Suppose, I am lifting a ball of mass $m$ very slowly from the ground to a height $h$. That is,for the whole time,approximately, $F_{me}=-F_g$ I am doing a work of $+mgh$, which is transferred to the body. Gravity is doing a work of $-mgh$. What is the significance of the negative work done by gra...

I'm trying to understand a result in Peskin and Schroeders chapter 3 on the dirac equation, during the free particle calculation for the case $p^0=E>0$. Peskin and schroeder derived this expression: $$u(p)=\left[\begin{matrix}\left((\sqrt{(E+p^3)}\times \frac{(I_{2x2}-\sigma^3)}{2}+\sqrt{(E-p^3)}...

I've started reading Landau-Lifshitz Mechanics, and I'm having trouble with the problem at the end of section 7. A particle of mass $m$ moving with velocity $\mathbf{v}_1$ leaves a half-space in which its potential energy is a constant $U_1$ and enters another in which its potential energy is a ...

I don’t think I have worded my question very well. Here’s another try… We are taught that pressure of a fluid increases with depth, usually given the explanation of the increasing weight from the column of water above our object as it goes deeper. Then suddenly, we are taught that pressure acts i...