]