<p>A ball rolls off the top of a stairway with horizontal velocity u. The steps are 0.1m high and 0.1m wide. The minimum velocity u with which that ball just hits the step 5 of the stairway will be<span class="equation">\(\sqrt{x}ms^{-1}\)</span>&nbsp;where x = ________[use g = 10 <span class="equation">\(m/s^2\)</span>​​​​​​​]​​​​​​​</p>