A pendulum is swinging back and forth. After ttt seconds, the horizontal distance from the bob to the place where it was released is given by \qquad H(t) = 7 - 7 \cos \left(\dfrac{2\pi (t-2)}{20}\right)H(t)=7−7cos( 20 2π(t−2) )H, left parenthesis, t, right parenthesis, equals, 7, minus, 7, cosine, left parenthesis, start fraction, 2, pi, left parenthesis, t, minus, 2, right parenthesis, divided by, 20, end fraction, right parenthesis. How often does the bob cross its midline? Give an exact answer