Falling with air resistance Cambridge International AS & A Level Physics revision
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In plain words
A skydiver doesn't keep speeding up for ever. The faster they fall, the harder the air pushes back, until it pushes back as hard as gravity pulls. From then on the speed stays the same. That steady speed is the terminal velocity.
Three things to know
- Drag increases with speed. At the moment of release the only force is the weight, so the acceleration is g.
- As the speed rises, drag rises, so the resultant force and the acceleration fall.
- When drag equals weight the resultant force is zero and the object moves at a constant terminal velocity.
Tips and tricks
- Give the whole chain: speed increases → drag increases → resultant force decreases → acceleration decreases → until drag = weight.
- On a velocity–time graph this is a curve that starts with gradient g and levels off.
It lands in your notebook with its questions as flashcards.
Falling with air resistance: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the acceleration of an object at the instant it is released from rest in air?
It isn't moving yet, so there is no drag.
What is the resultant force on an object falling at terminal velocity?
Drag and weight balance.
How does the drag on a falling object change as it speeds up?
Faster motion through the air means more resistance.
What happens to the acceleration of a falling object as it approaches terminal velocity?
The resultant force is shrinking.
Two balls of the same size are dropped; one is heavier. Which has the greater terminal velocity?
It needs more drag, and so more speed, to balance its larger weight.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 8 marks. Write your answers on paper, then check them.
Falling with air resistance
Cambridge International AS & A Level Physics 9702 · 8 marks · papermunch.org
Name ______________________________ Date ______________
Describe and explain the motion of a ball released from rest in air, until it reaches terminal velocity.[4]
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At first the only force is its weight, so it accelerates at g. As its speed increases the air resistance increases, so the resultant force and the acceleration decrease. When the air resistance equals the weight the resultant force is zero, and it falls at constant (terminal) velocity.
Sketch, in words, the velocity–time graph for the ball above.[2]
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A curve starting at the origin with gradient 9.81 m/s², getting less steep, and levelling off at the terminal velocity.
Explain why opening a parachute reduces a skydiver's terminal velocity.[2]
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It greatly increases the drag at any given speed, so drag equals weight at a much lower speed.
Answers: Falling with air resistance
- 1. At first the only force is its weight, so it accelerates at g. As its speed increases the air resistance increases, so the resultant force and the acceleration decrease. When the air resistance equals the weight the resultant force is zero, and it falls at constant (terminal) velocity.
- 2. A curve starting at the origin with gradient 9.81 m/s², getting less steep, and levelling off at the terminal velocity.
- 3. It greatly increases the drag at any given speed, so drag equals weight at a much lower speed.