Explain drag and terminal velocity
Describe how resistive forces affect falling motion and explain terminal velocity using force balance.
Section 3 · Dynamics
AS Level · Cambridge International Physics 9702, 2025–2027 syllabus, Topic 3. These original explanations and practice support theory study; they do not establish supervised practical competence or replace timed assessment and teacher review. Drag is treated qualitatively, and coefficient of restitution is outside this syllabus section.
Before you begin
- Use vectors and signed values consistently, and distinguish mass from weight.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the direction of drag on an object moving downward through still air?
What you will learn
- Describe friction, viscous drag and air resistance qualitatively
- Explain changing motion in a uniform gravitational field with air resistance
- Explain terminal velocity as constant speed when resultant force is zero
Resistive forces oppose relative motion
Friction acts between surfaces that tend to slide or roll relative to one another. A viscous or drag force acts as an object moves through a fluid, including air. These forces oppose the motion relative to the surface or fluid.
A simple syllabus-level model is that drag increases as speed increases. The exact coefficients and detailed drag equations are not required here. Keep the explanation qualitative unless a force value is provided.
A falling object can approach terminal velocity
Just after release, a falling object has small speed, so air resistance is small and weight is greater. It accelerates downward. As its speed increases, upward air resistance increases, so the resultant downward force and acceleration decrease.
At terminal velocity, air resistance balances weight. The resultant force is zero, so acceleration is zero and the object continues at constant downward velocity. It does not stop; the forces balance while it is moving.
Pause and explain
What is true once a falling object reaches terminal velocity?
Worked example
Describe how the speed and forces on a falling object change from release until it reaches terminal velocity.
Show the worked solution
- At release, speed and drag are small while weight acts downward.
- The resultant force is downward, so the object speeds up; increasing speed raises drag.
- When drag balances weight, resultant force and acceleration are zero, so speed is constant.
Answer The object accelerates downward initially. Its increasing speed raises air resistance, reducing the resultant downward force. Eventually drag equals weight, giving zero resultant force and constant terminal velocity.
Try a topic test
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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.
A falling object is moving downward and still speeding up. Compare its weight and air resistance at that instant.
Give me a hint
Use the direction of acceleration to infer the resultant force.
Compare my reasoning
- The acceleration is downward because its downward speed is increasing.
- The resultant force must therefore be downward.
- Weight downward is larger than air resistance upward.
Weight is greater than air resistance, so the resultant force and acceleration are downward.
Look for these in your work
- Acceleration direction identified
- Resultant direction linked to acceleration
- Forces compared correctly
A skydiver reaches terminal velocity before opening a parachute. State the force balance and what happens to acceleration.
Give me a hint
Terminal velocity means constant velocity, so use Newton's first law.
Compare my reasoning
- The skydiver continues moving downward at constant speed.
- Weight acts downward and air resistance acts upward.
- The forces are equal, so resultant force and acceleration are zero.
Weight equals air resistance; the skydiver continues downward with zero acceleration at constant terminal velocity.
Look for these in your work
- Both forces identified
- Balance stated
- Zero acceleration distinguished from motion
After a parachute opens, the skydiver's speed decreases. Explain the force comparison and the later motion.
Give me a hint
Consider the instant just after opening, then the eventual new steady speed.
Compare my reasoning
- At the initial high speed, the enlarged parachute makes air resistance greater than weight.
- The resultant force is upward, opposite the downward velocity, so the skydiver slows.
- As speed falls, drag falls until it balances weight at a lower terminal speed.
Initially drag exceeds weight, producing an upward resultant and downward deceleration. The skydiver slows until drag again equals weight, then falls at a lower constant terminal velocity.
Look for these in your work
- Initial force balance compared
- Opposite force and motion directions distinguished
- New terminal state described
Common mistakes
- Saying balanced forces mean the moving object must stop
- Saying drag grows while speed stays unchanged in the simple model
- Applying an unrequired numerical drag coefficient
What can you explain now?
What is the direction of drag on an object moving downward through still air?
Compare with the explanation
Upward
Drag opposes the object's motion relative to air.
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Make it stick
At terminal velocity, which quantities are zero and which remain non-zero?
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.