Through years of teaching, I have witnessed many mistakes that students tend to make. There is one very common mistake that physics students tend to make in exams. Cambridge A-level and O-level physics exams love to trick students by giving physics quantities in not-so-suitable units.
For example, Newton's second law states that the resultant force = mass*acceleration. If one object is 5grams and it experiences a resultant force of 10N, what is its acceleration?
Common and WRONG approach:
F=ma
10=5*a
so a = 2 m/s^2
The correct approach is to first change to m = 0.005 kg and then substitute in this value into the equation.
10 = 0.005*a
so a = 2000 m/s^2
And guess what? One wrong substitution will affect the whole question that is worth 10 to 20 marks. Students might get sympathy marks such as "error-carried-forward", but that is not enough for an A grade.
Therefore, to get to the point, remember to substitute the physical quantity with the right units, which are SI units for most of the physics equations. This could easily push your grade up by one. :)
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