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# FP3: directrix watch

1. the distance PS/ the distance PM (PS/PM) = e

what is e? I dont understand: is it the the distance between S & M?
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2. bump
3. (Original post by Maths&physics)
the distance PS/ the distance PM (PS/PM) = e

what is e? I dont understand: is it the the distance between S & M?
is a constant called the eccentricity. It is the ratio of as shown in your diagram.
4. (Original post by ManLike007)
is a constant called the eccentricity. It is the ratio of as shown in your diagram.
PS is the distance between points P & S?
5. (Original post by Maths&physics)
PS is the distance between points P & S?
Yes

is the distance between and

is the distance between and
6. (Original post by ManLike007)
Yes

is the distance between and

is the distance between and
what is a distance divided by a distance?
7. (Original post by Maths&physics)
what is a distance divided by a distance?
Distance divided by distance would give you no units so it's a ratio
8. (Original post by ManLike007)
Distance divided by distance would give you no units so it's a ratio
because the units would cancel?

superb.

the ratio will always be the same for the same ellipse but different if the ellipse is a different size?
9. (Original post by Maths&physics)
because the units would cancel?
Yh so the base unit for distance is meters so , hence it's dimensionless.

(Original post by Maths&physics)
superb.
Indeed

(Original post by Maths&physics)
the ratio will always be the same for the same ellipse but different if the ellipse is a different size?
I assume so yes (though you may want to check) so using your diagram, it's worth noting that

If , the point P describes an ellipse.

If , the point P describes a parabola.

If the point P describes a hyperbola.

I don't know this off the top of my head so credit to the FP3 text book.
10. (Original post by ManLike007)
Yh so the base unit for distance is meters so , hence it's dimensionless.

Indeed

I assume so yes (though you may want to check) so using your diagram, it's worth noting that

If , the point P describes an ellipse.

If , the point P describes a parabola.

If the point P describes a hyperbola.

I don't know this off the top of my head so credit to the FP3 text book.
thanks
11. (Original post by ManLike007)
Yh so the base unit for distance is meters so , hence it's dimensionless.

Indeed

I assume so yes (though you may want to check) so using your diagram, it's worth noting that

If , the point P describes an ellipse.

If , the point P describes a parabola.

If the point P describes a hyperbola.

I don't know this off the top of my head so credit to the FP3 text book.
How would e (the ratio) look like in relation to the graph? Thanks
12. bump
13. (Original post by ManLike007)
Yh so the base unit for distance is meters so , hence it's dimensionless.

Indeed

I assume so yes (though you may want to check) so using your diagram, it's worth noting that

If , the point P describes an ellipse.

If , the point P describes a parabola.

If the point P describes a hyperbola.

I don't know this off the top of my head so credit to the FP3 text book.
and is the x coordinate of the directrix a/e?
14. I was overthinking it. ive just watched a tutorial which explains it clearly. thanks

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