00:01
The first part of this question, we are going to derive the equation of motion.
00:09
So the system is like this.
00:10
It is a pulley and here is the string connecting to this little m.
00:17
And here it is the big m, the mass of coffee cup.
00:24
Here the angle is theta and here this is little r.
00:33
So now we write down the lagrangian which is kinetic energy minus potential energy.
00:42
The kinetic energy of this big m is 1 half, big m, r dot, square, and then plus 1 half.
00:55
Now we are dealing with this little m, and first we consider the radio motion, which contribute to 1 half little m, r dot, square, and then the angular motion which is one half little m bar theta dot squared.
01:19
This is the kinetic energy and we need to subtract the gravitational potential energy.
01:29
So for the big m, it lets this l to be the length of the length of, of string so that this term is big m g and then this distance downward which is l minus r i should put a negative sign because we are going downward and then the the kinetic energy of this little m which is negative m, gr, sine theta.
02:16
We can remove this because it's a constant.
02:33
So this is equal to one -half, capital m, r dot square, plus one -half little m, r -dot -square, plus one -half, have m r square theta dot square and then minus capital m grr and then plus little m gr sine theta this is our la grgion.
03:09
For the equation of motion first we look at d t over d t d l over d t d l over d d .r dot equal d l over d r the left hand side this is d over d t and l d l over d r dot.
03:39
First look at this term is contribute to big m r dot and then this term contribute to big m r dot and then this term contribute to big m a little m r dot.
03:52
So it is big m plus plus little m r double dot.
03:58
And the right hand side this is d over d r.
04:05
So we first look at this term...