Problems with UK A level Physics Marking Schemes

[ChrisXenon]

Hi Folks, I am a private tutor and one thing I teach is A level Physics (OCR Physics A).
Am I the only person who has been sometimes baffled and deeply disappointed with some of the questions/answers? Let me explain what I mean.

I see questions form which it's not possible to decide what kind of answer is required; when you look at the model answer to see that their interpretation is entirely arbitrary.

A recent example invites the student to describe the motion of a body whose upward velocity is decelerating due to gravity. The marking scheme expressly forbids the term "rate of acceleration", yes in the next question it allows that term.

I finally wrote to OCR to get clarification. They said:

"A dictionary definition of 'rate' states that it is a measure, typically measured against another quantity or measure, particularly against time, with examples such as crime rate, interest rate, mortality rate.
A common technical use of rate in science would be rate of flow, a volume in a given time."​

I completely understand that science demands a greater level of terminological accuracy than ordinary discourse, but I say it's entirely OK to say "rate of acceleration decreases". When I asked why they penalise it once but allow it in the second instance, they said it was unreasonable to penalise its use twice.

Here's another problem in the same question.

They also disallowed "acceleration" requiring "deceleration", but in my view, all deceleration is acceleration in the direction arbitrarily defined to be negative. You can say "the acceleration is negative" or you can say "the body is decelerating". So again, I think their position seems arbitrary and founded on nothing.

What do you think? It's important for me to get this right for my students.

Thanks
Chris

 

[PeroK]

Here's another problem in the same question.

They also disallowed "acceleration" requiring "deceleration", but in my view, all deceleration is acceleration in the direction arbitrarily defined to be negative. You can say "the acceleration is negative" or you can say "the body is decelerating". So again, I think their position seems arbitrary and founded on nothing.

What do you think? It's important for me to get this right for my students.

Thanks
Chris

1) In 1D motion whether the speed is increasing or decreasing is frame-dependent. Once you have established the reference frame, then talking about "deceleration" is fine. But, "acceleration" is always correct.

2) Again in 1D, wherher acceleration is "positive" of "negative" depends on the orientation of your reference frame. If "up" is "positive" then the acceleration due to gravity is negative, whether the object is decelerating on the way up or accelerating on the way down.

3) In 2D or 3D motion, acceleration is a vector, so strictly speaking the notions of "positive/negative" and "acceleration/deceleration" disappear and you have simply "acceleration".

The point I would emphasise, therefore, is that acceleration has a direction that is independent of the direction of the velocity (i.e. independent of the direction of motion).

 

[ChrisXenon]

1) In 1D motion whether the speed is increasing or decreasing is frame-dependent. Once you have established the reference frame, then talking about "deceleration" is fine. But, "acceleration" is always correct.

2) Again in 1D, wherher acceleration is "positive" of "negative" depends on the orientation of your reference frame. If "up" is "positive" then the acceleration due to gravity is negative, whether the object is decelerating on the way up or accelerating on the way down.

3) In 2D or 3D motion, acceleration is a vector, so strictly speaking the notions of "positive/negative" and "acceleration/deceleration" disappear and you have simply "acceleration".

The point I would emphasise, therefore, is that acceleration has a direction that is independent of the direction of the velocity (i.e. direction of motion).

Well-put - I agree - thank you.

But we both seem to be saying that the OCR are wrong, don't we?

 

[PeroK]

Well-put - I agree - thank you.

But we both seem to be saying that the OCR are wrong, don't we?

Acceleration is always the right term. You can't disallow that in a physics exam, IMHO.

 

[atyy]

Under gravity, the rate of acceleration is zero, since the acceleration is constant ~ 9.8 ms^-2.

 

[PeroK]

Under gravity, the rate of acceleration is zero, since the acceleration is constant ~ 9.8 ms^-2.

The question is whether you would deduct marks for saying "rate of acceleration", rather than "acceleration". That would seem a bit harsh to me.

 

[atyy]

The question is whether you would deduct marks for saying "rate of acceleration", rather than "acceleration". That would seem a bit harsh to me.

I think I would.

But on the second point, I would allow: negative acceleration = deceleration.

 

[PeroK]

I think I would.

But on the second point, I would allow: negative acceleration = deceleration.

In what sense is negative acceleration deceleration? The acceleration due to gravity doesn't change direction when an object changes direction.

Negative acceleration means acceleration in the negative direction, surely?

 

[atyy]

In what sense is negative acceleration deceleration? The acceleration due to gravity doesn't change direction when an object changes direction.

Negative acceleration means acceleration in the negative direction, surely?

Negative acceleration means acceleration in the negative direction, which is decelaration.

 

[PeroK]

Negative acceleration means acceleration in the negative direction, which is decelaration.

I would have said that "deceleration" means the speed is reducing. No?

PS "Deceleration" implies acceleration is in the opposite direction to the velocity. That's not necessarily in the negative direction. The velocity could be in the negative direction.

 

[atyy]

I would have said that "deceleration" means the speed is reducing. No?

Yes perhaps, I guess that is more common (deceleration refers to negative d|v|/dt), though I've always thought (deceleration = -dv/dt).

In any case, the body that is decelarating upwards is accelerating downwards.

 

[atyy]

I guess the problem is that acceleration has two meanings referring to the rate of change of speed, or to the rate of change of velocity.

Then if deceleration is the opposite of acceleration, one also has two meanings for deceleration. Deceleration is not usually defined in a technical sense, so that would argue for the primacy of its plain English sense. However, it is confusing in a technical context in which speed and velocity have different meanings, although in plain English they mean the same thing.

 

[ZapperZ]

In physics, rate of acceleration implies da/dt.

If you mean acceleration, then type “acceleration”. Trying to be “cute” and writing “rate of acceleration”, at best, creates this type of ambiguity in everyday usage, and at worst, an error in physics context.

Zz.

 

[Mark44]

In physics, rate of acceleration implies da/dt.

I agree. "Rate of acceleration" is a terrible abuse if "accleration" is what is intended.

 

[PeroK]

I agree. "Rate of acceleration" is a terrible abuse if "accleration" is what is intended.

I would say that "rate of acceleration" doesn't imply "rate of change of acceleration". "Rate" in general can also mean "value", as in "exchange rate" and "rate of inflation".

Of course, plain "acceleration" is better, but it seems such a small slip!

 

[ZapperZ]

I would say that "rate of acceleration" doesn't imply "rate of change of acceleration". "Rate" in general can also mean "value", as in "exchange rate" and "rate of inflation".

Of course, plain "acceleration" is better, but it seems such a small slip!

But it is an unnecessary slip that, at best, creates confusion and ambiguity. This can't be "better".

Again, we're talking about using such a word within a physics context. Outside of physics, several level of bastardization on many words are common. The word "theory" is one example. And I have heard many times when "rate of speed" was used when what was meant was "speed". Unnecessarily adding extra words to what was already a well-defined word, to simply mean THAT word in the first place, is mind-boggling.

Zz.

 

[CWatters]

Thanks for posting this topic. I have twins studying OCR Physics A level at the moment.

 

[PeroK]

But it is an unnecessary slip that, at best, creates confusion and ambiguity. This can't be "better".

Again, we're talking about using such a word within a physics context. Outside of physics, several level of bastardization on many words are common. The word "theory" is one example. And I have heard many times when "rate of speed" was used when what was meant was "speed". Unnecessarily adding extra words to what was already a well-defined word, to simply mean THAT word in the first place, is mind-boggling.

Zz.

Well, "flow rate" seems to be in common usage in physics and engineering. Why not simply "flow"?

But, "current rate" doesn't sound right at all.

The point is there's no fundamental physical or linguistic reason why it's "flow rate " and "current". It's down to convention.

And, although "rate of speed" sounds absurd, there is the idiom "at a rate of knots"!