Lesson overview
Acceleration, falling and drag: Why does a skydiver stop speeding up?
Drop a steel ball and a feather down a tall tube, then pump the air out and drop them again, and find the acceleration of free fall from the strobe images. Then time a ball sinking through glycerol with a stop-clock, find its terminal velocity, and watch the forces on the falling feather balance.
Suggested sequence
- Lesson 1 · Acceleration. Acceleration as the change in velocity per unit time, from data and from the gradient of a speed–time graph; deceleration and the friction in brakes; free fall with g = 9.8 m/s². Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Falling and drag. Drag in liquids and air resistance in gases; falling with and without them; terminal velocity and the parachute. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Acceleration
- Define acceleration as the change in velocity per unit time and use a = Δv / Δt, in m/s² (1.2.9)
- Tell from data or a speed–time graph whether an object has a constant or a changing acceleration (1.2.10)
- Calculate acceleration from the gradient of a speed–time graph (1.2.11)
- Know that a deceleration is a negative acceleration and use this in calculations; describe how solid friction acts against motion and heats the surfaces (1.2.12, 1.5.1.6)
- State that the acceleration of free fall g near the surface of the Earth is approximately constant and approximately 9.8 m/s² (1.2.8)
Part 2: Falling and drag
- Tell from data or a speed–time graph whether an object has a constant or a changing acceleration (1.2.10)
- Know that drag is friction in a liquid or a gas, acting against motion, and that drag in air is air resistance (1.5.1.7, 1.5.1.8)
- Describe the motion of objects falling in a uniform gravitational field with and without air or liquid resistance, including reaching terminal velocity (1.2.13)
Misconceptions to listen for
- “a = 8.0 ÷ 4.0 = 2.0 m/s².” — 8.0 m/s is her final speed, not the change. She started at 2.0 m/s, so her speed changed by 6.0 m/s.
- “a = 6.0 × 4.0 = 24 m/s².” — Acceleration is the change in speed per second, so divide the change by the time.
- “Car X has the constant acceleration because it goes faster.” — How fast a car goes says nothing about whether its speed changes steadily. Look at the change in speed in each second.
- “Both speeds go up every second, so both accelerations are constant.” — Constant acceleration means the speed goes up by the same amount every second. Car X's gains get smaller.
- “Her acceleration is 6 m/s², because the line reaches 6.” — 6 m/s is a speed. Acceleration is the change in speed divided by the time the change took.
- “Gradient = 3 ÷ 6 = 0.5 m/s².” — Rise ÷ run: the rise is up the speed axis (6 m/s), the run is along the time axis (3 s).
- “a = +2.5 m/s², because an acceleration is always positive.” — The bus is slowing down, so the change in velocity is negative and so is the acceleration: −2.5 m/s², a deceleration.
- “a = −2.5 m/s² means the bus is moving backwards.” — The bus keeps moving forwards. The minus sign says its speed is getting smaller.
- “Its speed is 9.8 m/s, the value of g.” — g is an acceleration: the speed gained in each second. After 2.0 s the stone has gained twice that.
- “A heavier stone would be going faster after 2.0 s.” — In free fall every object has the same acceleration, so every stone has the same speed after 2.0 s.
Scope
The acceleration of free fall g, approximately constant near the Earth and about 9.8 m/s² (1.2.8); acceleration as the change in velocity per unit time, a = Δv ÷ Δt (1.2.9); constant and changing acceleration from data and from a speed–time graph (1.2.10); acceleration from the gradient of a speed–time graph (1.2.11); deceleration as a negative acceleration (1.2.12); falling in a uniform gravitational field with and without air or liquid resistance, and terminal velocity (1.2.13); friction between solid surfaces, which impedes motion and causes heating (1.5.1.6); drag in liquids and air resistance in gases (1.5.1.7, 1.5.1.8). Resultant force, F = ma and Newton’s first law are in FORCE01.