Recognise drag in liquids and gases
Compare fluid resistance with solid contact friction and calculate simple opposed-force resultants.
Section P1.5.1 · Effects of forces
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 liquid from a gas.
- Find a resultant for collinear forces.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which of these is a gas-drag example?
What you will learn
- Identify resistance on objects moving through liquids and gases.
- Compare drag direction with motion relative to the surrounding fluid.
A surrounding fluid can resist relative motion
A moving object experiences frictional resistance or drag when passing through a liquid or gas. Water resistance opposes a swimmer or boat's motion relative to the water; air resistance opposes a cyclist or falling object's motion relative to the air. A liquid and a gas are both fluids, but their resistance effects need not have equal magnitudes.
Use the relative motion. In the examples the surrounding water or air is stationary, so drag opposes the object's stated travel. With a current or moving air, ground-relative velocity alone is insufficient to assign the fluid-resistance direction. No drag-coefficient formula or universal linear speed rule is assumed.
Use drag in the chosen body's force balance
A boat moving east through still water with 18 N eastward thrust and 12 N westward water drag has horizontal resultant 6 N east. A vehicle with 30 N driving force and 30 N resistance can move at a steady straight-line velocity even though drag continues to act.
A falling object's air resistance is upward while it moves downward through still air. An object moving upward has downward drag under the corresponding still-fluid assumptions. In liquid vertical motion, buoyancy may also matter; do not assume drag is the only non-weight force when a problem includes it. Extended motion lessons address terminal-force balance separately.
Sources for this lesson
Pause and explain
A boat has 18 N thrust east and 12 N water drag west. Which resultant is correct?
Worked example
A boat travels east through still water with 18 N eastward thrust and 12 N water resistance west. Find its horizontal resultant.
Show the worked solution
- Water drag opposes the stated relative motion.
- Choose east positive: 18 − 12 = 6 N.
- The horizontal resultant is eastward.
Answer 6 N east.
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.
A boat travels east through still water under 18 N thrust and 12 N opposite drag. Find resultant.
Give me a hint
Drag is opposite the relative motion in this stated setup.
Compare my reasoning
- Choose east positive.
- Resultant = 18 − 12 = 6 N.
- Its direction is east.
6 N east.
Look for these in your work
- Identify water drag.
- Find the signed sum.
A body moves downward through still air, and another moves upward through still water. Give each drag direction.
Give me a hint
Oppose each body's motion relative to its own fluid.
Compare my reasoning
- The first body's relative motion is downward.
- Its air drag is upward.
- The second body's upward relative motion gives downward water drag.
First: upward air drag; second: downward water drag.
Look for these in your work
- Treat the two media separately.
- Oppose the stated relative motions.
A car moves straight at constant speed with 30 N driving force and 30 N resistance. A student says drag must vanish at steady speed. Correct the claim.
Give me a hint
A zero resultant can contain equal non-zero forces.
Compare my reasoning
- The driving and resistance forces are equal and opposite.
- Their resultant is zero, so velocity can stay constant.
- The resistance remains present rather than vanishing.
Drag can remain non-zero and be balanced by the driving force at steady straight-line speed.
Look for these in your work
- Retain individual forces.
- Connect balance with unchanged velocity.
Common mistakes
- Drag acts in liquids as well as gases.
- Use motion relative to the fluid.
- Include other supplied forces rather than replacing them with drag.
What can you explain now?
Which way is air resistance on a downward-moving body in still air?
Compare with the explanation
Upward.
Resistance opposes its downward motion relative to the air.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Name a liquid-drag and gas-drag example and state the relative motion used to choose the resistance direction.
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.