# Torque due to tidal force

My textbook expects me to show the torque exerted on Earth by the Moon on the ocean, that is 'ahead' of the rotation of the earth due to friction between the Earth and its ocean. However, I wouldn't know how to show this properly. The best I've found so far are things like this:

But here they only include the side of the earth facing the Moon; how about the other side?

So I understand that without the friction of the tides, the Eart would have faces the Moon 'symmetrically', but because of friction, we get an asymmetrical situation. But how to conclude this line of reasoning?

Is it that the gravitational force by the Moon is much smaller on the side of the Earth that isn't facing the Moon, and therefore we only really need to consider the torque applied on the side facing the Moon?

## 1 Answer

There is only a torque because there is "too much" force on one side, and "not enough" force on the other side. If you compute the attraction on both bulges, you can consider these the sum of an attraction (due to same force on both) and a torque (due to the difference from the gradient in the gravitational field of the moon).