Choosing a formula is a matching problem. List what you are given, what you are asked for, and which quantity is missing, then choose the formula that contains the first two and leaves the missing one out.
This page is for students who can recite formulas but hesitate at the first step. It is one of the help pages for SPM Physics.
What does this sound like?
A student might say, “I know all the equations, but when I see a question I do not know where to begin.” Or, “I pick one, it does not work, and I pick another.”
Trying formulas at random uses up time and confidence. A fixed first step removes most of that.
What usually causes it?
Three academic causes show up.
- The quantities are not listed, so the question is read as a story and not as data.
- Hidden values in the wording, such as “from rest”, are not turned into numbers.
- Formulas are stored as separate facts instead of as a set with a pattern.
A worked example
A car speeds up from 5 m s⁻¹ to 20 m s⁻¹ in 6.0 s with constant acceleration. Find the distance it travels.
- List: initial velocity u = 5, final velocity v = 20, time t = 6.0, distance s = unknown. Acceleration a is not given and not needed.
- Match: the equation with u, v, t and s but no a is s = ½(u + v)t.
- Solve: s = ½ × 25 × 6.0 = 75 m.
- Check by a second route: a = (20 − 5) ÷ 6.0 = 2.5 m s⁻². Then s = ut + ½at² = 30 + ½ × 2.5 × 36 = 30 + 45 = 75 m. The routes agree.
What can you do on your own?
For motion problems, write the five quantities in a column and mark each as given, unknown or not needed.
| Quantity | Symbol | Status in the example |
|---|---|---|
| Initial velocity | u | Given, 5 |
| Final velocity | v | Given, 20 |
| Acceleration | a | Not needed |
| Time | t | Given, 6.0 |
| Distance | s | Asked |
Choose the equation that leaves out the “not needed” quantity. Try the same habit for other topics, such as electricity, where you list V, I, R and P. Motion graphs are covered in the motion graph explorer, and the chapter lesson is using equations of uniformly accelerated motion.
If the trouble is mixing up similar-looking formulas from different topics, see I memorise formulas but choose the wrong one.
When might one-to-one tuition help?
The list-and-match method is learnt by doing it on many different questions. If you still hesitate after practising it on a full set of questions, it may be worth having someone watch your first step.
A one-to-one teacher can ask what you list, what you skip, and why you chose the formula, then correct the first step before any arithmetic starts. See online one-to-one Physics tuition or the one-hour trial class (from RM50).
What to read next
Formula choice often breaks down when several topics share the same letter. Continue with I memorise formulas but choose the wrong one.