00:01
In this problem, you're given a force magnitude and your tunnel is always pointing away from some point in space.
00:10
So if this is the point you want to look at the force on an object here, that force is always away from it.
00:16
Always outward along the radio direction.
00:19
You might say, i've never seen anything like this before.
00:21
Yes, you have.
00:22
Just not outward.
00:23
You saw it inward.
00:24
Think of universal gravitation, always toward the center of the earth.
00:29
So you've seen it.
00:29
And you also knew that that force, this is not one of our squared.
00:36
That force was conservative.
00:38
This force also is conservative.
00:39
That's why they're asking you to get a potential energy function.
00:42
We could go through a lot.
00:43
We could do a whole other problem analyzing this in the same way to justify that.
00:51
That it is conservative that you can write a potential energy.
00:55
But we don't need to do that here.
00:58
So let us look at this.
01:00
Delta u the changing potential energy is defined so this is u r i'm going to go from infinity to r and that's defined as minus the integral from infinity to r f dot d r prime i have an r here i want to have a different symbol here so we don't have any confusion just a dummy index i'll be able so it's okay don't be bothered by that.
01:34
Now, dr prime vector is dr prime, r hat prime.
01:41
Just think of this as the r prime now.
01:44
So, anytime i dot a vector with a unit vector, i get the component in that direction.
01:49
So it becomes minus infinity to r, f, r prime, dr prime.
01:57
That's what i get.
01:59
Now, as i have drawn, that is a positive rp.
02:04
Prime components.
02:06
It's in the positive r hat prime direction.
02:10
So now i can put in, this is minus infinity to r, a, e minus b, r prime, d, r prime.
02:24
And now we can do the integral.
02:26
This is minus a, and the integral of e to minus b r prime is minus b, e to minus b, and our limits infinity to r.
02:38
And we can clean this up a little bit.
02:42
A over b, eta minus b r prime, infinity to r.
02:50
And i'll put in our limits.
02:52
A over b, e minus br, minus e to minus b times infinity, but that's one over an extremely large number.
03:04
Back to the infinite.
03:05
This term does not contribute.
03:07
This term is gone.
03:08
And let's not forget they tell us also that you, we're to take you at infinity to be zero.
03:12
So this is ur now.
03:17
So it becomes a over b, e minus b, r.
03:22
That's the functional form.
03:24
Now they ask us to put it right for position d.
03:28
R is equal to d.
03:29
So we would write u at d, a, e minus b, d over b...