Q: Confusing raw diameter measurements with the radius, or failing to square the radius when calculating cross-sectional areas ($ A = \pi r^2 $) for resistivity or Young Modulus equations.

Always write down $ r = d / 2 $ first, convert the radius from millimeters to meters ($ \times 10^{-3} $), and compute $ A $ explicitly as a separate step before substituting it into the main formula.

Q: Forgetting to double the percentage uncertainty of a value that is squared (like diameter in cross-sectional area) when calculating compound uncertainties.

Apply the fractional uncertainty power rule: if $ A = \pi r^2 $, the percentage uncertainty in $ A $ is equal to $ 2 \times $ the percentage uncertainty in $ r $.

Question 1

Confusing raw diameter measurements with the radius, or failing to square the radius when calculating cross-sectional areas ($ A = \pi r^2 $) for resistivity or Young Modulus equations.

Accepted Answer

Always write down $ r = d / 2 $ first, convert the radius from millimeters to meters ($ \times 10^{-3} $), and compute $ A $ explicitly as a separate step before substituting it into the main formula.

Question 2

Assuming the potential difference across a thermistor or LDR in a potential divider is independent of, or behaves the same as, other series components when physical conditions change.

Accepted Answer

Use a systematic chain of logic: 1) temperature increases, 2) thermistor resistance decreases, 3) total circuit resistance decreases, 4) circuit current increases, 5) potential difference across the fixed resistor increases (via $ V = IR $), 6) hence the potential difference across the thermistor decreases.

Question 3

Omitting the negative sign or failing to sum velocity magnitudes when computing change in momentum for an object that rebounds in the opposite direction.

Accepted Answer

Define a clear positive coordinate direction before starting the calculation. Write $ \Delta p = m(v - u) $. Since the final velocity is in the opposite direction, write it as a negative value, meaning the magnitudes of speed must sum: $ m(v + u) $.

Question 4

Drawing lines of best fit that are kinked, too thick, have multiple lines (feathering), or are forced through the origin rather than balancing the data points.

Accepted Answer

Use a sharp HB pencil and a clear plastic ruler. Balance points evenly on both sides of your line along its entire length. The line must be a single, continuous, straight line less than half a small square in thickness.

Question 5

Forgetting to double the percentage uncertainty of a value that is squared (like diameter in cross-sectional area) when calculating compound uncertainties.

Accepted Answer

Apply the fractional uncertainty power rule: if $ A = \pi r^2 $, the percentage uncertainty in $ A $ is equal to $ 2 \times $ the percentage uncertainty in $ r $.

Question 6

Stating Newton's Third Law action-reaction pairs as acting on the same physical body.

Accepted Answer

Explicitly state that Newton's Third Law forces are equal in magnitude, opposite in direction, of the same type, but act on two separate, interacting bodies.

Question 7

Stating Newton's Second Law of Motion as simply $ F = ma $.

Accepted Answer

Newton's Second Law must be defined as the force being proportional to the rate of change of momentum ($ F = \Delta p / \Delta t $). Note that $ F = ma $ is only a special case when mass remains constant.

Question 8

In Paper 3, recording raw length measurements from a ruler without a decimal place (e.g., writing $ 32\text{ cm} $ instead of $ 32.0\text{ cm} $ or $ 0.32\text{ m} $ instead of $ 0.320\text{ m} $).

Accepted Answer

Always record raw measurements to the maximum limit of resolution of the apparatus. A standard ruler's division is $ 1\text{ mm} $, so measurements must end in $ .0\text{ cm} $ or $ .5\text{ mm} $.

ฉบับ	เวลา	คะแนน	จำนวนข้อ	น้ำหนักคะแนน	ประเภทคำถาม
Paper 1 Multiple Choice	1ชม. 15นาที	40	40	31%	multipleChoice
Paper 2 Structured Questions	1ชม. 15นาที	60	6	46%	structured
Paper 3 Practical Skills	2ชม.	40	2	23%	practical

Physics (9702) เคล็ดลับการสอบ

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Table mode for roots & turning points

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Carry exact values with Ans & memory

Equation solver — to CHECK your working

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