WAEC Physics practical guide
Physics Paper 3 in the WASSCE is the practical paper. You carry out experiments in the laboratory, record your readings, plot a graph and draw a conclusion. Most of the marks come from how you record and process your readings, so careful practice before the exam makes a real difference.
This guide covers the skills every experiment needs, six experiments that come up often, and the precautions examiners accept. Check the current WAEC Physics syllabus and your timetable for the exact duration and question choice for your sitting.
What earns marks in the practical
- Readings recorded in a complete table, with units, to the precision of the instrument.
- Derived values such as T², sin i or 1/u worked out correctly and to a sensible number of figures.
- A graph with labelled axes, a good scale, accurate points and one line of best fit.
- A slope worked out from a large triangle, with the reading points shown.
- A final answer with the correct unit, and precautions that match what you actually did.
How to record readings
- Draw the table before you start. Give every column a heading with its unit, for example l (cm), t (s), T (s).
- Read each instrument to its smallest division: a metre rule to 0.1 cm, vernier calipers to 0.01 cm, a micrometer screw gauge to 0.01 mm, meters to the smallest division on the scale.
- Keep the same number of decimal places all the way down a column. 45.0 and 45 are not the same reading.
- Take the number of readings the question asks for, usually five or six, spread across the whole range allowed.
- Time many oscillations, not one, and repeat timings when you can. Record each timing and the average.
- Work out derived values to three significant figures, or three decimal places for sines and reciprocals, and keep that the same for the whole column.
How to plot the graph and find the slope
- "Plot T² against l" means T² on the vertical axis and l on the horizontal axis.
- Choose scales so your points cover at least half of the graph sheet in both directions. Use 1, 2 or 5 units per large square; avoid 3, 6 or 7, which make plotting slow and inaccurate.
- Label each axis with the quantity and its unit, and give the graph a title such as "Graph of T² against l".
- Plot each point as a small dot in a circle or a small cross, exactly where it belongs.
- Draw one straight line of best fit with points balanced on both sides. Never join the points dot to dot.
- For the slope, draw a large right-angled triangle on the line, covering at least half of it. Read both points from the line, not from your table, and show ΔY ÷ ΔX with units.
- Start the axes at zero only when the question asks for an intercept or the line should pass through the origin; otherwise start near your smallest reading.
Six experiments that come up often
1. Simple pendulum: finding g
Aim: To determine the acceleration due to gravity, g, using a simple pendulum.
Apparatus: Retort stand and clamp, two small wooden blocks (split cork), thread, pendulum bob, stopwatch, metre rule.
Method
- Clamp the thread between the blocks and measure the length l from the point of suspension to the centre of the bob.
- Pull the bob aside through a small angle, less than 10°, and release it.
- Time 20 complete oscillations, t, and work out the period T = t ÷ 20.
- Repeat for five more lengths and work out T² each time.
Table of readings
| S/N | l (cm) | t for 20 oscillations (s) | T (s) | T² (s²) |
|---|---|---|---|---|
| 1 | ||||
| 2 | ||||
| 3 |
Graph
Plot T² against l. You should get a straight line through the origin.
Result
From T = 2π√(l/g), T² = (4π²/g) l, so the slope s = 4π²/g.
Convert l to metres first, then g = 4π² ÷ s. Your answer should be close to 9.8 m/s².
Precautions
- I made sure the bob swung through a small angle, less than 10°.
- I started and stopped the stopwatch as the bob passed its lowest point.
- I made sure the bob swung in one plane, without spinning.
- I avoided parallax error by reading the metre rule with my eye level with the mark.
2. Spiral spring: Hooke's law and the spring constant
Aim: To verify Hooke's law and determine the spring constant, k, of a spiral spring.
Apparatus: Spiral spring, retort stand and clamp, pointer, metre rule, slotted masses with hanger.
Method
- Hang the spring with the pointer against the vertical metre rule and record the pointer reading P₀ with no load.
- Add a mass m, let the spring come to rest and record the new pointer reading P.
- Work out the extension e = P − P₀.
- Repeat for five more masses, then remove the masses and check the pointer returns to P₀.
Table of readings
| S/N | m (g) | P (cm) | e (cm) |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 |
Graph
Plot e against m. A straight line through the origin shows that extension is proportional to load, which verifies Hooke's law.
Result
The load is F = mg and F = ke, so e = (g/k) m and the slope s = g/k.
Convert m to kg and e to m, then k = g ÷ s, in N/m.
Precautions
- I waited for the spring to stop oscillating before taking each reading.
- I avoided parallax error by reading the pointer with my eye level with it.
- I did not load the spring beyond its elastic limit, and checked that the pointer returned to its first reading.
3. Rectangular glass block: refractive index
Aim: To determine the refractive index, n, of glass using a rectangular glass block.
Apparatus: Rectangular glass block, drawing board, white paper, drawing pins, four optical pins, protractor, ruler, sharp pencil.
Method
- Trace the outline of the block on the paper. Draw a normal at a point N on one long side and a ray AN at an angle of incidence i.
- Fix two pins on AN, as far apart as possible. Look through the opposite face and fix two more pins in line with the images of the first two.
- Remove the block, join the last two pin marks to the outline to get the emergent ray, and join that point to N to get the refracted ray. Measure the angle of refraction r.
- Repeat for i = 20°, 30°, 40°, 50° and 60°.
Table of readings
| S/N | i (°) | r (°) | sin i | sin r |
|---|---|---|---|---|
| 1 | ||||
| 2 | ||||
| 3 |
Graph
Plot sin i against sin r. You should get a straight line through the origin.
Result
Snell's law: sin i = n sin r, so the slope of the graph is n.
For glass, n is usually about 1.5.
Precautions
- I made sure the pins were upright and as far apart as possible.
- I looked at the bases of the pins when lining them up, to reduce error.
- I made sure the block did not move while the pins were being placed.
- I used a sharp pencil to draw thin lines.
4. Converging lens: focal length
Aim: To determine the focal length, f, of a converging lens.
Apparatus: Converging lens in a holder, illuminated object (cross-wire), white screen, metre rule.
Method
- Place the lens at a distance u from the object, greater than the focal length.
- Move the screen until a sharp image forms on it and measure the image distance v.
- Repeat for five more values of u.
- Work out 1/u and 1/v for each pair.
Table of readings
| S/N | u (cm) | v (cm) | 1/u (cm⁻¹) | 1/v (cm⁻¹) |
|---|---|---|---|---|
| 1 | ||||
| 2 | ||||
| 3 |
Graph
Plot 1/v against 1/u. You should get a straight line with a negative slope.
Result
From 1/f = 1/u + 1/v, each intercept of the line on the axes equals 1/f.
Read both intercepts, take their average and work out f = 1 ÷ intercept.
Precautions
- I made sure the object, the lens and the screen were in a straight line, with their centres at the same height.
- I moved the screen backwards and forwards to find the sharpest image.
- I avoided parallax error when reading the metre rule.
- I worked in a dim part of the room so the image was easy to see.
5. Metre rule: principle of moments
Aim: To determine an unknown mass using the principle of moments.
Apparatus: Metre rule, knife edge, thread, unknown mass Y, a set of known masses.
Method
- Balance the metre rule on the knife edge and record its balance point G. Keep the knife edge at G for the whole experiment.
- Hang the unknown mass Y at a fixed distance a from G.
- Hang a known mass m on the other side and move it until the rule balances. Record its distance d from G.
- Repeat with five more known masses and work out 1/m each time.
Table of readings
| S/N | m (g) | d (cm) | 1/m (g⁻¹) |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 |
Graph
Plot d against 1/m. You should get a straight line through the origin.
Result
At balance, the clockwise moment equals the anticlockwise moment: m × d = Y × a.
So d = Ya × (1/m), the slope is Ya, and Y = slope ÷ a.
Precautions
- I made sure the metre rule was horizontal and steady before each reading.
- I tied the threads tightly so the masses did not slide along the rule.
- I avoided parallax error when reading the positions of the threads.
6. Ohm's law: resistance of a resistor
Aim: To verify Ohm's law and determine the resistance, R, of a resistor.
Apparatus: Battery or accumulator, key, rheostat, ammeter, voltmeter, resistor, connecting wires.
Method
- Connect the battery, key, rheostat, ammeter and resistor in series, with the voltmeter across the resistor.
- Close the key, adjust the rheostat and read the current I and the voltage V.
- Open the key between readings.
- Repeat for five more settings of the rheostat.
Table of readings
| S/N | I (A) | V (V) |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 |
Graph
Plot V against I. A straight line through the origin verifies Ohm's law.
Result
V = IR, so the slope of the graph is the resistance R, in ohms.
Precautions
- I made sure all connections were tight and clean.
- I opened the key between readings so the resistor did not heat up.
- I checked the meters for zero error before starting.
- I avoided parallax error by reading the meters with my eye directly above the pointer.
How to write precautions examiners accept
A precaution is something you did to make your readings more reliable. Write it in the past tense and make it specific to the experiment, as in the examples above.
"I was careful", "I took accurate readings" and "I avoided errors" are too vague to earn marks. Say exactly what you did and why.
A precaution is not the same as a source of error. "Parallax error when reading the scale" is a source of error; "I read the scale with my eye level with the mark" is the precaution.
Common mistakes that lose marks
- Column headings without units.
- Different numbers of decimal places in one column.
- A tiny graph that uses only a corner of the sheet.
- Joining points dot to dot instead of drawing one line of best fit.
- Taking the slope from two table readings instead of a large triangle on the line.
- Forgetting to convert cm to m, or g to kg, before working out g or k.
- A final answer with no unit.
How to prepare in the weeks before the exam
- Do each of the six experiments above at least once in your school laboratory.
- Practise the table, graph and slope at home with readings from past class work, timing yourself.
- Learn two or three precautions for each experiment in the past-tense form above.
- Revise the theory behind each experiment, because short questions on it often follow the readings.
Practise with worked solutions
Try free exam-style questions with worked solutions, or book a tutor for one-to-one help. Your first lesson is free.
Frequently asked questions
What is WAEC Physics Paper 3?
Paper 3 is the practical paper of the WASSCE Physics exam. Candidates carry out experiments, record readings, plot graphs and answer short questions on the results. Check the current WAEC syllabus for the exact time and number of questions in your sitting.
How do I calculate g from a pendulum graph?
Plot T² against l with l in metres. The slope s equals 4π²/g, so g = 4π² ÷ s.
How many readings should I take?
Follow the question. Most practical questions ask for five or six sets of readings spread across the allowed range.
Should my graph start from the origin?
Only when the question asks for an intercept or the line is expected to pass through the origin. Otherwise choose a scale that starts near your smallest reading so the points fill the sheet.
What precautions are accepted in the WAEC Physics practical?
Specific actions written in the past tense, such as "I avoided parallax error by reading the scale with my eye level with the mark" or "I opened the key between readings to prevent heating". Vague statements such as "I was careful" do not earn marks.