Question

A grandfather pulls his grandson, whose mass is 20kg and who is sitting on a swing with ropes of length 2.50m, backward and releases him from rest. The speed of the grandson at the bottom of the swinging motion is 3.0 m/s. What is the angle (in degrees measured relative to the vertical) from which he is released?

Answer 1

Try using conssrvation of energy.

Hint: The change in elevation for an angle A from vertical is
L (1 - cos A)
where L is the rope length.