The changing motion of falling objects
Teaching Guidance for 11-14
Drag forces and gravity forces add to zero
Wrong Track: As things fall they must keep speeding up because gravity pulls them down.
Right Lines: On Earth, all falling objects move through the atmosphere. The resulting movement through the air creates a frictional force which acts against the motion. Objects reach a constant speed when the upward force equals the downward force of gravity.
Demonstrating terminal speed
Thinking about the learning
The point here is for pupils to appreciate that objects speed up or accelerate as they fall under the influence of gravity, but also to recognise that they may achieve a constant terminal speed because of the effects of retarding forces.
Pupils know that you cannot take away or reduce the gravity force and therefore tend to assume that a falling object speeds up continuously. Furthermore, within the limits of pupil experience few falling objects reach a constant speed.
Thinking about the teaching
To move pupils onto the right lines you might set up some laboratory demonstrations to illustrate terminal speed. These might include objects falling in a tall column of water or low density objects such as polystyrene balls in free-fall.