How did Physics M/J '17 52 go?

[rahimshah98]

Oh, i used max E -E to find the uncertainty, i.e. 1/min gradient - 1/gradient.
Gradient was around 0.14 with error of about 0.006. So, 7% look pretty low to me

 

[Shafin Ahmed]

For question 1, I used slow motion video camera with timer alongside a dot marked on turntable edge to calculate "t", hence f=1/t at the point when mass starts to slide. Variable power supply to vary motor's rotation speed.

For question 2, most people had gradient near 0.140 ( Mine was 0.143) . The y-intercept was around 4.2 .
For last 2 questions, my uncertainties were around i)9% and ii)23%.

For people getting over 100% (could be the right answer too , who knows.), it could be DUE TO WRONG MEASUREMENTS OF THE VALUE OF EACH SMALL SQUARE ( there was a tricky part somewhere in counting the squares) or maybe LARGE ERROR BARS.

 

[rahimshah98]

^ Shouldn't f be 2pi/t because it was circular motion?

 

[Shafin Ahmed]

I think I wrote the wrong dependant and independant variable. I wrote keep mass of cube constant and density. Current and potential difference across motor should be constant. Measure r with ruler and m with balance. Clean surface of turntable. Use safety screen to protect hair being trapped in turntable. And then of motor to avoid overheating. I said use alternate power supply with variable frequency signal generator. Reading frequency of generator. Measuring r when cube stabilizes. I wrote plot a graph between 1/r and f. Relationship valid if straight line from origin. Keep current constant using rheostat and ammeter. Cathode Ray Oscilloscope can be used to measure frequency to by multiplying number of blocks with timebase settings and then doing 1/t to get f. I wrote that pass a small enough frequency to produce measurable value of r and repeat experiment for values of Frequency and take average r. I already feel quite upset, but have I salvaged some marks or E.C.f ? Because I thought it said cube does not move relative to motion of turntable

The frequency that the question mentioned was "frequency of the turntable" and not frequency of the power supply as far as I remember. The frequency of power supply and frequency of turntable are likely different things

 

[Shafin Ahmed]

^ Shouldn't f be 2pi/t because it was circular motion?

I thought of the same thing at first (circumference or 2 pi/ time ) and even wrote it on paper but then realised that circumference / time = velocity . While 2 pi / time = ANGULAR VELOCITY.
Frequency should most probably be 1 / (time taken for one complete rotation of turntable)

 

[rahimshah98]

I wrote 1/t in the paper myself but my friends said that it was 2pi/t. Thanks for the clarification!!

 

[rahimshah98]

man this is confusing, i hope I get ecf atleast. Gonna get A/B now. or in worst case C.

Well, P5 has the lowest weightage while P4 counts for about 50% of your entire A2 grade.
So, you can do well next week and get the A*!

 

[Rizwan Javed]

I got a y-intercept of 4.6 +- 3, and the percentage uncertainty in Q to be 70sth Am I wrong? Otherwise, Q1 went fantastic!

 

[Qaussain]

I got exact y intercept =5.20 ,the absolute error was about ±4.68 (Due to much error range of P (9.0-34), it was abovious that uncertainity would b larger.,
Q1, can b two way.. you vary f and measure r,or you vary r you measure f,
For uncertainity in Q i got 101%, in E =11.8%

 

[Tozu Fofo]

The frequency that the question mentioned was "frequency of the turntable" and not frequency of the power supply as far as I remember. The frequency of power supply and frequency of turntable are likely different things

I think you are mistaken mate, if (angular) speed increases so does frequency as it is equal to 2pi f

 

[Tozu Fofo]

Oh, i used max E -E to find the uncertainty, i.e. 1/min gradient - 1/gradient.
Gradient was around 0.14 with error of about 0.006. So, 7% look pretty low to me

Bro, look at your figures 1/0.134 - 1/0.14 = 0.32 divide by E ( 1/0.14) times 100 = 4.48 %

 

[Shafin Ahmed]

I messed up Q2. I used x2-x1/y2-y1 to find the gradient instead of vice versa. Now my Y-intercept, value of E and value of Q and all the uncertainties are wrong because of that stupid mistake

I think you are mistaken mate, if (angular) speed increases so does frequency as it is equal to 2pi f

Sorry buddy, its just my opinion but I think that in your answer you used variable frequency signal generator, no problem with that. You can change rotational speed using that. No problem here either. However, as I mentioned before, in your answer, you are varying the frequency of power supply but you DID NOT MEASURE the FREQUENCY OF "TURNTABLE", which means how many times the TURNTABLE rotates per second.

About the screenshot you posted, thats a complex thing and not in the syllabus of A levels (variable frequency signal generator ) and also it wasn't what I meant to talk about.

 

[Shafin Ahmed]

I messed up Q2. I used x2-x1/y2-y1 to find the gradient instead of vice versa. Now my Y-intercept, value of E and value of Q and all the uncertainties are wrong because of that stupid mistake

I did the same mistake in AS when I had a panic attack and used x2-x1/y2-y1 . However, dont worry, you should get the ECF marks if the methods in later questions are correct.

 

[Tozu Fofo]

Sorry buddy, its just my opinion but I think that in your answer you used variable frequency signal generator, no problem with that. You can change rotational speed using that. No problem here either. However, as I mentioned before, in your answer, you are varying the frequency of power supply but you DID NOT MEASURE the FREQUENCY OF "TURNTABLE", which means how many times the TURNTABLE rotates per second.

About the screenshot you posted, thats a complex thing and not in the syllabus of A levels (variable frequency signal generator ) and also it wasn't what I meant to talk about.

My young Padawan, you cannot question the sensei, sit down... class has began.

 

[Zaina Tariq]

Same but percentage uncertainty not that high for me was of 7%

most of us got this right.

 

[Zaina Tariq]

My young Padawan, you cannot question the sensei, sit down... class has began.

sensei I myself was confused seeing the question so I analaysed it carefully and applied F=mrw^2(4pie^2×frequency^2) which helped me alot solving it.

 

[Zaina Tariq]

Well, P5 has the lowest weightage while P4 counts for about 50% of your entire A2 grade.
So, you can do well next week and get the A*!

buddy I hope so bcuse I spent alot of time on P4. its going to be very tough after seeing P5.

 

[Zaina Tariq]

People can we discuss question number 1 in detail cause Q2 we will prolly get ecf and will get around 10-12 marks who got their Y-intercept wrong. Also Question 1 had alot of diff points to gain marks, I will start with what I wrote.
☆Freq is IV.
☆r is DV.
☆mass to be constant also the distance from centre of turntable to radius of mass M, at starting point should be samw.
☆freq could be varied by increasing or decreasing(I used oscilloscope to vary freq). I chosed to decrease bcusw that sounded easier to me and I plotted r againt 1/f which means as I decreased the freq, r moved was greater.
☆then telling them how to get my gradient and then our constant K.
☆I measured change in r using a meter ruler and I also stated to repeat experiment several time and average our r moved to prevent any errors.
☆safety precautions: I stated that motor heats up so take breaks btw experiment to let it cool down. or used heat resistant gloves.

Guys that how I wrote it and I cant remember what else I wrote but I will be loosing marks bcusw of not stating *measurimg mass of block*

 

[rahimshah98]

You are absolutely right. I multiplied the absolute error by 100 instead of the fractional uncertainty.
I dunno how to feel about that.

Bro, look at your figures 1/0.134 - 1/0.14 = 0.32 divide by E ( 1/0.14) times 100 = 4.48 %

 

[rahimshah98]

1. An oscillope can measure frequency but can't vary it. You need a signal generator for that.
2. I am not sure about your safety precaution. I recommended a protective screen to prevent the mass from flying off the turntable and hitting someone.

People can we discuss question number 1 in detail cause Q2 we will prolly get ecf and will get around 10-12 marks who got their Y-intercept wrong. Also Question 1 had alot of diff points to gain marks, I will start with what I wrote.
☆Freq is IV.
☆r is DV.
☆mass to be constant also the distance from centre of turntable to radius of mass M, at starting point should be samw.
☆freq could be varied by increasing or decreasing(I used oscilloscope to vary freq). I chosed to decrease bcusw that sounded easier to me and I plotted r againt 1/f which means as I decreased the freq, r moved was greater.
☆then telling them how to get my gradient and then our constant K.
☆I measured change in r using a meter ruler and I also stated to repeat experiment several time and average our r moved to prevent any errors.
☆safety precautions: I stated that motor heats up so take breaks btw experiment to let it cool down. or used heat resistant gloves.

Guys that how I wrote it and I cant remember what else I wrote but I will be loosing marks bcusw of not stating *measurimg mass of block*