A sequence to develop ideas about falling
Classroom Activity for 5-11
Based on the Physics Narrative and the Teaching and Learning Issues
Falling, and perhaps also rising, are so common as occurrences that you'll perhaps need to make them a bit strange, to draw attention to particular features, in order to make the accounts of falling that you present seem worthwhile. Otherwise the phenomena may just seem to be
obvious and not in need of any kind of explanation.
We'd strongly suggest that you restrict the situations you use for such introductory work to one dimension, so straight up and down. That'll mean selecting suitable examples, and so steering the discussion away from topics such as runaway cars and plummeting mountain bikes.
In some cases the way in which the falling happens is there to support enhanced movement in another direction: the advantage to tree seeds in falling slowly is precisely that so they can travel sideways further in the longer time enabled by the slower fall. However we suggest keeping the links at about this kind of level, and not trying to account for the path of the motion in any detail.
To extend the ideas, or even to see how securely children can make the links between the forces and the motion you might like to explore
falling upwards, or rising in fluids. Apart from extending the ideas, the phenomena are interesting, often rather beautiful, and have many links.
Meeting reality: valuable experiences
- Dropping things in air: streamlined and irregularly shaped
- Dropping things in water: not into water
- Dropping things in a vacuum – at least a clip
- Rising in fluids, as a kind of inverse falling
A sequence for developing the idea
There are two essential elements interwoven here:
- A description of how the falling thing moves
- An account of why the falling thing has that motion
The end point, perhaps on later studies is to weave these two stories together, relating the changes in motion to the forces acting on the object. For your benefit, here is a summary for falling: in fluids and in a vacuum. In this sequence you'll start to develop the descriptions of the forces acting, and the changes in speeds, without necessarily bringing them together.
The ideas are developed in the Physics Narrative.
Here is one sequence for developing the idea:
In this activity children assemble a collection of falling things, and start to describe the motion. Seeds look a fertile starting point here, but you might explore falling in all kinds of fluids, and for many kinds of objects.
These can be stories with or without data, but should mostly descriptive, exploring what might be causing the falling at each stage: so a focus on forces.
This explores the idea of falling on different continents . Apart from giving opportunities to amass a wide range of interesting examples, you can rather directly address the difficulties of what it means to
fall down, as there is a known difficulty with the idea of down. Depicted on a globe, with the gravity arrows added, this can be both interesting and a powerful learning experience.
By starting just outside the atmosphere of Earth this activity enables you to tackle the known difficulty that many children are rather persistent in believing that the force of gravity only extends as far as the atmosphere. Extending the
pull of gravity to other planets, and the solar system itself therefore provides a helpful corrective to this error. You should be prepared to deflect questions about why Earth does not fall into the Sun.
This activity opens up the difficulties of relative motion, and gets to the heart of what it is to say
That's moving. You might use a clip of a skydiver falling, as shot by another skydiver, or do some careful representing and consequent discussion.
Increasing and constant speeds
The difference between seeds and stones might be a natural starting point, but the end point is to describe (qualitatively) the changes in motion during a single fall. Few things fall at the same speed for the whole of their journey.
Some careful noticing and recording of simple objects falling through fluids other than air extends the opportunities to notice the changes in speed as the object falls, and to practice describing this change over the fall.
This is something of an interlude, or extension, or reinforcement. The process of
falling up, as shown by bubbles in cooking oil, is the mirror of falling, also with variations in speed, and with some really interesting things to notice and record.
Not everything falls, and this activity provides a bridge to thinking about the forces acting on the supported objects.
This will not be for everyone, but links the description of motion to the varying force acting. Children will meet this again later, so there is no need to squeeze it here.
In this activity you can explore the idea of a
This activity is included in case the suggestion is made that bigger things must fall faster. It may be that you deflect the discussion, using some of the dialogue, rather than running it as a full-fledged activity.
Messages from research and practice: specific tripwires for this idea
The local nature of the idea of
down is problematic, as is action at a distance. There are activities to tackle the former, but not the latter, which is probably best left to a later stage.
All motion and therefore all speed is relative. Our everyday talking does not take this into consideration that this does lead difficulties if we're not careful. Be aware and be careful.
The idea of average speed can cause difficulty and it's probably worth being aware of this in order to avoid those difficulties rather than trying to address them explicitly at this stage.
Force equals motion: no motion equals no force
Force equals motion: motion equals force
It is early days, but it's worth starting out on the journey of trying to keep force and speed or velocity separate.
Force causes a change in motion, not a motion.
Separating ideas of gravity and atmosphere
Since the ideas of gravity, falling and down are inextricably linked and many people tend to associate the force of gravity only with actions on objects inside the atmosphere, this particular difficulty is probably worth tackling head-on. There are activities aimed at working with this difficulty.
Do heavier things fall faster?
There is an almost universal temptation to think that heavier things will fall faster. It's perhaps worth tackling this head-on somewhat later in the sequence.
These challenges and some suggestions for working with them are more fully explained in the Teaching and Learning Issues.
Representing and reasoning: doing physics
You have to bring together three separate chains of thought, all of which most naturally use arrows to represent what is happening.
- the gravity force acting on an object
- what counts as down for an object
- the speed of the object
Looking through forces spectacles
It requires an act of imagination to re-describe the world in terms of forces acting on objects. This is a process that you only begin here, but it's probably good idea to be aware of the endpoint. This particularly true since we make extensive use of representing gravity forces using arrows.
Arrows are also used to describe motion, and provide a very concise way of doing so, so it's a good idea to keep the arrows rather separate in style.
- speeds can change
- forces change speeds
- what we call
downdepends on gravity
- gravity is an omnipresent force, found everywhere in the universe
- choosing particular snapshots of a motion is a fruitful way to describe the motion
Find out more from the Physics Narrative.