A correct formula can still give a wrong answer if the situation is outside the model behind it. This section trains four checks that catch that problem before you commit to the final line.
It extends force and motion I in SPM Physics. If you are still choosing equations, read using equations of uniformly accelerated motion first.
When does a correct formula give a wrong answer?
Take this example. A trolley moves at 6.0 m s⁻¹ and slows at 2.0 m s⁻². A student is asked for its position after 5.0 s.
Using s = ut + ½at², the student writes s = 6 × 5 − ½ × 2 × 25 = 5 m. The algebra is correct. But the trolley stops after 3.0 s, having travelled 9 m, and then stays at rest on the track. The true position at 5.0 s is 9 m.
The formula assumed constant acceleration for the whole 5.0 s. That assumption failed at t = 3.0 s, which is why the answer was wrong while the arithmetic was right.
How are the lessons organised?
Each lesson teaches one check.
- Drawing a labelled system boundary before choosing a conservation equation decides what is inside the model.
- Explaining why constant speed does not always mean constant velocity separates a scalar from a vector.
- Using dimensions to detect an incorrect formula substitution checks the units of both sides.
- Rejecting a mathematically valid answer that violates the stated physical setup removes roots the situation cannot have.
The integrated practice set mixes the four.
Who should start where?
If you apply conservation of momentum or energy without saying what is included, start with the boundary lesson. If you call circular motion “unaccelerated”, read the velocity lesson. If your formulas sometimes come out with odd units, use the unit-check lesson and the units and significant figure checker.
If you want a teacher to ask those follow-up questions about your own answers, see online one-to-one Physics tuition.