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# Physics question: AS watch

1. the last 2 questions
in the mark scheme it says that the acceleration reduce, but if density decrease, shouldn't the acceleration increase?
Attached Images

2. (Original post by etomac)
It's a past ppr (screenshot - GIF), can I post it?
Try at least.
3. no is not that i don't know, its that I don't agree with the markscheme
4. (Original post by etomac)
no is not that i don't know, its that I don't agree with the markscheme
I think what was meant was "try to post the screenshot".
5. o right
hope i won't get banned or anything : )
see the first post, i am editing it
6. (Original post by etomac)
no is not that i don't know, its that I don't agree with the markscheme
The mark schemes you see aren't necessarily the ones used to mark the exam, often mistakes are spotted and ammended mark schemes are used.
7. done
please check, maybe i am wrong
8. (Original post by etomac)
the last 2 questions
in the mark scheme it says that the acceleration reduce, but if density decrease, shouldn't the acceleration increase?
the acceleration is proportional to density (of the air outside), so it wil reduce with height
9. i thought
upthrust + drag force = weight
density of fluid x volume x gravity + 6 pi radius x coefficent of viscosity x VELOCITY = density of the thing falling x volume x gravity

so if density of fluid decrease and other thing keep constant, velocity will increase
10. (Original post by etomac)
i thought
upthrust + drag force = weight
density of fluid x volume x gravity + 6 pi radius x coefficent of viscosity x VELOCITY = density of the thing falling x volume x gravity

so if density of fluid decrease and other thing keep constant, velocity will increase
that equation only applies in a balanced situation. but there is an acceleration, so the forces aren't balanced. you have to use Newton's Second Law.
11. (Original post by elpaw)
that equation only applies in a balanced situation. but there is an acceleration, so the forces aren't balanced. you have to use Newton's Second Law.
Resultant Force On An Object = (Mass Of Object) * (Acceleration In Direction Of Resultant Force)

Abbreviated to: F = ma
12. so how does density relates to that?
there is less atoms per volume so it should move faster
13. density = mass/volume
mass = volume x density
mass = 2500x1.3
F = ma
F = 2500x1.3x9.8
=~32000N
14. (Original post by etomac)
so how does density relates to that?
there is less atoms per volume so it should move faster
The greater the density, the greater the number of atoms per unit volume.
15. b) Drag
c) arrow going left
16. yes i know
the question says density decreases --> less atoms per volume but the acceleration decreases
17. (Original post by etomac)
yes i know
the question says density decreases --> less atoms per volume but the acceleration decreases
Which part is this on now sorry?
18. the last part

Mathematician told be that it is because

density = m/v
force = ma or m = force/a
density = force/a/v = Fv/a

therefore density is inversly propotional to acceleration
19. (Original post by etomac)
yes i know
the question says density decreases --> less atoms per volume but the acceleration decreases
In reducing the density of the air outside the balloon, you are reducing the buoyancy of the balloon therefore reducing its accelleration.
20. density = d
volume = V
mass = m
velocity = v
coeff of viscocity=k

=> dVg -6πrkv -mg = m dv/dt

you have to solve that differential equation, and you will see that reducing d will decrease |acceleration|.

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