# Physics A level mark scheme

1. Feb 27, 2017

### Cozy_Powell

Hello!
This is a general question about graphs, but it can be seen specifically on paper 9702/33 February/March 2016, question 1. d) i).
On the mark scheme, can be read the following:
"Scatter of points must be no more than ± 5° from a straight line in the y (θ ) direction".

Thanks!

2. Mar 3, 2017

### Merlin3189

I think that they are simply saying that scatter should be less than 5 units on the vertical scale: and in this particular question the vertical scale is theta per degree.
You can measure scatter in other ways, but this is a simple way.

I'd expect this in a practical exam. You seem to be suggesting this is a theory paper, "a question ABOUT graphs". So either they're marking your ability to plot points accurately (which I doubt) or they are giving a rough and ready criterion for an acceptable best straight line - you need to get a line that passes within 5 vertical units of all given points.

Edit: I think you are using Cambridge board, but I've not found any relevant comments in their specs.
In a sample mark scheme they say "All points in the table must be plotted .... Scatter of points must be less than 0.1 m−1 from a straight line on the <vertical> axis. (ii) Line of best fi t: [1] Judge by balance of all points on the grid about the candidate’s line .... There must be an even distribution of points either side of the line along the full length. Allow one anomalous point only if clearly indicated (i.e. circled or labelled) by the candidate."
So here the scatter is a measure of accuracy in a practical exam. again it is referred to the maximum deviation on the vertical scale.

Last edited: Mar 3, 2017
3. Mar 4, 2017

### Cozy_Powell

Sorry for not providing the direct link for my question on my first post.

Yes, it's from Cambridge. And yes, it is the same type of question from which you copied the sample mark scheme. But I still am not able to understand what they mean... I think it may be some kind of misinterpretation from my part, even after your thoroughly explanation.

In your example, or on the following one (which is similar),

http://papers.xtremepapers.com/CIE/...nd AS Level/Physics (9702)/9702_s15_qp_31.pdf

http://papers.xtremepapers.com/CIE/...nd AS Level/Physics (9702)/9702_s15_ms_31.pdf

Question 1. e) i).

Which line is the straight line on "...from a straight line"?
My first thought was that this could be the line of best fit, but that is not correct, because that regards the next question.
Or, are we measuring the scattering or deviation in relation to what?

I hope I could explain properly my doubt...

Thanks again!

4. Mar 6, 2017

### Merlin3189

I think the first question marks the accuracy of your points by saying that they should lie nearly in a straight line, allowing +/- 5 units random variation. There is no particular line specified, but a best fit straight line would likely be a good one for the marker to use.

In the second question you are marked on how well you draw in a line of best fit. Even if you failed to get the accuracy mark in (i) you could still draw a good best straight line and get this mark. Since you are told it should fit a straight line, you only have to choose the best one, with points balanced either side of it. (I sometimes imagine the line being attached with elastic bands to the points. Doesn't work in all cases, but seems to work ok when the points are evenly spread along the line.)

You could also get the first mark for accuracy of points and then fail to draw a good straight line, though I'd have thought you'd have to be a bit careless to do that.

5. Mar 8, 2017

### Cozy_Powell

It makes sense now! :)

Many thanks Merlin3189!!!

6. Mar 8, 2017

### sophiecentaur

Have I missed something? I can't see a link to the question. It sounds to me that the graph exercise could have regular steps in y values and that the only difficult aspect of plotting the points is where they appear in the x direction. The mark scheme wording could be better. It would be normal to specify accuracy in terms of x and y error bars which I think is the answer you need.
You have interpreted the wording in the opposite way to me and I can see that's just as valid. The OP is confusing.