Explain falling motion and terminal velocity
Compare ideal free fall with resistance that grows until the forces balance.
Section P1.2 · Motion
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish zero acceleration from rest.
- Recognise that forces have directions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Can an object move when its acceleration is zero?
What you will learn
- Describe falling motion without resistance.
- Explain changing acceleration and terminal velocity in air or liquid.
Ideal and resistive falls differ
In a uniform near-Earth gravitational field with no resistance, a dropped object accelerates downward approximately constantly at g = 9.8 m/s². A speed–time graph is a straight rising line. There is no terminal speed in that ideal uniform-field, no-resistance model; speed keeps increasing during the modelled fall.
In air, a dropped object's initial resistance is small because its speed is small. As downward speed rises, upward air resistance increases. With weight approximately unchanged, the downward resultant decreases and acceleration decreases. The speed–time curve still rises, but becomes less steep.
Balance means constant velocity, not no force
When upward resistance equals weight, the resultant force is zero and the object continues downward at constant terminal velocity. Weight and drag remain present. Zero acceleration at this stage does not mean the object has stopped or that g is zero. The terminal speed depends on the object and medium rather than being a universal 9.8 m/s.
In a liquid, upthrust may also act upward. For a sinking body, increasing liquid resistance can bring the total upward force, resistance plus upthrust, into balance with weight. Its resulting terminal motion is again constant downward velocity. The visual uses a schematic speed curve approaching a horizontal limit; it is not a measured dataset or a formula for a particular object.
Sources for this lesson
Pause and explain
At terminal velocity in air, what is true?
Worked example
A falling object in air reaches constant downward speed. Explain the forces and acceleration.
Show the worked solution
- Weight continues to act downward.
- Upward air resistance equals weight, giving zero resultant.
- Acceleration is zero; the object continues at terminal velocity.
Answer Balanced weight and drag; zero acceleration with continued downward motion.
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.
Describe a dropped object's speed and acceleration without resistance in a uniform near-Earth field.
Give me a hint
Use the ideal gravitational model rather than terminal-speed reasoning.
Compare my reasoning
- Speed increases from its release value.
- Acceleration stays approximately 9.8 m/s² downward.
- The speed–time line rises straight with no terminal limit in the model.
Increasing speed at approximately constant downward acceleration; no terminal velocity.
Look for these in your work
- State the no-resistance assumption.
- Distinguish speed and acceleration.
Explain why a falling object's speed curve in air rises but flattens toward terminal velocity.
Give me a hint
Track resistance, resultant force and acceleration in that order.
Compare my reasoning
- Resistance increases as speed rises.
- The downward resultant and acceleration decrease.
- At balance, acceleration is zero and velocity stays constant.
Increasing drag reduces acceleration until drag balances weight.
Look for these in your work
- Connect the force changes.
- Explain the horizontal limiting speed.
A body sinks steadily through a liquid. Explain the balance including upthrust and why it is still moving.
Give me a hint
Add the upward forces before comparing them with weight.
Compare my reasoning
- Weight acts downward; upthrust and resistance act upward.
- At terminal motion, weight equals upthrust plus resistance.
- The resultant and acceleration are zero, while downward velocity remains constant.
Weight = upthrust + liquid resistance; constant downward velocity.
Look for these in your work
- Include both upward forces.
- Distinguish balance from rest.
Common mistakes
- At terminal velocity gravity and drag have not disappeared.
- Zero acceleration does not mean zero speed.
- In liquid, include upthrust where relevant.
What can you explain now?
Does ideal free fall without resistance have terminal velocity in a uniform field?
Compare with the explanation
No.
Gravity continues to produce constant downward acceleration in that model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch ideal and resistive fall speed graphs and explain terminal balance in both air and liquid.
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.