# Tidal Forces

## Main Question or Discussion Point

Hi,

I'm not sure if I'm at the right place to write this message, so please excuse me if I'm not.

I have read a bit about tidal forces, but I still don’t understand what they are, how they act on different body masses, etc. So, I thought I could find answers here. I would greatly appreciate it if someone could refer me to a good link about the subject or, if you want, explain it to me.

Thanks a lot

Happy holidays

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vanesch
Staff Emeritus
Gold Member
In Newtonian gravity, tidal forces are the gradient in the gravitational acceleration. An extended body which is kind of elastic and undergoes a gravitational field with a gradient will not feel the same weight per unit of mass everywhere, and hence will undergo a deformation (as clearly the total force per unit of mass must be identical if the body is not to be ripped apart, hence there must be mechanical tensions in the body).

In general relativity, tidal forces are all there is to gravity.

my two cents:

if you have an extended body, some part of the body experiences slightly different force due to the slight difference of position to the gravity causing object. Let's say if you have a rod, the end closer to the earth will experience a greater force than the end farther away from the earth.

In the frame of that accelerating rod, the end farther away from the earth will accelerate more slowly. In order for that end to "catch" up (so that the rod doesn't fall apart), there must be a tension in the rod to make up for the difference so that the different parts of the rod accelerate uniformly.

vanesch
Staff Emeritus