15. A child with a weight of 400 N sits in a playground swing. The swing is attached to a pair of ropes, each with a length of 2 m. As the child swings, she follows a circular path. At her lowest point, when the ropes are perfectly vertical, the child is at a height of 0.5 m above the ground. Let us use the ground as our reference point for gravitational potential energy. Calculate the gravitational potential energy of the child-Earth system, relative to the ground, when the ropes make an angle of 30° relative to the vertical.

15. A child with a weight of 400 N sits in a playground swing. The swing is attached to a pair of ropes, each with a length of 2 m. As the child swings, she follows a circular path. At her lowest point, when the ropes are perfectly vertical, the child is at a height of 0.5 m above the ground. Let us use the ground as our reference point for gravitational potential energy. Calculate the gravitational potential energy of the child-Earth system, relative to the ground, when the ropes make an angle of 30° relative to the vertical.