What's the source of centripetal force?

It can be anything.

- - In a planet's orbit, it's gravitation, between the planet

and the sun.

- - In an atom, it's electrostatic, between the positive nucleus

and the negative electron.

- - In a yo-yo, it's the tension in the string.

- - Inside a car that's turning a corner, it's the friction between

the seat of your pants and the upholstery.

- - On a merry-go-round, it's your grip on the horse you're sitting on.