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Physics: Post your doubts here!

hussamh10

hussamh10

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Peter Check said:
I have a serious problem with uncertainties, almost all the sums i solve are wrong! For instance in question 1b(ii) 21/M/J/11 http://www.xtremepapers.com/papers/...nd AS Level/Physics (9702)/9702_s11_qp_21.pdf
I do not get how the uncertainty in A = 2 × (0.01 / 0.38) × 100 = 5.3 % I guess we multiply by 2 because of pie r square.
But because it is the radius why don't we halve the diameter's percentage uncertainty?
Also how do i decide between which significant figures, if nothing is mentioned, for instance in the question where the percentage uncertainties are .17% and .27, why is it not .17% and .267% or .20% and .27%
Click to expand...
final answer is always in 1 S.f both of absolute error and the value
 
R

raamish

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How do we take out speed of seperation and speed of approach. Its required to tell if collisions are inelastic or not
 
Yousuf Ykr

Yousuf Ykr

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Can anyone help me with these past papers' questions
M/J 02- 7b
O/N 02- 9b, 2c
O/N 03- 3 c
M/J 04- 1b
O/N 04- 1b (ii)
M/J 05- 5c, 6b(iii)
O/N 05- 3c
 
Peter Check

Peter Check

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hussamh10 said:
final answer is always in 1 S.f both of absolute error and the value
Click to expand...
the final answer right? But what if I use my percentage uncertainty as lets say 2.6666667 instead of 2.7(this is not the final answer) so when I use 2.666667 to add to the other two percentages, I will get a different answer!
 
Peter Check

Peter Check

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hussamh10 said:
Always use the formula Pi (D)^2/4 donot use Pi R^2 it adds complicatons i have been through this problem many times and my teacher reccomended to use the other formula .......
Click to expand...
What is that formula for? If it is for area, I am getting a different answer for the area, using that formula.
 
Unicorn

Unicorn

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applepie1996 said:
CAN SOMEONE PLEASE ANSWER QUESTIONS 6b and 6c
Click to expand...

for question 6b
they tell u that the distance between where sound is heard is 32.4 and if u look closely in fig 6.1 you will see it is a node
and the distance between 2 nodes is 0.5ƛ
and we know s=f * ƛ
so 0.5ƛ=32.4cm so ƛ=64.8cm =0.648

therefore 0.648*512 =331.78 -> 332m/s

for 6c
distance between a node and antinode it is 0.25ƛ which is half distance between 2 nodes
so 32.4/2 =16.2

and the length of air is 15.7 so 16.2-15.7 =0.5 above the end of the tube or simply 16.2 cm above water level

hope it helped
 
Jaf

Jaf

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Peter Check said:
What is that formula for? If it is for area, I am getting a different answer for the area, using that formula.
Click to expand...
You shouldn't. It's the same formula.
Cross-section area of the wire = πr² or π(d/2)² ---> π d²/2² ---> πd/4

We're not halving the uncertainty because we shouldn't! Think of how an uncertainty occurs when you're measuring the diameter. It is because of the markings in the ruler. You're halving the value read by the same ruler so the uncertainty will not change.
You will only multiply fractional uncertainties by a number if the value is raised to a power of the same number in the equation you're dealing with (the number could be a fraction too).
 
Peter Check

Peter Check

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A question on planning an apparatus to find the young modulus
A question part states that some has measured the original length of a wire, its extension, the load at extension, and deduced the cross-area. Now the question is Describe how the measurements taken can be used to determine the Young modulus.

A part of the answer in ms is:
plot a graph of force against extension (B1)
determine gradient of graph for F / e (B1)
calculate E from E = F l / e A or gradient × l / A (B1)

Is it necessary to plot a graph?? I mean I have all the values, can i just plug it into the equation for young modulus??
 
Peter Check

Peter Check

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Jaf said:
You shouldn't. It's the same formula.
Cross-section area of the wire = πr² or π(d/2)² ---> π d²/2² ---> πd/4

We're not halving the uncertainty because we shouldn't! Think of how an uncertainty occurs when you're measuring the diameter. It is because of the markings in the ruler. You're halving the value read by the same ruler so the uncertainty will not change.
You will only multiply fractional uncertainties by a number if the value is raised to a power of the same number in the equation you're dealing with (the number could be a fraction too).
Click to expand...
okay yeah, i thought the formula was pieD^.5
 
applepie1996

applepie1996

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Unicorn said:
for question 6b
they tell u that the distance between where sound is heard is 32.4 and if u look closely in fig 6.1 you will see it is a node
and the distance between 2 nodes is 0.5ƛ
and we know s=f * ƛ
so 0.5ƛ=32.4cm so ƛ=64.8cm =0.648

therefore 0.648*512 =331.78 -> 332m/s

for 6c
distance between a node and antinode it is 0.25ƛ which is half distance between 2 nodes
so 32.4/2 =16.2

and the length of air is 15.7 so 16.2-15.7 =0.5 above the end of the tube or simply 16.2 cm above water level

hope it helped
Click to expand...
but there is only one node so y r u taking the distance between 2 nodes and thnx!!!
 
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