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1.6.1States of Matter

Drag Forces — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 7 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Drag and Fluids

1.6.1 Drag Forces

A fluid is a gas or a liquid, and its particles are free to move around.

Fluid
  • A gas or a liquid, whose particles are free to move and collide with any object passing through them.
Drag
  • A frictional force on an object moving through a fluid, produced by collisions with the fluid particles, which opposes the motion.
Air resistance
  • A type of drag acting on an object moving through air, produced by collisions with air particles.
Density effect
  • A denser fluid produces greater drag, which is why moving through water is harder than moving through air.

Part 2 of 5Drag and Speed

The drag force increases as the object moves faster through the fluid.

1
An object moving through a fluid collides with more particles every second as its speed increases.
2
Each collision transfers momentum to the object, so the total drag force increases with the speed of the object.
3
At higher speeds the drag force is much larger, which is why running feels harder than walking on the same day.
4
The drag force always acts opposite to the direction of motion, so it slows the object down.

Part 3 of 5Effects of Drag

Effects of Drag
Deceleration
  • Drag reduces the resultant force on an object, so it slows the object when the driving force is small.
Heating
  • Work done against drag transfers kinetic energy to the thermal stores of the object and the fluid.
Viscosity
  • A more viscous liquid, such as washing-up liquid, produces more drag, so a marble falls through it more slowly.
Shape and area
  • A larger area facing the direction of motion increases drag, so a flat plate moves through air more slowly than a streamlined shape.

Part 4 of 5Worked Example

Question Explain why a parachutist falling at high speed experiences a larger drag force than when falling slowly.

  1. At higher speed, the parachutist meets more air particles every second.
  2. Each collision with an air particle exerts a backward force on the parachutist.
  3. More collisions per second means a larger total drag force, opposite to the motion.

Answer A higher speed means more air particles collide with the parachutist each second, so the drag force is larger.

Part 5 of 5Exam Checklist & Tips

1.6.1 – Key Points for the Exam

  • Drag A frictional force that opposes the motion of an object moving through a fluid.
  • Fluid A gas or a liquid, whose particles can move freely.
  • Air resistance The drag force acting on an object moving through air.
  • Friction A contact force that opposes motion between two surfaces, or a surface and a fluid.

Common Mistakes

  • Saying drag occurs only in air; it occurs in any fluid, including liquids.
  • Stating that drag stays constant as speed increases; the drag force increases with speed.
  • Describing drag as a non-contact force; it results from collisions with fluid particles.
  • Forgetting that drag acts opposite to the direction of motion.

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