I think it is a well-known fact here that the Earth is slowly losing rotational velocity over time (some 1.4 milliseconds of the day are gained per hundred years). Eventually, this means that the Earth will become tidally locked with the Sun, meaning that one face of the Earth will always face the Sun and one will face away, much like our relationship with our own moon.
My question may perhaps be more related to physics, but why does the Earth lose rotational velocity? We know from Newton's laws that an object in motion stays in motion unless acted upon by an outside force. Well, what is the outside force? Is it the Sun's pull on us? Is it the Moon's pull? Is it another planet?