Concept:
For uniformly accelerated or retarded motion, we use:
\(\displaystyle v^2=u^2+2as\)
where,
\(u=\) initial velocity
\(v=\) final velocity
\(a=\) acceleration
\(s=\) distance
Step 1: Write the given values.
Initial velocity:
\(\displaystyle u=20\ \text{m/s}\)
Final velocity:
\(\displaystyle v=0\ \text{m/s}\)
Retardation:
\(\displaystyle a=-5\ \text{m/s}^2\)
Step 2: Apply the formula.
\(\displaystyle v^2=u^2+2as\)
Substitute the values:
\(\displaystyle 0^2=20^2+2(-5)s\)
\(\displaystyle 0=400-10s\)
\(\displaystyle 10s=400\)
\(\displaystyle s=40\ \text{m}\)
Step 3: Final conclusion.
Hence, the stopping distance is:
\(\displaystyle \boxed{40\ \text{m}}\)