IGCSE Physics / Movement and position
Acceleration
Edexcel International GCSE Physics (4PH1), chapter 1: Movement and position, specification point 1.6, 1.1. Free lesson.
Read the revision notes · IGCSE Physics past papers
Acceleration measures how quickly velocity changes. Use it to calculate an acceleration, a final velocity or a time, and remember that slowing down gives a negative acceleration.
What acceleration means
Acceleration is the change in velocity each second. If a car gains the same amount of velocity every second, its acceleration is constant. Gaining 3 m/s each second is an acceleration of 3 m/s². Acceleration is measured in metres per second squared (m/s²). Velocity is measured in metres per second (m/s).
How to use the acceleration equation
Use a = (v − u)/t. The numerator, v − u, is the change in velocity: always subtract the initial velocity from the final velocity. The equation is on the equation list, but you need to know what each symbol means. a is acceleration, in m/s². v is final velocity, at the end, in m/s. u is initial velocity, at the start, in m/s. From rest, u = 0. t is time taken, in seconds (s).
How to calculate acceleration
Find the change in velocity first, then divide by the time. This shows how much velocity the car gained during the time, rather than how fast it was going at the end.
Why you must subtract the initial velocity
Acceleration depends on how much velocity changes, not just the final velocity. The car already had a velocity of 8 m/s, so it gained 12 m/s, not 20 m/s. Incorrect: a = 20/4 = 5 m/s². Correct: a = (20 − 8)/4 = 3 m/s².
How to find time when an object slows down
Slowing down is called deceleration. Treat it as negative acceleration. A car that brakes at 5 m/s² has a = −5 m/s². If it stops, its final velocity is v = 0.
How to find final velocity
From rest means the initial velocity is zero. If an object accelerates by 10 m/s each second for 3 seconds, its velocity increases by 30 m/s.