# Action definition

1. Jan 24, 2009

### o_neg

Hi,

why is the action defined as the integral of the Lagrangian over time?
i don't see the meaning of integral on the (kinetic - potential) energy

thanks,
ori.

2. Jan 24, 2009

### clem

The integral has no meaning by itself, but setting its variation equal to zero leads to the equations of motion.

3. Jan 24, 2009

### phreak

You can see the integral as a way of measuring how much of something is used. So in this case, the integral measures how much (K-U) energy is being used up, and the path that a particle takes minimizes this (this isn't technically correct... it takes a 'stationary' path, but looking at it this way could be beneficial to the student visually).

4. Jan 25, 2009

### o_neg

thanks for the replay.

I found a detailed explanation in the Feynman lectures on physics vol 2