Forumers, ask your Physics questions here ! (compilation)

• Flying grenade

  5 Jul 16, 02:06

  Please feel free to post your Physics questions here ! Let us all ask, discuss, and contribute in this Homework Forum @ SGForums ! :D

   

   

• Flying grenade

  5 Jul 16, 02:07

  [For H2 Physics(and H2 math)]

  MightyBiscuits asked : Why is it that for a small θ , sinθ = tanθ = θ?

  read and study this : https://en.m.wikipedia.org/wiki/Small-angle_approximation

• Flying grenade

  5 Jul 16, 02:25

  [For H2 Physics]

  MightyBiscuits posted regarding Work,Energy,Power & Forces qn,

  here: http://sgforums.com/forums/2297/topics/491885?page=1

  addressed by eagle and Flying grenade, with extension questions by eagle

• MightyBiscuits

  5 Jul 16, 23:20

  Thank you very much. May I also enquire if this also applies in a small change in mass?

• eagle

  6 Jul 16, 00:35

  Small angles approximations will be taught in your J1 Maths. 

  You have to go through Differentiation, Applications of differentiation, Maclaurin's Series first before learning small angles approximations. Most schools shouldn't have reached yet.

  Small angle approximations in Physics are generally not tested at H2 Physics level anymore. If they do test, they should provide you with the approximation. The question you have shared is a very very old question, probably outdated. 

• Flying grenade

  6 Jul 16, 02:43

  [For H2 Physics]

  MightyBiscuits asked :

  https://www.instagram.com/p/BHcq0BRhvew/?taken-by=lolwerwdr

  For CS Toh physics practice Qns 4(ii) (Oscillations)

   

  Flying grenade's suggested solution

  Qn already given you the equation for the Period of oscillation of a spring-mass balance.

  T=2π(m/k)^0.5

  Qn already given u the values required, both m and k.

  find period of oscillation before and after the astronaut gains 0.5kg by simple substitution and calculation.

  for % change in period of oscillation, take (finalT - initialT)/initialT x 100%. 

   

  from wiki : " The small-angle approximation is a useful simplification of the basic trigonometric functions which is approximately true in thelimit where the angle approaches zero." 

  for this question scenario, whereby we attempt to calculate mass by method of oscillations, small angle approximations is inapplicable and irrelevant. 

• Flying grenade

  6 Jul 16, 02:53

  Originally posted by MightyBiscuits:

  Thank you very much. May I also enquire if this also applies in a small change in mass?

  BTW MightyBiscuits, does Cstoh's physics practice qns book come with worked solutions either at the back of the book, after the chapter, or anywhere in the book ?

• Flying grenade

  6 Jul 16, 03:04

  For H2 Physics, and actually all subjects examined by SEAB&Singapore-Cambridge examinations,

  the Syllabus(which can be found at SEAB's website) , is highly useful.

  peruse and digest the great content outline and assessment objectives, which is truely a great summary,overview,and synopsis, and also Examples of the application of concepts and information in real life, making the Syllabus a very interesting, useful, and valuable read.

• eagle

  6 Jul 16, 15:46

  Hi,

  the syllabus, while great, isn't an accurate representation of the question patterns that might be tested. Relying on it solely is pretty dangerous. It's a good guideline, not a summary.

  Thanks :)

• Flying grenade

  6 Jul 16, 16:19

  Thanks eagle !!!

  yes i agree for physics, chemistry ,biology and maths,

  exposing to and doing many questions is very important. 

  exposing to new, different, atypical and unusual questions are fun and satisfying and ultimately helps very much in the preparation towards the national examinations. 

• iSean

  18 Jul 16, 20:18

  Thermal Physics - Continuous Flow
  
  

  Just wondering how do they make the temperature change constant though? Despite adjusting to increase the flow rate of water and voltage of heating coil for 300 seconds? New input of water is always replaces the heat away from the water.

  The temperature change shouldn't be constant though? 

  Or is they just want to collect the constant the change of temperature by adjusting it only? Like Set 1 : Ti 25'C, Tf 35'C and Set 2 is adjusted to Ti 30'C and Tf 45'C

  Also is it even possible to increase flow rate? As I thought mass flow in a tube is always constant in Fluid Dynamics? 

• eagle

  19 Jul 16, 09:22

  I don't understand the first part of your question. The temperature change refers to the difference in temperature between the input and output of the tube. 

  It is possible to increase flow rate. Imagine this: you turn on your tap a little, vs turn on a lot. There is no such rule that mass flow is always constant in Fluid Dynamics. The closest you have, for water which is incompressible, is the continuity principle, where the volume in equals the volume out per unit time.

  More volume in per unit time means more mass flows through in per unit time. Consequently, the thermal energy required per unit time (and hence power) needed to raise the temperature between the input and output will be higher, because heat capacity is proportional to mass. 

   

• iSean

  26 Jul 16, 21:33

  Actually I'm quite uncertain regarding the steps of doing this experiment to determine specific heat capacity. 

  From what I can see here is my time is fixed, and my temperature change is fixed.
  What are changed are my flow rate and power output.

  Procedures:
  1. Set up the apparatus as shown. 
  
  Collection of Set I 
  2. Run water at a constant flow rate until the two thermometers at water input and output zone reach a constant value, And this will be me "temperature change"?
  3. Then I start timing for 300s? 
  4.  Also collect Mass of Liquid collected, M1,  Voltage, V1 and Current, I1 at this moment for 300s?
  
  Collection of Set II
  5. Run water at a higher flow rate, and adjust power by rheostat, until the two themometers reaches the same temperature change as Set I, then I collect the data again?
  
  The book didn't really explain how they do the procedure tho.

• Flying grenade

  26 Jul 16, 21:40

  Hi iSean, may i ask u from which book/source are the questions in the 2 pictures u posted above from?

• domefossil

  27 Jul 16, 04:16

  Basic sf question (H2):

  The fractional uncertainty of a product or quotient of 2 quantities is the sum of the fractional uncertainties of those 2 quantities, i.e.

  
  

  r = xy or r = x/y

  dr/r = dx/x + dy/y

  
  

  e.g. Two lengths are reported as: x = 18.5 +/- 0.5 mm, y = 12.5 +/- 0.5 mm

  
  

  dr/r = 0.5/18.5 + 0.5/12.5 = 0.067

  r = 18.5 x 12.5 = 231.25

  dr = 0.067 x 231.25 = 15.49 = 20 mm (1 sf)

  
  

  r = 230 +/- 20 mm

  
  

  (the underlines are mine)

  
  

  I'm confused as to how the rounding ups come about.

  How did 231.25 become 230? Where does the 2 sf come from? Why not 3 sf and 231 since it's multiplication and the quantity with the lowest sf has 3 sf?

  Shouldn't dr/r be 0.07? 0.5/18.5 = 0.03 (1 sf, following number of sf in 0.5 for division), 0.5/12.5 = 0.04, 0.03 + 0.04 = 0.07
• Flying grenade

  27 Jul 16, 16:15

  in response to domefossil's question

  for this qn, the value of the quantity should be rounded off to no decimal places , i.e. 231mm(0 dp) . This is because, the uncertainty of this quantity have no decimal places.

  if for example, x is given to be x=18.0 , and y=12.5, then your final value of r must be rounded off according to the no. of decimal places of the uncertainty.

  i didn't calculate your numerical working, assuming your numerical working is correct, the value of Δr you have calculated should be correct, i.e. 20mm(1sf round up)

  Δr/r =Δx/x + Δy/y is the right formula to use. 

  you cannot calculate Δx/x and Δy/y and round up to 1sf first then add (even if u did, which is the wrong way, you'll still get 20mm(1sf) coincidentally) , 

  only round up 1sf after u get Δr . 

• Flying grenade

  27 Jul 16, 16:36

  Cambridge will unlikely ask you for the calculated value of fractional uncertainty. 

  They will most probably ask you to calculate the value of actual uncertainty. 

  If they did ask , the value of fractional uncertainty should be given as 0.067(2sf round up)

   

• Flying grenade

  27 Jul 16, 17:13

  percentage uncertainty must be rounded up to 2sf.

• domefossil

  28 Jul 16, 04:20

  
  

  Originally posted by Flying grenade:

  percentage uncertainty must be rounded up to 1sf.

  For this question only or as a rule of thumb?

  Not necessarily specific to this question, is there a rule on how many sig figs to use for raw quantities? Is it ok to present a half answer like Δr/r as 0.5/18.5 + 0.5/12.5 = 0.067027027, if it's not the final answer to the question? Thanks bro.

• Flying grenade

  28 Jul 16, 08:05

  percentage uncertainty must always be rounded up to 2sf.

  if it's not the final answer to the question, you can give that long string of values. or u can give all intermediate workings to be 5sf.

  My Apologies, if Cambridge do ask you for fractional uncertainty, give it to 2sf(round up)

  intermediate workings for fractional uncertainties should be given as 5sf or more sf,

  cannot round up to 1sf for intermediate workings , lest your final calculated value will be inaccurate and thus incorrect.

• eagle

  28 Jul 16, 14:42

  Originally posted by iSean:

  Actually I'm quite uncertain regarding the steps of doing this experiment to determine specific heat capacity. 

  From what I can see here is my time is fixed, and my temperature change is fixed.
  What are changed are my flow rate and power output.

  Procedures:
  1. Set up the apparatus as shown. 
  
  Collection of Set I 
  2. Run water at a constant flow rate until the two thermometers at water input and output zone reach a constant value, And this will be me "temperature change"?
  3. Then I start timing for 300s? 
  4.  Also collect Mass of Liquid collected, M1,  Voltage, V1 and Current, I1 at this moment for 300s?
  
  Collection of Set II
  5. Run water at a higher flow rate, and adjust power by rheostat, until the two themometers reaches the same temperature change as Set I, then I collect the data again?
  
  The book didn't really explain how they do the procedure tho.

  Hi iSean, 

  the procedure you have listed is already extremely clear. How much clearer do you need?

  In my opinion, you are lacking certain Physics concepts to be unable to understand the procedures. 

  I notice that you did not specify the level of the question. 
  Are you an O level student? 
  Or if you are an A Level student, did you take Pure Physics during O Level, or just combined science?

  Your physics concepts that seems to be lacking from O Level are 
  1) Work Energy Power
  2) Thermal Properties of Matter 
  3) Circuits as well as Practical Electricity
  and the combination of these topics together.

  You have too many concepts missing, and no indication of which level of study you are at, so I have no idea where to start advising you too. :x

• senga

  29 Jul 16, 18:19

  New guy here. I have a question for Eagle. So yesterday I ask ultimaonline how I should go about studying for a levels chem. He recommend I get CS Toh study guide and start doing yearly TYS. Should I use this approach for chemistry and math??

• UltimaOnline

  29 Jul 16, 18:29

  Originally posted by senga:

  New guy here. I have a question for Eagle. So yesterday I ask ultimaonline how I should go about studying for a levels chem. He recommend I get CS Toh study guide and start doing yearly TYS. Should I use this approach for chemistry and math??

  
  Yes, you should use this approach for H2 Chemistry. What you mean to ask Eagle, is should you use this approach for H2 Physics and H2 Math. Be careful, careless mistakes can easily cost you a grade difference in the A levels.
• senga

  29 Jul 16, 18:38

  Opps. I meant to say physics. Thanks for pointing it out.

• iSean

  29 Jul 16, 20:37

  Originally posted by Flying grenade:

  Hi iSean, may i ask u from which book/source are the questions in the 2 pictures u posted above from?

  CIE A Level Physics Coursebook by Mike Crundell of Hodder Education
  Here is the link.
  http://gceguide.com/Books/Cambridge%20International%20AS%20and%20A%20Level%20Physics%202nd%20ed.pdf