When there is no force acting on a moving object, it just
keeps on moving in a straight line. It takes force to pull
the object away from a straight line.
Whenever anything moves in a curve, there's a force on it,
pulling it toward the center of the curve. That's called a
"centripetal" force.
I think the easiest example to understand is spinning a stone
or a yo-yo around your head on the end of a string. The object
is moving in a circle, and the force that's keeping it on the circle
is the tension in the string. If the string suddenly breaks, then
that force suddenly goes away, and the object suddenly stops
moving in a circle ... it takes off in a straight line, in the direction
it was moving on the circle at the instant when the string broke
and the centripetal force stopped.
But examples are everywhere ... wherever you see something
that's moving in a curve and not in a straight line.
A fun one to talk about is things in orbit ... the Earth around the
Sun, the Moon around the Earth, Jupiter's moons around Jupiter,
and TV satellites and the International Space Station around the
Earth. An orbiting object might not exactly move in a circle, but it
moves in a path that's always curved, so there's a centripetal force
acting on it all the time. Where does that force come from ?
Where ?
Why, it's the force of gravity between the two objects, always
trying to pull them together !
