Unseen errors in official GCE pastpapers?

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Intpj
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#1
Report Thread starter 6 years ago
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I think that both B and A can be correct for a falling body.
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Plagioclase
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#2
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(Original post by Intpj)
I think that both B and A can be correct for a falling body.
It's not just a falling body, it's falling at terminal velocity. The upthrust is basically constant and since it's falling through air, it's going to be very small. So when the body is falling at terminal velocity, the drag will be a lot bigger. If this were simply any falling body then yes, you'd be right, but it's falling at terminal velocity through air.
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Intpj
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(Original post by Plagioclase)
It's not just a falling body, it's falling at terminal velocity. The upthrust is basically constant and since it's falling through air, it's going to be very small. So when the body is falling at terminal velocity, the drag will be a lot bigger. If this were simply any falling body then yes, you'd be right, but it's falling at terminal velocity through air.
Drag, like you say, increases with velocity and hence, it is maximum at Vmax or terminal velocity.However, terminal velocity can be extremely small (0.001ms-1 for eg). So can be the drag in a NEARLY MOTIONLESS body.
Take for example a helium balloon with upthrust = weight. No motion at all.
Let upthrust be 10N and weight 10N.
Now, sprinkle a grain of sand on the balloon. Let Mgrain= 0.00001g and Wgrain=0.0001N
At terminal velocity....
Upthrust =10N
Total Weight=10N + 0.0001N
Air resistance=0.0001N
And still velocity is downwards.
I think this concludes for B. Please note A can also be correct
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Plagioclase
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(Original post by Intpj)
Drag, like you say, increases with velocity and hence, it is maximum at Vmax or terminal velocity.However, terminal velocity can be extremely small (0.001ms-1 for eg). So can be the drag in a NEARLY MOTIONLESS body.
Take for example a helium balloon with upthrust = weight. No motion at all.
Let upthrust be 10N and weight 10N.
Now, sprinkle a grain of sand on the balloon. Let Mgrain= 0.00001g and Wgrain=0.0001N
At terminal velocity....
Upthrust =10N
Total Weight=10N + 0.0001N
Air resistance=0.0001N
And still velocity is downwards.
I think this concludes for B. Please note A can also be correct
I think you're being a bit pedantic here. I mean okay, you've very carefully constructed an obscure scenario where you may be right but I think it's fairly obvious what the question was implying. You do have to use a certain degree of common sense in exams. I've also come across questions in physics which you could pick holes in but picking holes isn't going to get you any marks. The question is talking about a ball falling through air, and any sensible person (taking into account that this is a fairly simplistic GCSE or A Level question) would think of a normal ball (that is not filled with a lighter-than-air substance since this is not implied in the question at all) that weighs a significant amount and does not displace a significant mass of air and will therefore have a comparatively significant amount of drag before it reaches terminal velocity in air.
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Intpj
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Report Thread starter 6 years ago
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I fully agree with you. To score well in an exam, one must think like an examiner. That is my weakness and I always get confused in exams.
Thanks for helping.
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Plagioclase
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#6
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(Original post by Intpj)
I fully agree with you. To score well in an exam, one must think like an examiner. That is my weakness and I always get confused in exams.
Thanks for helping.
Yes, definitely. It's a very sad thing that the exams system is so prescriptive but unfortunately, the fact of the matter is that examiners have a list of things they can give you marks for and if your answer, no matter how correct, isn't on there, you don't get the marks. Learning how to think like an examiner is one of the key things behind getting good grades!

Good luck.
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