7.3. a)
We take the zero level of the potential energy to be at the lowest point of the arc. When the string is held horizontally initially, the position is 2 m above the zero level. Therefore,
b)
From the sketch, we see that at an angle of 
, the ball is at
a vertical height of 
above the lowest point
of the arc. Thus,
c)
At the lowest point, the zero level has been reached, so the potential energy of the swing is zero.