Most marks lost on a physics test are not lost to not knowing the physics. They are lost to a missing unit, a dropped direction, a sign flipped halfway through, or the right equation applied to the wrong interval. A consistent method costs about thirty seconds per question and reclaims most of those marks.
- Read it twice and write down what is asked. Underline the quantity wanted and its unit. A surprising number of errors are answering a different question from the one printed.
- Draw something. A free-body diagram, a motion sketch, a circuit, a wave. It costs ten seconds and makes half the errors impossible.
- List the given values with symbols and units, and mark the unknown. Convert everything to base SI units at this point — km/h to m/s, grams to kilograms, minutes to seconds.
- Choose the equation, using the missing-variable method for kinematics and the destination for everything else. Write the equation down before substituting anything.
- Rearrange first, then substitute. Untangling numbers is where arithmetic errors breed.
- Check the answer: correct unit, sensible size, direction stated if it is a vector, and significant figures matched to the data.
Four checks that catch most errors
| Check | What to ask | What it catches |
|---|---|---|
| Units | Do the units of my working give the unit I want? | Wrong rearrangement, unconverted values |
| Magnitude | Is this a believable size for a person, car or building? | Factor-of-10 slips, missed conversions |
| Direction | Does a vector answer have a direction in words? | Incomplete vector answers |
| Sign | Does the sign match the convention I declared? | Flipped conventions, deltas written backwards |
The magnitude check is the most valuable habit in the course
A person cannot run at . A dropped ball does not land at after falling ten metres. A household appliance does not draw . If a number looks absurd it almost certainly is, and finding that in ten seconds is far cheaper than losing the marks.
Reference points worth carrying in your head
| Quantity | Familiar value | Use it to sanity-check |
|---|---|---|
| Walking speed | Any pedestrian velocity | |
| Highway speed | Vehicle speeds | |
| A person's weight | – | Any force involving a human |
| Speed of sound in air | at room temperature | Wave and echo answers |
| Household voltage | in Canada | Circuit questions |
| Typical appliance current | – | Anything electrical |
| Energy to boil a kettle | About | Thermal calculations |
Traps that recur across the strands
AKinematics and forces
- Negative acceleration does not mean slowing down — compare it with the velocity's sign
- Kinematic equations need uniform acceleration; split the motion if it changes
- At the top of a throw but still
- takes the net force, not the applied one
- Weight and normal force are balanced forces, not a third-law pair
BEnergy, waves and circuits
- A force perpendicular to the motion does zero work
- has no — a phase change has no temperature change
- Echo problems are round trips: the factor of 2
- Closed pipes have odd harmonics only
- Parallel resistance needs the final reciprocal
Significant figures without losing marks
- Multiplying or dividing: the answer keeps the fewest significant figures of the values used.
- Adding or subtracting: the answer keeps the fewest decimal places.
- Carry extra digits through the middle of a multi-step problem and round only at the very end.
- Exact values do not limit anything — the 2 in , or a count of 4 half-lives, has infinite significant figures.
- When in doubt, three is usually safe for physics, and matches .
Show the substitution line
Write the equation, then write it again with the numbers in place, then give the answer. Part marks are awarded for a correct equation and a correct substitution even when the arithmetic goes wrong — and an answer with no working earns nothing if it is wrong.
Calculator settings
This course works entirely in degrees. Check the mode before you start — a calculator left in radians turns into , and every work and vector question afterwards is wrong. Check it again after anyone else borrows the calculator.