Practical skills in core practicals 9 and 10

Core practical 9: force and change in momentum

• Speed from a light gate: v = card length / time. Measure the card with vernier calipers; the logger times to a fraction of a millisecond.
• Percentage uncertainty in v = % in card length + % in time.
• Compensate for friction by tilting the runway until a pushed trolley moves at constant velocity (equal times at two gates).
• Δp = mΔv; plot Δp against Δt (constant force): gradient = F. With a force sensor, the area under the force-time graph should equal mΔv; the logger's high sampling rate captures the short collision.

Core practical 10: collisions with ICT

• Camera directly overhead, a scale (metre rule) in the field of view at the height of the spheres, high frame rate.
• Time between frames = 1 / frame rate; speed = distance moved / (frames / frame rate).
• Use positions just before and just after the collision, as friction slows the spheres.
• Compare total momentum before and after in two perpendicular directions; use percentage difference to judge conservation, and kinetic energy to judge elasticity.

Worked example

A card 50.0 mm long (± 0.05 mm) blocks a light gate for 0.0250 s (± 0.0001 s). Find v and its percentage uncertainty.

1. v = 0.0500 / 0.0250 = 2.00 m/s
2. % in length = 0.05 / 50.0 x 100 = 0.1%; % in time = 0.0001 / 0.0250 x 100 = 0.4%
3. % in v = 0.1 + 0.4 = 0.5%

Answer: v = 2.00 m/s ± 0.5%

Check yourself

1. A 25.0 mm card blocks a light gate for 0.0410 s. Find the speed in m/s, to 3 s.f.
   Show answer

   0.610 m/s. v = 0.0250 / 0.0410 = 0.610 m/s.

2. The card length has a percentage uncertainty of 0.2% and the time 0.5%. Find the percentage uncertainty in v.
   Show answer

   0.7 %. Division: 0.2 + 0.5 = 0.7%.

3. A 0.500 kg trolley's speed rises from 0.400 m/s to 0.900 m/s in 0.84 s. Find the average resultant force in N, to 3 s.f.
   Show answer

   0.298 N. Δp = 0.500 x 0.500 = 0.250 kg m/s; F = 0.250 / 0.84 = 0.298 N.

4. A video at 50 frames per second shows a sphere moving 0.180 m in 12 frames. Find its speed in m/s.
   Show answer

   0.75 m/s. Time = 12 / 50 = 0.24 s; v = 0.180 / 0.24 = 0.75 m/s.

5. Total momentum before a collision is 0.0400 kg m/s and after it is 0.0388 kg m/s. Find the percentage difference, using the value before as the reference, to 2 s.f.
   Show answer

   3.0 %. |0.0388 - 0.0400| / 0.0400 x 100 = 3.0%.

6. How is a runway checked to be friction-compensated in core practical 9?
   • A pushed trolley gives equal times at two light gates
   • The trolley stays still when released
   • The runway is measured to be level
   • The trolley accelerates down the slope
   Show answer

   A pushed trolley gives equal times at two light gates. Equal times mean constant velocity, so friction is balanced by the weight component.

7. In core practical 10, why must the scale be placed at the same height as the spheres?
   • So the frame rate is correct
   • So the spheres roll straight
   • Objects at different distances from the camera appear different sizes, so the scale would be wrong
   • To reduce friction
   Show answer

   Objects at different distances from the camera appear different sizes, so the scale would be wrong. Perspective changes apparent size with distance from the lens.

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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 WPH16/01, October 2024

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