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# OCR B (Advancing Physics) - Section C watch

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1. ne1 know of a reason 2 measure the acceleration of something
2. (Original post by rpotter)
ne1 know of a reason 2 measure the acceleration of something
police want to use it for speed traps.
3. Isn't that just measuring velocity not acceleration
4. For seeing whether rockets will make it out of the atomosphere?
5. (Original post by Mathemagician)
thats not a formula
yeah yeah it's late, and i was trying to be helpful

Anyone know whether more lines per mm on a diffraction grating brings maxima closer together or further apart? This is really confusing me!
6. It moves them further apart. n lambda = d sin theta. If the number of lines per mm is increased, then d is decreased hence sin theta increases.
7. (Original post by dinkymints)
yeah yeah it's late, and i was trying to be helpful

Anyone know whether more lines per mm on a diffraction grating brings maxima closer together or further apart? This is really confusing me!

more lines per mm on a diffraction grating brings the maxima further apart.

lambda/d = sinx

since sin x is smaller, lamda/d must be smaller.

and since the wavelength of the light is the same, d must increase.
8. Thanks. So on that stupid LED question from UP June 2004 (which I'm sure someone has seen!) where there's light from an orange LED passing through a grating producing the following pattern:

red green (space) central maximum (space) green red

if the diffraction grating was replaced by another with twice the number of lines per mm, the green dots would move the same distance again away from the central maximum?

Also, what does the presence of just 2 dots, green and red, in 1st order spectrum tell you about the light waves coming from the LED?

And why is the intensity of the central maximum greater than the intensity of either red or green in first order?

Thanks, if anyone has any ideas... this q completely baffled and confused me!
9. the red light nd green light have different wavelengths
so,
sinθ=nλ/d
n is teh same so the angle through which it is defracted is different hence, two dots
hence the orange light is made up from two different parts
(i think)
10. Surely orange = red+yellow though, not red+green? Or am I just being nitpicky here?
11. (Original post by dinkymints)

if the diffraction grating was replaced by another with twice the number of lines per mm, the green dots would move the same distance again away from the central maximum?
yes
(Original post by dinkymints)
Also, what does the presence of just 2 dots, green and red, in 1st order spectrum tell you about the light waves coming from the LED?
that there are 2 different wavelengths from the source that superimpose to give these 2 colours.

(Original post by dinkymints)
And why is the intensity of the central maximum greater than the intensity of either red or green in first order?
because the resultant phasor for the light to arrive tothe centre is big (all the phasor arrows line up), so squaring this would make the probability big aswell. For the red or green dot, there is a phase difference so there will still be some destructive interference (little).
12. (Original post by dinkymints)
Surely orange = red+yellow though, not red+green? Or am I just being nitpicky here?
stupid drunken physics examiners.
13. Since this seems to be the general OCR B revision thread I have a few questions...

1. "A perticular material breaks after plastic deformation. It requires a large energy to create new surface area, but does NOT fracture by crack propogation" -- is it stiff or tough? Why?

2. Attached
Attached Images

14. (Original post by Setup)
Since this seems to be the general OCR B revision thread I have a few questions...

1. "A perticular material breaks after plastic deformation. It requires a large energy to create new surface area, but does NOT fracture by crack propogation" -- is it stiff or tough? Why?

2. Attached
I would guess at stiff, as no crack propogation. Probably wrong though... argh...

*edit* Actually, no, brittle materials fracture by crack propogation, so tough?
15. That's what I thought but then it can be stiff because stiff stuff doesn't bend easily... I am so screwed tomorrow lol
16. (Original post by Setup)
That's what I thought but then it can be stiff because stiff stuff doesn't bend easily... I am so screwed tomorrow lol
lol, me too.
By definition, tough materials require high energy per unit c-s area to create new fracture surface.
17. Ah then tough is right.

New query after going through a UP paper:

Apple of mass 0.15 kg falls from rest through a vertical height of 2.8m and knocks someone out. Calculate the speed at which it strikes the person (g = 9.8 N/kg).
18. (Original post by Setup)
Ah then tough is right.

New query after going through a UP paper:

Apple of mass 0.15 kg falls from rest through a vertical height of 2.8m and knocks someone out. Calculate the speed at which it strikes the person (g = 9.8 N/kg).
suvat? s=2.8, u=0, v and t unknown, a=9.8ms^2

v^2 = u^2 + 2*a*s
19. (Original post by Setup)
Ah then tough is right.

New query after going through a UP paper:

Apple of mass 0.15 kg falls from rest through a vertical height of 2.8m and knocks someone out. Calculate the speed at which it strikes the person (g = 9.8 N/kg).
u = 0
s = 2.8m
v = ?
a = 9.8ms^-2

v² = u² + 2as

v = 7.408...ms^-1

why is the mass of the apple given?
20. (Original post by Mathemagician)
u = 0
s = 2.8m
v = ?
a = 9.8ms^-2

v² = u² + 2as

v = 7.408...ms^-1

why is the mass of the apple given?
For another part of the question maybe...

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Updated: June 10, 2005
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