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1.2.6Motion, Forces and Energy

Freefall — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 17 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.2.6 Freefall — IGCSE Physics Notes — IGCSE PHYSICS notes: Describe free fall, the acceleration of free fall and terminal velocity.Without air resistance — all objects fall with the same acceleration whatever their massValue — g is about 9.8 m/s² near the Earth's surfaceMeaning …

  • Gravitational field strength
  • Only force
  • What the symbols mean
  • Negligible air resistance
  • No terminal velocity
  • Rising part

Part 1 of 7Acceleration of free fall

1Without air resistance
  • Without air resistance all objects fall with the same acceleration whatever their mass
2Value
  • Value g is about 9.8 m/s² near the Earth's surface
3Meaning
  • Meaning velocity increases by 9.8 m/s for every second of fall
4Gravitational field strength
  • Gravitational field strength g in N/kg also gives weight using W = mg

Part 2 of 7Free fall in a vacuum

1Vacuum
  • Vacuum no matter, so no air or liquid resistance
2Only force
  • Only force the weight, W = mg
    What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)
3Acceleration
  • Acceleration a = F ÷ m = mg ÷ m = g, so the masses cancel
4Moon test
  • Moon test a hammer and a feather dropped on the Moon landed together
5Negligible air resistance
  • Negligible air resistance then the motion is approximately the same as in a vacuum
6No terminal velocity
  • No terminal velocity constant acceleration continues

Part 3 of 7Falling through air

Air resistance
Air resistance a frictional force from collisions with air particles
Speed
  • Speed air resistance increases as speed increases
Forces
  • Forces weight acts downwards and air resistance acts upwards
Terminal velocity
  • Terminal velocity reached when air resistance equals weight, so there is no resultant force and constant speed

Part 4 of 7Skydiver stages

1
At the jump only weight acts, so the skydiver accelerates downwards.
2
As speed increases, air resistance increases and the resultant force and acceleration decrease.
3
Air resistance equals weight, forces balance and terminal velocity is reached.
4
The parachute opens and its larger area greatly increases air resistance.
5
Air resistance exceeds weight, the resultant force acts upwards and the skydiver decelerates.
6
As speed falls, air resistance falls until it equals weight again.
7
A new, slower terminal velocity is reached.

Part 5 of 7Falling through a liquid

Drag
  • Drag the liquid exerts a frictional force that opposes motion
Speed
  • Speed drag increases as the speed increases
Terminal velocity
  • Terminal velocity reached when drag equals weight

Part 6 of 7Graph of a skydive

Graph of a skydive
Rising part
  • Rising part speed increases as the skydiver accelerates
Horizontal parts
  • Horizontal parts periods of terminal velocity
After the parachute
  • After the parachute speed falls to the lower terminal velocity

Part 7 of 7More from the specification

1Acceleration of Freefall
  • In the absence of air resistance, all falling objects accelerate downwards at the same constant rate near Earth's surface regardless of their mass.
    • (g approx 9.8 m/s2
2Interpreting Speed-Time Graphs for Freefall
  • On a speed-time graph, a straight line with a constant positive gradient represents uniform acceleration,
  • where the numerical value of the gradient equals gravitational acceleration g.
3Calculating Distance Travelled
  • The total vertical distance travelled by a falling body during freefall is equal to the area under the speed-time graph.
    • (Area = 1/2 * base * height)
4Curvature on a Distance-Time Graph
  • On a distance-time graph, an object accelerating under gravity produces a parabolic curved line with an increasing gradient, representing a continuous change in speed over time.
5Gradient
  • Slope of a graph line.
6Distance
  • Total length of path travelled, a scalar.
7Speed-time graph
  • Graph of speed against time.
8Distance-time graph
  • Graph of distance against time showing movement.
9Average speed
  • Total distance divided by total time.
10Gradient triangle
  • A right-angled triangle drawn on a line to find Δy and Δx.

Key terms

TermMeaning
Free fallFalling under the force of weight alone.
gAcceleration of free fall, about 9.8 m/s².
Air resistanceFrictional force from air on a moving object.
Terminal velocityConstant speed when resistance equals weight.
VacuumSpace with no matter.
DragResistive force from a liquid or gas.

Avoid these mistakes

1
Thinking heavier objects fall faster without air resistance — all fall with the same acceleration.
2
Confusing air resistance with air pressure — they are different.
3
Thinking a skydiver moves upwards when the parachute opens — the skydiver decelerates while still moving downwards.
4
Thinking terminal velocity means no forces — the forces are balanced.

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