00:01
Okay, so we have a particle moving under this force law.
00:03
So we know the second law, newton's second law, tells us that mr double dot is minus kr, like that.
00:19
We want to show that angular momentum is conserved.
00:23
So angular momentum is r cross p, which is r cross mr dot.
00:42
Alright, so if we take its derivative, so the mass is constant, then we get r dot cross r dot using the product rule.
01:08
Okay, the first of those terms is zero, because it's just crossing a vector into itself.
01:18
And then the second term, m times r double dot from the newton's second law is just kr.
01:30
So it's minus r cross kr.
01:38
K is not a vector, it's a scalar.
01:43
And that equals zero.
01:47
So therefore, l is conserved.
01:59
Part b, so the motion lies in a plane.
02:09
So at t equals zero, we get x equals a, y equals zero, x dot equals zero, y dot equals some velocity v.
02:30
Okay, so our two equations, because we're in the xy plane, we get mx double dot equals minus kx, and my double dot is minus ky...