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1.6.2States of Matter

Terminal Velocity — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Forces on a Falling Object

A falling object is subject to two main forces, which act in opposite directions.

Weight
  • Weight The constant downward force of gravity on the object, which does not change during the fall.
Drag
  • Drag An upward force opposing the motion, which increases as the speed of the object increases.
Resultant force
  • Resultant force The difference between weight and drag, which causes acceleration while the weight is larger than the drag.
Terminal velocity
  • Terminal velocity The constant maximum speed reached when drag equals weight, so the resultant force is zero.

Part 2 of 5Stages of a Fall

The motion of a falling object can be divided into stages, each with a different resultant force.

1
At release the only force is weight, because there is no speed yet and so no drag, so the object accelerates downwards.
2
As speed increases, drag increases, so the resultant force and the acceleration both decrease.
3
When drag equals weight, the resultant force is zero and the object continues to fall at a constant terminal velocity.
4
Opening a parachute increases the surface area, so the drag rises and a new, lower terminal velocity is reached.

Part 3 of 5Reading the Velocity-Time Graph

Reading the Velocity-Time Graph
Steep start
  • Steep start A steep initial slope shows a large acceleration, when the drag is still small.
Decreasing gradient
  • Decreasing gradient The gradient becomes smaller as drag grows, showing that the acceleration falls towards zero.
Flat section
  • Flat section A horizontal section shows terminal velocity, when the velocity no longer changes.
Parachute opening
  • Parachute opening A sudden drop in velocity shows a sharp increase in drag when the parachute opens.

Part 4 of 5Worked Example

Question

  • A falling object has a weight of 800 N.
  • At one moment, the drag acting on it is 600 N.
  • Calculate the resultant force and state its direction.

  1. Weight acts downwards800 N down.
  2. Drag acts upwards600 N up.
  3. Resultant force = 800 − 600 = 200 N, in the direction of the larger force.
  4. The resultant acts downwards, so the object is still accelerating and has not yet reached terminal velocity.

Answer The resultant force is 200 N downwards.

Part 5 of 5Exam Checklist & Tips

1.6.2 – Key Points for the Exam

  • Terminal velocity The constant maximum speed reached when the drag on a falling object equals its weight.
  • Drag The frictional force opposing the motion of an object through a fluid.
  • Weight The gravitational force acting on an object with mass.
  • Resultant force The single force equivalent to all the forces acting on an object.

Common Mistakes

  • Saying the object stops when it reaches terminal velocity; it continues to fall at a constant speed.
  • Assuming the drag is constant during the fall; it increases with speed.
  • Claiming the object is accelerating at terminal velocity; the resultant force is zero there.
  • Thinking a parachute raises the terminal velocity; it lowers it by increasing the drag.

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Physics 0625 & 9702 SUVAT Mastery Graph Interpretation

Constant Acceleration Equations (SUVAT) & Slopes

v = u + at  |  s = ut + ½at2  |  v2 = u2 + 2as  |  s = ½(u + v)t

  • Displacement-Time Graph: Gradient = Velocity (curve = accelerating).
  • Velocity-Time Graph: Gradient = Acceleration; Area under curve = Distance travelled / Displacement.
  • Free Fall in vacuum: a = g ≈ 9.81 m/s2 downwards (air resistance negligible).
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