Close your eyes and try to remember what it feels like to be in a swing, moving to and fro, with varyings force between the swing and your body.
Note how the experience of the body changes between feeling light and feeling heavier than normal. Can you feel when you pass the lowest point or when you turn in the highest points? How would you expect the water level to move if you would take along a mug of water or a half-filled bottle on the swing?
The figure shows examples of possible "free-body diagrams" for a swing, swinging up to 45°. Which of the diagrams gives the best representation for the turning point and for the lowest point?
Check your choices by considering free-body diagrams that explicitly show F=ma.
The graph shows an example with G-force along the three axes of the smartphone resting on the bottom of the swing in the photo.
Question to ponder: Why is only one of the coordinates significantly different from zero?
Read more about force and accelerations in swings and pendulum rides (with video abstract).