Help with edexcel a2 questions Watch

rahul03
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Hi,

Can someone please explain how to solve these questions. They are from past papers of edexcel.



correct answer is C






correct answer is B





correct answer is C



Thank you

rahul
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lerjj
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Ok, well for the first one you appear to know the equation as you've written it down.

It it was equally bright, but twice as far away, it must shine 4 times brighter as intensity drops of with an inverse square. However, it appears only half as bright, and if two things are equal distance away and one appears half as bright then that one has half the intensity. Combine those for 1/2X4=2 (I may have gotten confused between 'brightness' and 'intensity' in that paragraph. Just read it in a way that makes sense )


For the second one, you need to know the definition of simple harmonic motion. An object is in simple harmonic motion if and only if it has an acceleration that is opposite and proportional to it's displacement. The trampoline example fails this as it's acceleration is proportional only to gravity. Alternatively, SHM occurs in closed systems where a restoring force is present, whereas a trampoline requires you to continually jump.

I'm hideous with EM to be honest. Post you're working, and make sure you convert cm to m when you calculate area!
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Stonebridge
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3rd Question

Formula for force is
F=BIL on a single wire (do you know this one?)
as there are 50 turns then multiply by 50.
You are given B, I and L

Gives answer C
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Protoxylic
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(Original post by lerjj)
Ok, well for the first one you appear to know the equation as you've written it down.

It it was equally bright, but twice as far away, it must shine 4 times brighter as intensity drops of with an inverse square. However, it appears only half as bright, and if two things are equal distance away and one appears half as bright then that one has half the intensity. Combine those for 1/2X4=2 (I may have gotten confused between 'brightness' and 'intensity' in that paragraph. Just read it in a way that makes sense )


For the second one, you need to know the definition of simple harmonic motion. An object is in simple harmonic motion if and only if it has an acceleration that is opposite and proportional to it's displacement. The trampoline example fails this as it's acceleration is proportional only to gravity. Alternatively, SHM occurs in closed systems where a restoring force is present, whereas a trampoline requires you to continually jump.

I'm hideous with EM to be honest. Post you're working, and make sure you convert cm to m when you calculate area!

For the SHM question, acceleration due to gravity is held constant at 9.81ms^-2 assuming no resistive forces. So the acceleration due to gravity is not proportional to the displacement, however, the trampoline (assuming the person is in contact with the trampoline the whole upstroke) is proportional to its displacement because it is just an application of hookes law, F=-kx. So shouldn't it be the trampoline's force that is making the child accelerate that does show SHM, but gravity that does not? (This is assuming a uniform gravitational field as dh is small)
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lerjj
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(Original post by Protoxylic)
For the SHM question, acceleration due to gravity is held constant at 9.81ms^-2 assuming no resistive forces. So the acceleration due to gravity is not proportional to the displacement, however, the trampoline (assuming the person is in contact with the trampoline the whole upstroke) is proportional to its displacement because it is just an application of hookes law, F=-kx. So shouldn't it be the trampoline's force that is making the child accelerate that does show SHM, but gravity that does not? (This is assuming a uniform gravitational field as dh is small)
I guess so? The question is asking about the entire system though, so for most of the journey the restoring force is constant at gravity, but for the other part it is proportional to displacement against the trampoline, yes. But the entire motion is not SHM, only part of it.
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Protoxylic
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(Original post by lerjj)
I guess so? The question is asking about the entire system though, so for most of the journey the restoring force is constant at gravity, but for the other part it is proportional to displacement against the trampoline, yes. But the entire motion is not SHM, only part of it.
Yes, I understand that the whole system is not SHM, but I was just confused at the point where you said acceleration was only proportional to gravity. Which is true, but the statement just reads oddly.
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lerjj
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(Original post by Protoxylic)
Yes, I understand that the whole system is not SHM, but I was just confused at the point where you said acceleration was only proportional to gravity. Which is true, but the statement just reads oddly.
I guess I was only thinking about the part of the journey where you're not in contact with the trampoline. Additionally, you do actually have to jump when on a trampoline, so I'm not 100% sure that that part is pure SHM.
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Protoxylic
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(Original post by lerjj)
I guess I was only thinking about the part of the journey where you're not in contact with the trampoline. Additionally, you do actually have to jump when on a trampoline, so I'm not 100% sure that that part is pure SHM.
I think it is SHM if and only if the child is in contact with the trampoline from positive amplitude to zero displacement. If, at any point, the child is no longer in contact with the trampoline when the child is still at a finite positive displacement (taking up to be negative and down as positive), then SHM won't apply for the trampoline.
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lerjj
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(Original post by Protoxylic)
I think it is SHM if and only if the child is in contact with the trampoline from positive amplitude to zero displacement. If, at any point, the child is no longer in contact with the trampoline when the child is still at a finite positive displacement (taking up to be negative and down as positive), then SHM won't apply for the trampoline.
This is somewhat academic as the other three examples are just the standard spring-mass system and a pendulum. I also haven't yet covered SHM in class, so I'll defer to your above analysis.
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Protoxylic
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(Original post by lerjj)
This is somewhat academic as the other three examples are just the standard spring-mass system and a pendulum. I also haven't yet covered SHM in class, so I'll defer to your above analysis.
Yeah it is easy to perform a process of elimination on the other three examples, but I always like to look further and assess why. I have just about finished SHM and resonance in class so I'm just applying my knowledge here.
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Stonebridge
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It should be noted that the bungee cord example will only be SHM so long as the bungee rope is taut. If it goes slack when the jumper rebounds then the situation is the same as when the child leaves the trampoline mat and it isn't SHM.
I guess you have to assume this to be the case.
Like many multiple choice questions we get on here, this isn't one of the best.
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rahul03
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(Original post by Stonebridge)
3rd Question

Formula for force is
F=BIL on a single wire (do you know this one?)
as there are 50 turns then multiply by 50.
You are given B, I and L

Gives answer C
Hi Stonebridge,

Thank you for replying.

I'm little confused. Usually I find the force by taking into consideration the angle that the wire makes with the magnetic field. However, your method which works fine, seems different and I would like to know about the explanation. When do we use the relevant angle provided in the question?

Thanks,
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Stonebridge
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(Original post by rahul03)
Hi Stonebridge,

Thank you for replying.

I'm little confused. Usually I find the force by taking into consideration the angle that the wire makes with the magnetic field. However, your method which works fine, seems different and I would like to know about the explanation. When do we use the relevant angle provided in the question?

Thanks,

You are asked for the force on QS.
It's the wire PQ that makes the angle with the field.
QS is perpendicular to the field.
The angle is not actually relevant in this question and is rather confusing. (Another poor question. :rolleyes: )

The angle would have been relevant in a question either about the force on PQ or a question about the emf generated in the (spinning) coil.
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rahul03
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There appears to be an angle that side QS is making with the magnetic field as marked on the picture. The side QS doesn't seem to be perpendicular to the field. Can you please explain...

Thanks
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Stonebridge
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(Original post by rahul03)

There appears to be an angle that side QS is making with the magnetic field as marked on the picture. The side QS doesn't seem to be perpendicular to the field. Can you please explain...

Thanks
Explain what?
That is not the diagram in the question.
It's one you've made up. (And is incorrect)
Maybe you could explain what you've done there.

I think you've not read the question carefully.
Figure 2 is a view of the coil from above, looking down on PQ.
It's not a view from the side, which is what you seem to have drawn.
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