All subjects · Physics · WPH14 (Unit 4)

Core practical 10: analysing collisions with ICT

Core practical 10 uses video analysis to study collisions between small spheres, such as ball bearings rolling on a table, and to test conservation of momentum in two dimensions.

Method

Mark out a flat, level table top. Place a metre rule or grid in the field of view as a scale.
Mount a camera directly above, pointing straight down, to avoid parallax and perspective errors.
Roll one sphere into a stationary one and record the collision at a high frame rate.
Measure each sphere's mass on a balance.

Analysis with ICT

Step through the video frame by frame in video analysis software (or a spreadsheet of positions).
Mark each sphere's position in successive frames. The time between frames is 1 / frame rate.
velocity = distance moved / time; find the x and y components before and after.
Compare total momentum in x and in y before and after, and compare total kinetic energy to decide whether the collision is elastic.

Errors and improvements

Friction and rolling slow the spheres, so measure velocities just before and just after the collision.
A higher frame rate gives more positions and less motion blur.
Use the same scale for all measurements, at the height of the spheres.

Worked example

A video is taken at 50 frames per second. A sphere moves 0.12 m in 6 frames. Find its speed.

Time = 6 / 50 = 0.12 s
v = 0.12 / 0.12

Answer: v = 1.0 m/s

Key idea: Camera overhead with a scale in view; frame rate gives the time; resolve velocities; compare momentum in x and y.

Check yourself

A video at 50 frames per second shows a sphere moving 0.20 m in 10 frames. Find its speed in m/s.
Show answer
1 m/s. Time = 10 / 50 = 0.20 s; v = 0.20 / 0.20 = 1.0 m/s.
A camera records at 240 frames per second. Find the time between frames in ms, to 3 s.f.
Show answer
4.17 ms. Δt = 1 / 240 s = 4.17 ms.
At 30 frames per second, a ball bearing moves 4.5 cm between consecutive frames. Find its speed in m/s.
Show answer
1.35 m/s. v = 0.045 x 30 = 1.35 m/s.
A 0.050 kg sphere at 0.80 m/s hits a stationary 0.040 kg sphere head-on and continues at 0.30 m/s. Predict the speed of the 0.040 kg sphere in m/s.
Show answer
0.625 m/s. 0.040 = 0.015 + 0.040v, so v = 0.025 / 0.040 = 0.625 m/s.
Why is a metre rule placed in the camera's field of view?
- To keep the spheres moving in straight lines
- To convert distances on the video into real distances
- To measure the frame rate
- To reduce friction
Show answer
To convert distances on the video into real distances. The software needs a known length to set the scale.
Why is the camera placed directly over the table, pointing straight down?
- To make the spheres move faster
- To increase the frame rate
- To light the table evenly
- To avoid parallax and perspective errors in the positions
Show answer
To avoid parallax and perspective errors in the positions. Viewing at an angle distorts distances in the image.
Why is a high frame rate useful?
- It makes the collision elastic
- It gives more position readings and less motion blur
- It removes friction
- It increases the momentum
Show answer
It gives more position readings and less motion blur. More frames per second means shorter time intervals and sharper images.

Practise with instant feedback

Sources

Last updated 7 November 2026. Spotted a mistake? Tell us.