Step 1: Identify the given quantities.
Mass of the particle:
\[
m=400\ \text{g}=0.4\ \text{kg}
\]
Restoring force:
\[
F=-kx
\]
Given:
\[
k=10\ \text{Nm}^{-1}
\]
Speed at the centre of oscillation:
\[
v_{\max}=10\ \text{ms}^{-1}
\]
In S.H.M., speed is maximum at the mean position.
Step 2: Find angular frequency.
For simple harmonic motion,
\[
\omega=\sqrt{\frac{k}{m}}
\]
Substituting values,
\[
\omega=\sqrt{\frac{10}{0.4}}
\]
\[
=\sqrt{25}
\]
\[
=5\ \text{rad/s}
\]
Step 3: Use the formula for maximum speed in S.H.M.
In S.H.M.,
\[
v_{\max}=\omega A
\]
where
\[
A=\text{amplitude}
\]
So,
\[
A=\frac{v_{\max}}{\omega}
\]
Substituting values,
\[
A=\frac{10}{5}
\]
\[
A=2\ \text{m}
\]
Step 4: Final conclusion.
Hence, the amplitude of motion is
\[
\boxed{2\ \text{m}}
\]