Integrating to get r^2 = l^2 + a^2

  • Thread starter Lapidus
  • Start date
  • Tags
    Integrating
In summary: On the left, we can use the substitution x = r^2 - a^2, so that dx = 2r dr. This gives ##\int \frac{dx}{2 \sqrt{x}} = \sqrt{x} + C_1 = \ln |r + \sqrt{r^2 - a^2}| + C_1 = dl##.Solving for r, we get r + \sqrt{r^2 - a^2} = e^{dl - C_1}, which can be rewritten as r^2 = e^{2(dl - C_1)} - a^2.Now, we set l = 0 and r = a. This gives us a^2 = e
  • #1
Lapidus
344
12
I'm reading a physics book( "Einstein's gravity in a nutshell" Zee) and at page 126 the author says:

(dr/dl)^2 + a^2 / r^2 = 1

gives r^2 = l^2 + a^2

where we absorbed an integration constant into l by setting l=0 when r=a.

Can someone explain what's going on here? How do I have to "massage" the first equation to make an integration that gives the second equation? What does Zee mean with absorbing an integration constant?

Thanks for any respons!
 
Physics news on Phys.org
  • #2
Lapidus said:
I'm reading a physics book( "Einstein's gravity in a nutshell" Zee) and at page 126 the author says:

(dr/dl)^2 + a^2 / r^2 = 1

gives r^2 = l^2 + a^2

where we absorbed an integration constant into l by setting l=0 when r=a.

Can someone explain what's going on here? How do I have to "massage" the first equation to make an integration that gives the second equation? What does Zee mean with absorbing an integration constant?

Thanks for any respons!
Rewrite the DE as dr/dl = ##\pm \sqrt{1 - \frac{a^2}{r^2}} = \pm \frac{\sqrt{r^2 - a^2}}{|r|}##
This equation is separable, with a fairly easy integration. The work is simpler if you can assume that r > 0 and dr/dl > 0.
 
  • #3
Mark44 said:
Rewrite the DE as dr/dl = ##\pm \sqrt{1 - \frac{a^2}{r^2}} = \pm \frac{\sqrt{r^2 - a^2}}{|r|}##
This equation is separable, with a fairly easy integration. The work is simpler if you can assume that r > 0 and dr/dl > 0.

Thanks, Mark. But I am afraid, I do not follow. For example, how does the l^2 emerge?
 
Last edited:
  • #4
If it's reasonable to assume that dr/dl > 0 and r > 0, we can write the DE as ##dr/dl = \frac{\sqrt{r^2 - a^2}}{r}##.

Separating this, we get ##\frac{r dr}{\sqrt{r^2 - a^2}} = dl##.

Now integrate both sides.
 
  • Like
Likes Lapidus

FAQ: Integrating to get r^2 = l^2 + a^2

What does the equation "r^2 = l^2 + a^2" represent?

The equation represents the Pythagorean theorem, which states that in a right triangle, the square of the length of the hypotenuse (r) is equal to the sum of the squares of the other two sides (l and a).

How is this equation related to integration?

The equation is related to integration because it can be used to find the length of a curve on a graph. By integrating the equation, we can find the distance along the curve between two points (r) by breaking it down into smaller segments (l and a) and using the Pythagorean theorem to calculate each segment.

What is the purpose of integrating to get r^2 = l^2 + a^2?

The purpose of integrating to get r^2 = l^2 + a^2 is to find the length of a curve on a graph, which is useful in many fields such as physics, engineering, and economics. It allows us to calculate distances and areas that cannot be easily measured with traditional methods.

Can this equation be applied to any curve?

Yes, this equation can be applied to any curve as long as it is a continuous function. This means that the curve has no breaks or gaps and can be represented by a single equation.

Are there other methods for finding the length of a curve?

Yes, there are other methods for finding the length of a curve, such as using the arc length formula or approximating the curve with straight lines and calculating the sum of their lengths. However, integrating to get r^2 = l^2 + a^2 is often the most accurate and efficient method.

Similar threads

Replies
7
Views
2K
Replies
10
Views
2K
Replies
1
Views
1K
Replies
6
Views
3K
Replies
4
Views
2K
Replies
45
Views
5K
Replies
1
Views
2K
Back
Top