When the Lagrangians are equals?

1. Jul 9, 2013

I’m not very good with english, it isn’t my native language...., but I’m going to explain my question....

I’m reading the first book of Landau's series ,it’s about clasical mechanics.
In the second chapter you can find a problem about the conservation's theorem

the problem says The first problem says:

Find the ratio of the times in the same path for particles having different masses but the same potential energy.

the solution is: t'/t=sqrt(m'/m)

My tentative solution is supposing that the lagrangian for both paths are the same...

then:

L'=L

1/2m'v'2-U=1/2mv2-U

Finally:

t'/t=sqrt(m'/m)

BUT, It’s that correct?

and why the lagrangians are the same??? I’m not sure about the real concept (or meaning) of the lagrangian of a system...

thanks...

2. Jul 11, 2013

Khashishi

It would be helpful if you posted the problem since we don't all have a copy of Landau. Maybe this should be in the homework help section.