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# The Physics PHYA2 thread! 5th June 2013 watch

• View Poll Results: What mark do you think you got out of 70?
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1. (Original post by posthumus)
mgh = 1/2 mv^2

m cancel out

EDIT: It is a pretty stupid question.... but there's more friction for bigger masses. They've made it negligible in this question.
But friction is due to the surface. However, the question is 'The world record for a high dive into deep water is 54 m.'. So, clearly friction is negligible.
2. (Original post by StalkeR47)
But friction is due to the surface. However, the question is 'The world record for a high dive into deep water is 54 m.'. So, clearly friction is negligible.
The force provided by friction is dependent on the medium and the force downwards (mg). So if you did take friction into account then the mass would make a difference.
3. (Original post by StalkeR47)
Hi! I was also stuck on this question. The question is relatively easy but the fact that no GPE is converted to KE is confusing. I think little GPE is converted into KE and rest is wasted as heat, sound due to friction and air resistance.
That question also is a bit weird, they stress that there is constant velocity so Ek=1/2mv^2, 'v' and 'm' are constant so kinetic energy can't really increase.

Atleast that's what I think, your thoughts?
4. (Original post by NedStark)
That question also is a bit weird, they stress that there is constant velocity so Ek=1/2mv^2, 'v' and 'm' are constant so kinetic energy can't really increase.

Atleast that's what I think, your thoughts?
Sorry, I thought you are talking about question 2. I just edited the answer on my initial post. I think because there is no surface, there would be not heat or thermal energy wasted as Ke. So, all of the gpe is transferred to ke.
5. (Original post by StalkeR47)
Sorry, I thought you are talking about question 2. I just edited the answer on my initial post. I think because there is no surface, there would be not heat or thermal energy wasted as Ke. So, all of the gpe is transferred to ke.
I was referring to Q2 as well (in this post). I'm leaving question 1d for now.
6. Is this how this standing wave will be travelling?

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7. (Original post by NedStark)
That question also is a bit weird, they stress that there is constant velocity so Ek=1/2mv^2, 'v' and 'm' are constant so kinetic energy can't really increase.

Atleast that's what I think, your thoughts?
They want you to think that so the calculation is easier. Obviously the person will accelerate a bit at the start But Terminal velocity will be reached fairly quickly... so average velocity is constant and equal to the gravitational potential energy the diver had 54m above.

The reason why mass is negligible is best explained if you look at the equations equated to each other:

mgh = 1/2 mv^2

mass can cancel out: v=root(2gh)

velocity (and hence KE) is independent of the mass.

Also the reason they want you to consider air resistance negligible is that if there was friction. This would be dependent on the mass because F=mg contributes to force provided by friction.
8. (Original post by NedStark)
I was referring to Q2 as well (in this post). I'm leaving question 1d for now.
For Q2) You are Correct! This question is an absolute stupid! As you move downhill, there is always a gain in Ke is you have any GPE left. However, as you move downhill and cover a great distance, gain in ke reduces and most of the GPE is wasted as heat and sound due to the friction like I said in my first post. Is that ok?
9. (Original post by Jimmy20002012)
Is this how this standing wave will be travelling?

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What are those red marks? But the diagram is correct for standing/stationary wave.
10. Not looking forward to this exam...I want it to be mainly mechanics and minimal waves!
11. Just done all the end of unit questions in the textbook... ouch
12. (Original post by g.k.galloway)
Not looking forward to this exam...I want it to be mainly mechanics and minimal waves!
Why mechanics? Most people do not like mechanics. HAHA.
13. (Original post by SortYourLife)
Just done all the end of unit questions in the textbook... ouch
What about past papers? All of them?
14. (Original post by StalkeR47)
What are those red marks? But the diagram is correct for standing/stationary wave.
They are my attempt of arrows showing the way the wave will travel, will they travel in that direction?

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15. (Original post by g.k.galloway)
Not looking forward to this exam...I want it to be mainly mechanics and minimal waves!
Would love another 6 marker on anything but an experiment. Waves would be nice.

I'm guessing it'll be 6 marks on energy changes on an object. You heard it here first.
16. (Original post by StalkeR47)
Why mechanics? Most people do not like mechanics. HAHA.
I guess I'm not 'most people' then

The main issue for me is I have biology tomorrow and chemistry on tuesday so revision time for physics is hard to find!
17. (Original post by StalkeR47)
Why mechanics? Most people do not like mechanics. HAHA.

(Original post by g.k.galloway)
Not looking forward to this exam...I want it to be mainly mechanics and minimal waves!
I love the mechanics aha :') hate the waves. bleurgh, no good with written answers. mechanics doesnt have any of that
18. (Original post by NedStark)
Would love another 6 marker on anything but an experiment. Waves would be nice.

I'm guessing it'll be 6 marks on energy changes on an object. You heard it here first.
19. (Original post by Jimmy20002012)
They are my attempt of arrows showing the way the wave will travel, will they travel in that direction?

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A standing or stationary wave travel in opposite direction (In phase) so they cancel each other out (destructive) and they will produce 0 amplitude. They do not travel in 1 direction but opposite to each other.
20. (Original post by Qari)
For the v were you on about vernea caliber?
that was it! thanks aha

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