How foucoults pendulum works?

Answer 1

Newton's three laws of physics describe the concept of force. The laws of force are only valid in "inertial reference frames," which means that they are only correct according to observers that are not accelerating (i.e. observers that are either standing still, or moving with constant speed).

The Earth is NOT an inertial reference frame (though for most cases it can be approximated as one, since the acceleration of an object on Earth is small). Since Earth is accelerating, Newton's laws are not 100% valid according to someone that is rotating with Earth. What this means is that a person on Earth, measuring the force on some object on Earth, will find that there are forces acting on the object which have no actual cause via interaction with other objects.

Due to Earth's rotation, an object moving along the surface of Earth with some velocity will appear to have a force acting on it which causes it to travel in a circle. A Foucalt pendulum exhibits this phenomenon. The pendulum seems to change its swinging direction at the same rate as Earth's rotation. See Wikipedia.org/wiki/Foucault_pendulum for some animations on this.