Recognise near-Earth free-fall acceleration
Use approximately 9.8 m/s² for near-Earth free fall while distinguishing acceleration from speed.
Section P1.2 · Motion
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Distinguish a rate from a total.
- Recognise the direction toward the Earth.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Does constant acceleration imply constant speed?
What you will learn
- State the approximate near-Earth value of g.
- Interpret its unit as a velocity-change rate.
g is not a falling speed
Near the Earth's surface the free-fall acceleration g is approximately constant at 9.8 m/s², directed downward. The unit means the downward velocity changes by approximately 9.8 m/s in each second of ideal free fall. It does not mean every falling object always has speed 9.8 m/s.
Free fall describes motion under gravity alone in the model. For a small height compared with Earth's size, treating g as constant is useful. The real gravitational field varies with location and height, so the near-surface value is approximate rather than a universal exact constant.
Keep the Core requirement in view
A dropped object begins with its initial release velocity and then changes velocity. The Core requirement here is knowing the approximate constant acceleration, not applying Supplement acceleration calculations. A change in mass alone does not change the ideal gravitational acceleration in this near-Earth model.
Real objects can also experience air or liquid resistance. Their observed acceleration may then differ from g. The separately labelled Extended falling-motion lesson explains the resistance and terminal-velocity model; do not treat an observed constant falling speed as proof that gravitational acceleration itself has become zero.
Sources for this lesson
Pause and explain
Which correctly states the near-Earth free-fall quantity?
Worked example
A learner says every near-Earth falling object has constant speed 9.8 m/s. Correct the statement.
Show the worked solution
- 9.8 m/s² is an acceleration, not a speed.
- It approximately describes free fall near Earth's surface.
- Velocity changes with time; resistance can affect actual falling motion.
Answer Near-Earth free-fall acceleration is approximately 9.8 m/s² downward.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the approximate near-Earth free-fall acceleration with its unit and direction.
Give me a hint
The quantity is acceleration rather than a single speed.
Compare my reasoning
- Use the approximate value 9.8.
- Use m/s².
- State downward, toward the Earth.
Approximately 9.8 m/s² downward.
Look for these in your work
- Use an acceleration unit.
- Include direction and approximation.
Explain why 9.8 m/s is not the correct unit statement for g.
Give me a hint
Acceleration describes a velocity change per second.
Compare my reasoning
- m/s describes speed or a velocity magnitude.
- g describes a velocity-change rate.
- Its unit is (m/s)/s = m/s².
g uses m/s², a rate of velocity change.
Look for these in your work
- Distinguish the quantities.
- Explain the extra time denominator.
A falling object in air has an observed constant speed. Does that establish g = 0?
Give me a hint
The observed motion may involve forces other than gravity.
Compare my reasoning
- g describes the gravitational acceleration model.
- Air resistance can affect the actual motion.
- Constant observed speed does not imply absence of Earth's gravitational field.
No; air resistance can alter the actual acceleration while gravity remains.
Look for these in your work
- Separate g from observed motion.
- Recognise the model limitation.
Common mistakes
- Use m/s² rather than m/s for g.
- Near-Earth g is approximate, not identical everywhere.
- Air resistance can change an object's actual acceleration.
What can you explain now?
What approximate acceleration describes free fall near Earth's surface?
Compare with the explanation
9.8 m/s² downward.
The approximation refers to acceleration under gravity, not a fixed speed.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
State the value, unit and direction of g and explain why it is not a universal falling speed.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.