A computer reaction test starts when a participant selects Start. After a random delay, a grey circle turns green. The participant presses and releases the space bar. What should the computer measure to determine the participant's reaction time?
the time between the circle turning green and the space bar being pressed
the time between selecting Start and the space bar being pressed
the time between the space bar being pressed and released
the time between selecting Start and the circle turning green
126 exam-style questions on Edexcel GCSE Physics 2.5 Stopping and braking distances, covering 2.5.1 Stopping distance and reaction time, 2.5.2 Factors affecting stopping distance, 2.5.3 Dangers of large decelerations, and 2.5.4 Braking distance and initial speed. Each one has a worked solution and a mark scheme showing where the marks go.