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Speed, velocity and acceleration Cambridge IGCSE Physics (9–1) revision

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In plain words

Speed is how far you go each second. Acceleration is how quickly your speed changes: a car that goes from 0 to 20 m/s in 5 s gains 4 m/s every second, so its acceleration is 4 m/s².

Three things to know

  1. Average speed = total distance ÷ total time. Velocity is speed in a stated direction.
  2. Acceleration = change in velocity ÷ time taken. Its unit is m/s². Slowing down is a negative acceleration.
  3. Near the Earth's surface a falling object with no air resistance accelerates at about 9.8 m/s².

Worked example

A cyclist speeds up from 4 m/s to 10 m/s in 3 s. Find the acceleration.

  1. Change in velocity = 10 − 4 = 6 m/s.
  2. Acceleration = change in velocity ÷ time = 6 ÷ 3.
  3. = 2 m/s².

Tips and tricks

  • Write the formula first, then put the numbers in, then give the answer with its unit. Even with a slip in the arithmetic, the formula and the substitution usually earn marks.
  • Slowing down is a negative acceleration. Don't lose the minus sign.
6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Speed, velocity and acceleration: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. A car travels 150 km in 2.5 hours. What is its average speed?
    • 37.5 km/h
    • 60 km/h (the answer)
    • 152.5 km/h
    • 375 km/h

    150 ÷ 2.5 = 60 km/h.

  2. What does an acceleration of 3 m/s² mean?
    • The object moves 3 m every second.
    • The velocity changes by 3 m/s every second. (the answer)
    • The object takes 3 s to travel 1 m.
    • The speed is 3 m/s.

    Acceleration is the change in velocity each second.

  3. A ball is dropped from rest and falls freely for 2.0 s. Taking g as 9.8 m/s², what is its speed?
    • 4.9 m/s
    • 9.8 m/s
    • 19.6 m/s (the answer)
    • 39.2 m/s

    Speed gained = acceleration × time = 9.8 × 2.0.

  4. A bus slows from 20 m/s to 5 m/s in 5 s. What is its acceleration?
    • −3 m/s² (the answer)
    • −4 m/s²
    • 3 m/s²
    • 5 m/s²

    (5 − 20) ÷ 5 = −3 m/s². The minus sign shows it is slowing down.

  5. What is the difference between speed and velocity?
    • Velocity is measured in different units.
    • Speed is always greater than velocity.
    • Velocity includes direction. (the answer)
    • Speed includes direction.

    Velocity is speed in a given direction, so it is a vector.

  6. A sprinter runs 100 m in 12.5 s. What is her average speed?
    • 0.125 m/s
    • 1.25 m/s
    • 8.0 m/s (the answer)
    • 12.5 m/s

    100 ÷ 12.5 = 8.0 m/s.

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