Lesson focus: A falling object reaches terminal velocityThe top steady speed a falling object reaches when air resistance balances its weight. when dragA resistive force from a fluid (air or water) that acts opposite to the direction of motion and increases as speed increases. has grown to equal its weight, making resultant forceThe overall force on an object when all the forces acting on it are added together. and acceleration zero.
Explain each stage of a fall using weight, drag, resultant force and acceleration.
Build on earlier learning
Air resistance and egg dropYear 7 explored air resistance and parachutes. This lesson explains terminal velocity using force balance and changing speed.
You will be successful when you can…
- Draw falling force diagrams.
- Explain changing drag.
- Explain terminal velocity.
- Describe parachute deployment.
Words that carry the science
- drag
- A resistive force opposing motion through a fluid.
- terminal velocity
- Constant velocity reached when resultant force becomes zero.
- free fall
- Motion when gravity is the only significant force.
- resultant force
- Vector sum of all forces on an object.
- cross-sectional area
- Area presented perpendicular to motion through the fluid.
- fluid
- A substance such as a liquid or gas that can flow.
Weight initially dominates
01Weight initially dominates
Just after release, speed and dragA resistive force from a fluid (air or water) that acts opposite to the direction of motion and increases as speed increases. are small, while weight is approximately constant. The resultant forceThe overall force on an object when all the forces acting on it are added together. is downward, so the object accelerates downward.
As speed rises, collisions and interactions with air increase drag. The downward resultant and acceleration become smaller.
02Terminal velocity is dynamic force balance
When upward drag equals downward weight, resultant force is zero. Acceleration becomes zero and the object continues at constant terminal velocityThe top steady speed a falling object reaches when air resistance balances its weight..
Forces have not disappeared. Equal opposite forces continue acting while the object moves.
03A parachute creates a new force imbalance
Opening a parachute greatly increases cross-sectional areaThe area of the front of an object facing the way it moves; a bigger area means more air resistance. and drag. Drag can temporarily exceed weight, producing an upward acceleration while the person is still moving downward, so their speed decreases.
As speed falls, drag falls until it again equals weight at a lower terminal velocity.
Falling with air resistance
Air resistance grows as the skydiver speeds up. When it balances the weight, the speed stops changing: terminal velocity. Opening the parachute makes the air resistance much bigger.
Controls
Explain a force sequence
A skydiver of weight 750 N experiences 300 N drag.
- Take downward as positive.
- Resultant = 750 − 300 = 450 N downward.
- The resultant is in the same direction as velocity.
- Therefore the skydiver speeds up downward, though acceleration is less than at release.
How to make the reasoning visible: As drag approaches 750 N, resultant force and acceleration approach zero.
Track a model fall
Use the forces to infer the motion at successive moments.
| Stage | Weight / N | Drag / N | Motion change |
|---|---|---|---|
| release | 750 down | 0 | accelerates downward most |
| falling faster | 750 down | 400 up | still speeds up downward |
| first terminal | 750 down | 750 up | constant high downward velocity |
| parachute opens | 750 down | 1500 up | slows while moving downward |
| new terminal | 750 down | 750 up | constant lower downward velocity |
Why can upward acceleration occur while motion is downward?
Why is the second terminal velocity lower?
Check your understanding
Answer from memory, then check against the suggested answer.
What causes drag to rise during a fall?
What is true at terminal velocity?
Does weight vanish at terminal velocity?
A dense small ball and a broad paper cone have equal weight. Which likely has lower terminal speed?
Why is early fall often approximated as free fall?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A skydiver jumps from a plane and later opens a parachute.
Explain how the skydiver reaches terminal velocity.
(3)Explain what happens when the parachute opens.
(3)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., narrate the changing force balance rather than saying only that air resistance increases.