More Physics Clangers


Teachingscienceinallweathers highlights some disappointing Physics errors on the DfE’s National Curriculum documents for Science which have remained uncorrected for over a year (see here, here and here).

This would be bad form on, say, a school website. For an organisation that is in charge of a national education system whose elected leaders do not hesitate to label schools as “coasting” and “lacking rigour”, it is unbelievably shabby and smacks of arrogant, lazy hypocrisy. And these documents are no longer drafts: the DfE website says that “these programmes of study are issued by law; you must follow them unless there’s a good reason not to. All local-authority-maintained schools in England must teach . . . key stage 4 from September 2016.”

Some of the persistent errors highlighted by Teachingscienceinallweathers (and others, including @DrDav, @HRogerson and @miss_m_w) are:

1. The formula for kinetic energy is given as “0.5 x mass x (acceleration)^2” instead of “0.5 x mass x (velocity)^2” [p.37]

2. The formula for weight is given as “gravity force = mass x gravity constant” instead of using the correct scientific terminology of “weight = mass x gravitational field strength”. As Teachingscienceinallweathers points out, the magnitude of gravitational field strength is anything but a “gravity constant”, even near to the Earth’s surface. Similarly, stating that “potential energy = mass x height x gravity constant (g)” [p.37] invites confusion between the constant “big G” the Universal Gravitational Constant (which is genuinely a constant) and “little g” which, as noted above, is not.

3. “Charge flow = current x time” [p.37]: the phrase “charge flow” is confusing in this context. Very often, the phrase “flow of charge” is used as a synonym for “current”. I would argue that “Charge transferred = current x time” would be preferable in this case.

4. “Interpret enclosed areas in distance-time and velocity-time graphs” [p.32]: the area enclosed by a velocity-time graph represents the change in displacement; the area enclosed by a distance-time graph represents . . . erm, nothing with any physical significance, as far as I know.

I would argue that the writers of science examination questions and science specifications have tended towards the prolix over the last two decades, and I, for one, would welcome the return to the more concise but rigorous style of writing of yesteryear when an exam question could begin “A monochromatic ray of light is incident on a plane mirror at an angle of 30 degrees to the normal…” and students were expected to draw an appropriate diagram because the language was clear, formal and unambiguous.

That may indeed have been the intention of the National Curriculum writers, but they are some way from achieving it. In fact, this document is nowhere close.

My own personal bête noire is:

explain with examples that motion in a circular orbit involves constant speed but changing velocity (qualitative only) [p.31]

There is no indication that the writers intend to restrict the meaning of orbit to the celestial sense, and so it seems that it refers to motion on a circular path in general. And therein lies the problem: it might be true in cases where the radius and angular velocity are constant, but the writers do not specify this. Are they considering the motion of an object whirled on a string? Motion in a vertical circle? Motion in a horizontal circle? They don’t say. It is a fair generalisation to say that it is hard to set up motion in a vertical circle that features uniform speed without variable torque to compensate for the transfer of k.e. to g.p.e. and vice versa.

“Explanations of circular motion restricted to examples involving constant speed to introduce the concept of centripetal acceleration as a result of changing direction of velocity” is far from perfect but is, I think, more useful than the original.

In short, those who call for rigour should display rigour.

2 thoughts on “More Physics Clangers

  1. missironfistatheist June 28, 2015 / 8:10 pm

    “Ye cannae change the laws of Physics.”

    Oh yes we can!

    Not said by Obama.

    Or a Classicist = )


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