Feynman Lectures: The Random Walk Explained

[luuurey]

There is a chapter in Feynman Lectures on Physics called The Random Walk(41-4). I understand everything till the paragraph right after equation 41.18. I have no idea what he is trying to say. There is an equation 41.19, which is diff. eq. for object that is forced and is in a environment that causes resistance... that is not eq. of particle in a gas. Than he is saying that average of \left\langle x v_x \right\rangle does not change because it doesn't remember where it was before.. I don't understand that formulation. I am also not sure why \left\langle x F_x \right\rangle = 0.

Could someone help me please ?
Thank you very much.

 

[Simon Bridge]

The Feynman lectures were not very effective when Feynman was giving them - they tend to the over-clever - I don't think they are that great now. You are best advised to read them alongside or after a more contemporary text... they tend to work best as a refresher course for people who have already done the basic physics by another method.

However, your questions can be approached by considering the converse. i.e. what does it mean for <xF> not zero, and <xv> not constant? What do xv and xF measure?