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# AQA Physics PHYA5 - Thursday 18th June 2015 [Exam Discussion Thread] watch

1. Question for turning points.

Special relativity:
Is the length for the observer always shorter than that for the person travelling?

Is the rest mass always smaller than the mass due to its speed

Is the time always shorter for the moving particle

Basically
Is length observer<moving object

Mass : m0<m

Time t<to

I know this is confusing but can someone just simply lay it out for me please

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2. (Original post by gcsestuff)
Question for turning points.

Special relativity:
Is the length for the observer always shorter than that for the person travelling?

Is the rest mass always smaller than the mass due to its speed

Is the time always shorter for the moving particle

Basically
Is length observer<moving object

Mass : m0<m

Time t<to

I know this is confusing but can someone just simply lay it out for me please

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That's right, but i assume for the length you are referring to the length of the moving object.

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3. (Original post by Mehrdad jafari)
That's right, but i assume for the length you are referring to the length of the moving object.

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Yeah that's what I meant! Just wanted to know as a mental check so I know I'm right

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4. (Original post by gcsestuff)
Yeah that's what I meant! Just wanted to know as a mental check so I know I'm right

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Oh cool. Yeah i know, when you make a comparison the results seem to contradict each other as in m>m0 whereas t<t0 because you expect t0<t as in the case with mass. I prefer to think about them to distinguish them so for example i know that time slows down for a uniformly moving object

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5. i really dont understand what is meant by critical mass in fission reaction, can someone explain it to me pls
6. (Original post by Mehrdad jafari)
Oh cool. Yeah i know, when you make a comparison the results seem to contradict each other as in m>m0 whereas t<t0 because you expect t0<t as in the case with mass. I prefer to think about them to distinguish them so for example i know that time slows down for a uniformly moving object

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Yeah that's what I'm trying to do

I always think about the space ship and time so I know moving objects time is slower but what do you think about for the other

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7. (Original post by coqthepoliceman)
i really dont understand what is meant by critical mass in fission reaction, can someone explain it to me pls
The critical mass is the smallest mass of fissile material in the fuel rods required to provide a sustainable chain reaction.

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8. (Original post by coqthepoliceman)
i really dont understand what is meant by critical mass in fission reaction, can someone explain it to me pls
Critical mass is- the minimum amount of mass required for a self sustaining nuclear fission reactor. This means that it has enough fissionable material in the fuel so that when a thermal neutron hit the fuel rod it has a good chance of hitting a fissionable nuclei.

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9. (Original post by coqthepoliceman)
i really dont understand what is meant by critical mass in fission reaction, can someone explain it to me pls
In order to maintain a reaction rate of one neutron per fission to cause another fission.

So you'll need a critical mass of fuel to maintain this... If you go below it than the nuclear reactor will cease to function you go above then it would go out of control.

Correct me if I'm wrong please
10. Can someone help explain Q 2b on the june 2011 paper please?
11. thanks for clearing that up, just couldnt understand the wording of the book
12. (Original post by MSB47)
In order to maintain a reaction rate of one neutron per fission to cause another fission.

So you'll need a critical mass of fuel to maintain this... If you go below it than the nuclear reactor will cease to function you go above then it would go out of control.

Correct me if I'm wrong please
You are correct. However, something to note, in actual fission reactors they use super critical mass (i.e. greater than the critical mass) and use control rods to limit the reaction rate by raising or lowering the control rods into the reactor.
13. (Original post by gcsestuff)
Yeah that's what I'm trying to do

I always think about the space ship and time so I know moving objects time is slower but what do you think about for the other

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So the same with others. A moving body has kinetic energy and so has a greater mass than its rest mass.
With the length it's quite tricky thinking about it. Of course a moving body shrinks in length but also the distance the moving body travels shrinks because as the time in the moving frame of reference slows down this means that the distance the moving frame travels has to shrink so that the stationary observer will see that the moving frame travels the same distance in the same time, if that makes sense but I'm not really sure about the length contraction of the moving frame itself

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14. (Original post by _Caz_)
X
Can I ask how it got the value for (a) in your Astro telescope CGP question?

I got this, which I know I wrong according to the mark scheme you posted, but if it's a single 25m dish, surely you just do the wavelength over 25m?

Even when I do the wavelength over 25x27 (as seen on the right) it gets a number which is out by a factor of ten?

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15. For those doing Turning Points is muon decay really important?
16. (Original post by Disney0702)
For those doing Turning Points is muon decay really important?
Yes, we do need to for the evidence of time dilation, unfortunately

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17. Came across this question, am I missing something because I used the equation given in the data sheet, which has (1-v^2/c^2)^-0.5, however they did not have the minus power?

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18. Can someone explain where equation Theta = Sr comes from??
i dont understand how it makes sense when using it in calculations and what do the S and r represent??

thanks
19. (Original post by SuperMushroom)
Can someone explain where equation Theta = Sr comes from??
i dont understand how it makes sense when using it in calculations and what do the S and r represent??

thanks
http://www.mathwarehouse.com/trigono...a-equation.php

For a small angle approximation, the arc length is almost like a triangle, meaning we can simply treat it as theta = S/r

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20. (Original post by CD223)
http://www.mathwarehouse.com/trigono...a-equation.php

For a small angle approximation, the arc length is almost like a triangle, meaning we can simply treat it as theta = S/r

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Ill have a look at this now, thanks

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