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5.6.14Forces

Terminal Velocity

5 min read 4 parts Lesson 50 of 67 Unit quiz

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Section 1 of 4Forces on a Falling Object

Two forces act on a falling object: its weight and the air resistance opposing its motion.

Weight
  • Weight Weight acts downwards on a falling object and is caused by the gravitational pull of the Earth.
Air resistance
  • Air resistance Air resistance, also called drag, acts upwards against the motion and increases as the object's speed increases.
Resultant force
  • Resultant force The resultant force is the weight minus the air resistance, so it becomes smaller as air resistance builds up.
Acceleration
  • Acceleration Acceleration is proportional to the resultant force, so the object speeds up more slowly as its speed rises.

Section 2 of 4Reaching Terminal Velocity

The speed of a falling object changes in a predictable sequence of stages.

1
At the start, the object has zero speed, so there is no air resistance and the resultant force is equal to its full weight.
2
As the object speeds up, air resistance increases, so the resultant force and the acceleration both decrease.
3
When air resistance becomes equal to the weight, the resultant force is zero, so the object stops accelerating.
4
The object then keeps falling at a constant speed, which is called its terminal velocity.

Section 3 of 4Factors Affecting Terminal Velocity

Changing the object or the fluid it falls through changes the terminal velocity.

Factors Affecting Terminal Velocity
Surface area
  • Surface area A larger surface area increases air resistance, so the terminal velocity is lower.
Shape
  • Shape A streamlined shape reduces air resistance, so the object reaches a higher terminal velocity.
Mass
  • Mass A heavier object has a greater weight, so it must reach a higher speed before drag equals its weight.
Fluid
  • Fluid Falling through a denser fluid such as water usually gives a lower terminal velocity than falling through air.

Section 4 of 4Exam Checklist & Tips

5.6.14 – Key Points for the Exam

  • Terminal velocity The constant maximum speed reached when drag is equal to weight.
  • Air resistance A frictional force that opposes the motion of an object moving through air.
  • Drag Another name for air resistance or fluid resistance.
  • Resultant force The overall force on an object after all the forces acting on it are combined.
  • Equilibrium A state in which the forces on an object are balanced, so the net force is zero.

Common Mistakes

  • Saying the object stops falling at terminal velocity; it keeps falling at a constant speed.
  • Stating that weight decreases at terminal velocity;
    • weight stays the same, but drag grows until it balances the weight.
  • Confusing terminal velocity with the speed at the start of a fall, which is zero.
  • Believing that a parachute increases the weight of the falling object;
    • it increases air resistance, which lowers the terminal velocity.
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).