Required practical: measuring g by free fall with light gates

Motion Along a Straight Line — AQA A-Level Physics

Drop a steel ball-bearing from an electromagnet through two light gates.
Independent variable: height h between the light gates. Dependent variable: time t.
Use the SUVAT equation s = ut + 0.5at^2, rearranged to 2h/$t = gt + 2u$.
Plot 2h/t on the y-axis against t on the x-axis.
The gradient of the straight-line graph gives g.
The y-intercept gives 2u (twice the initial speed at the first gate).
Repeat each height at least 3 times and take an average t.

Sources of error

Systematic: residual magnetism after switching off the electromagnet delays release, making t too large.
Random: parallax error reading the ruler, ball not falling centrally through light gates.
Reduce random errorAn error that causes readings to scatter unpredictably above and below the true value. Can be reduced by averaging repeated measurements. by repeating measurements 3-5 times per height and averaging.

Examiner Tips and Tricks

You must know the graph axes (2h/t vs t), why it produces a straight line, and how to extract g from the gradient.
This is a commonly examined required practical.