Practical skills in core practicals 1 to 3

Core practical 1: free fall

• Measure h with a metre rule checked vertical with a set square, from the bottom of the ball.
• Plot h against t²: gradient g/2. A delay in release adds a constant time to every reading (systematic), so the line misses the origin; plotting sqrt(h) against t still gives g from the gradient (g = 2 x gradient²).
• Electronic timing removes reaction time; a small dense ball makes air resistance negligible.

Core practical 2: viscosity

• Measure the ball's diameter with a micrometer (or vernier calipers for larger balls), at several orientations, correcting zero error; the diameter is squared in η = 2r^2g(ρ_s - ρ_l)/9v, so its uncertainty matters most (compounding it is a Unit 6 skill).
• To show terminal velocity has been reached, measure the time between two pairs of equally spaced rubber bands: equal times mean constant speed.
• Record the liquid's temperature: viscosity depends strongly on it.
• Compare your η with a data-book value using limits from your own percentage uncertainty.

Core practical 3: Young modulus

• A long, thin wire gives a large extension, so a small percentage uncertainty in the extension.
• Micrometer readings of diameter at several places and orientations; A = πd²/4 doubles its percentage uncertainty.
• Plot force against extension; E = gradient x original length / A.
• Safety: eye protection in case the wire snaps; sand tray under the masses.

Worked example

A graph of force against extension for a 2.00 m wire of diameter 0.38 mm has gradient 1.10 x 10⁴ N/m. Find E.

1. A = π(0.38 x 10^-3)² / 4 = 1.134 x 10^-7 m² (keep this unrounded value in the calculator)
2. E = gradient x l / A = 1.10 x 10⁴ x 2.00 / 1.134 x 10^-7 = 1.94 x 10¹¹ Pa

Answer: E = 1.94 x 10¹¹ Pa, close to the value for steel

Check yourself

1. In core practical 1, a graph of h against t² has gradient 4.85 m/s². Find g in m/s².
   Show answer

   9.70 m/s². g = 2 x gradient = 9.70 m/s².

2. How can a student show that a falling ball has reached terminal velocity in core practical 2?
   • Measure its speed once near the bottom
   • Use a longer tube
   • Time it between two pairs of equally spaced marks and check the times are equal
   • Check that the ball has stopped accelerating by eye
   Show answer

   Time it between two pairs of equally spaced marks and check the times are equal. Equal times over equal distances show the speed is constant.

3. A ball falls between two bands 0.100 m apart in 1.25 s. Find its speed in m/s, to 3 s.f.
   Show answer

   0.0800 m/s. v = 0.100 / 1.25 = 0.0800 m/s.

4. A 1.80 m wire has cross-sectional area 1.00 x 10^-7 m². Its force-extension graph has gradient 1.20 x 10⁴ N/m. Find E, in units of 10¹¹ Pa, to 3 s.f.
   Show answer

   2.16 x 10¹¹ Pa. E = 1.20 x 10⁴ x 1.80 / 1.00 x 10^-7 = 2.16 x 10¹¹ Pa.

5. A wire's diameter is 0.38 mm, measured with a micrometer of resolution 0.01 mm. Find the percentage uncertainty in the diameter, using half the resolution, to 2 s.f.
   Show answer

   1.3 %. Uncertainty = 0.005 mm; 0.005 / 0.38 x 100 = 1.3%.

6. Why is a long, thin wire used in core practical 3?
   • It is less likely to snap
   • It makes the Young modulus larger
   • It removes the need to measure the diameter
   • It gives a larger extension, so a smaller percentage uncertainty in the extension
   Show answer

   It gives a larger extension, so a smaller percentage uncertainty in the extension. Extension is proportional to length and inversely proportional to area, so a long thin wire stretches more for the same load.

7. In core practical 1, the electromagnet releases the ball slightly after the timer starts. What effect does this have on g calculated from single readings using g = 2h/t²?
   • Every value of g is too large
   • Every value of g is too small
   • The values scatter randomly
   • There is no effect
   Show answer

   Every value of g is too small. Each measured t is too long, so 2h/t² is too small: a systematic error.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)
• Pearson examiners' report: IAL Physics WPH13/01, October 2024

Last updated 8 November 2026. Spotted a mistake? Tell us.