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Exploring motion - Physics narrative
Physics Narrative for 5-11
A Physics Narrative presents a storyline, showing a coherent path through a topic. The storyline developed here provides a series of coherent and rigorous explanations, while also providing insights into the teaching and learning challenges. It is aimed at teachers but at a level that could be used with students.
It is constructed from various kinds of nuggets: an introduction to the topic; sequenced expositions (comprehensive descriptions and explanations of an idea within this topic); and, sometimes optional extensions (those providing more information, and those taking you more deeply into the subject).
Natural and unnatural descriptions
Ideas about movement and speed are part of the language and experience of most children. But, without thinking, they tend to assume that they're at rest and everything moves around them. This is
common sense, but can be deceptive. That's particularly true when thinking about astronomical movements.
It's easy enough to imagine that the Moon orbits around the Earth, although somewhat harder to explain why the appearance of the moon changes.
It's altogether harder to imagine that the Earth is orbiting around the Sun and that the Earth is spinning on its axis. Yet you'll need these two to make sense of the motion of the astronomical objects (stars and planets) and to account for day and night.
So its worth taking care to explicitly select a point of view, so laying firm foundations for clear thinking in the future.
Movement and speed
Changing distances, speed and relative movement
When asked about the speed limit on the motorway children will readily use the language of
70 miles per hour. Speeding motorists are recorded on police cameras and reported in the news. Speed records for the fastest bird or animal, car or plane hold children's attention.
But there are subtleties. What I (Charlie) record as passing at 13 metre / second, you (Alice) may record as stationary (so a speed of 0). The reason may be simple: Alice is driving along in her car, and Charlie is standing on an overhead footbridge. The recording is a record of the speed of Alice's handbag, on the front seat of the car.
Alice notices no movement from her point of view: the handbag is at all times the same number of metres away from her. That's why she records the speed as 0. If Alice says
that's at rest, she means nothing more or less than it's just moving with her.
Charlie notices the handbag getting 13 metre closer to the shadow of the bridge on the road every couple of seconds. That's why he records the speed as 13 metre / second.
Now consider Bob, cycling along at half of the car's speed, and overtaken by Alice. What will he record for the speed of that handbag?
Bob won't record what Alice does, nor what Charlie does. Each individual with a unique point of view will record a different speed. These will only agree with each other if they are moving together. Duncan, cycling along with Bob, agrees with Bob, for example. Elizabeth, sitting in the rear seat of the car, agrees with Alice. Fayed, standing with Charlie, agrees with him.
Choose a point of view to describe moving things.
So far as we can tell, there is no absolute point of view. No absolute
At rest means staying the same distance from me.
Talking about speed
What exactly do we mean by speed?
Speed is about movement, about travelling. But speed is not just about a distance travelled or a time taken. It is a way of reporting a rate of progress. It specifies how much distance is covered during a particular time interval. That is the increasing or decreasing separation between Alice and the object that Alice is reporting on.
Perhaps the simplest definition is:
Alice: Speed is the distance moved per second.
From this definition we can see that metres per second (metre / second) is a useful unit for speed. However, Alice might use any unit of distance and any unit of time. Here are a few examples:
What is common to these examples is the idea of a rate of progress – two points move so that their separation changes with time. One or more points is re-positioned. The distance between them alters as time ticks by.
You will find in physics textbooks formal definitions such as:
- A caterpillar might have a speed of 4 millimetre / second.
- Your fingernails might grow at a speed of 2 millimetre each week.
- A jet fighter might travel at a speed of 1000 kilometre in each hour.
Speed is the rate of change of distance with time.
We'd suggest that you stick with standard units of of metres and seconds in the early stages, avoiding quirky favourites such as
furlongs per fortnight.
Converting from one set of units to another often involves arithmetic that may serve to obscure rather than to reveal.
Let's talk forces
Pushing and pulling
I was sitting in a train by the seaside quite recently. We had come to a halt and a steam engine went by pushing a line of four or so carriages. A small child sitting nearby called out excitedly:
Engine push wagons, push, push wagons!
The child's mother responded:
Yes, that's right, the big engine is pushing the wagons.
Ideas of pushing and pulling are common in everyday talk and are used from an early age as this example demonstrates. The word
force is used in a whole range of different ways: people refer to
force of habit or
forcing things open or
armed forces. In a tight corner you might argue:
you can't force me to do that!
In the sciences the concept of force is used in a more limited way and the good news is that the scientific way of thinking about forces is pretty close to everyday understanding. For example, when young children say:
I am lifting the bag or
we are pushing the trolley, they are starting to use the language of forces in ways which any scientist would recognise.
From such starting points, pupils need help in developing and applying a scientific description. In one sense this is not difficult: we are not expecting essays. A good starting point is to recognise that in describing situations where forces are acting, it is helpful to focus on three facets:
- What is the force exerted by?
- What kind of force is acting?
- What is the force acting on?
Let's apply these three questions to the example: "The engine pushes the wagons."
- The force is exerted by the engine.
- The force is a push.
- The force acts on the wagons.
This simple statement might be expressed slightly differently:
The push exerted by the engine acts on the wagons.
acts on is a good way of linking the force to whatever it is that feels the action of that force. Alternatively you might choose to link to what is providing the force. We suggest the consistent use of the word
exerts, in which case:
The engine exerts a push on the wagons.
Let's look at a second example:
The woman pulls on the rope.
- What exerts the force? The woman.
- What kind of force is acting? A pull.
- What is the force acting on? The rope.
You can develop your understanding of the kinds of forces through these next three resources.
But for now expect to find a force wherever something affects an object and you might have the same effect by pushing or pulling. Your push or pull can be identical to the action of the inanimate surroundings (the environment of the object) exerting a force on an object.
Looking through forces spectacles
Onions don't come from the market with arrows attached labelled:
I'm being pulled down by gravity with a force of about 3 newton
I weigh 3 newton.
The fact that forces aren't visible, labelled and ready to be described offers a challenge to the imagination. You must learn to recognise where forces are acting. So change your perspective: moving from the physical (green panes) to a theoretical (blue panes) description.
If you look at things you'd find in the kitchen, you can spot lots of different objects. Each of these has many different facets that you can describe: the materials from which objects are made; the ways in which they move around; the forces acting on these objects; the colours of the objects. In this topic, for one object at a time, we'll be describing the forces – and only the forces.
An everyday scene
So let's take an everyday scene.
This might look like a person carrying some shopping and indeed that is just what it is.
But view the scene through
forces spectacles, shifting from the physical to the theoretical, to find out how a physicist might
see the situation when looking at this scene through forces spectacles.
The left hand re-description is too complex – there are still several objects: the shopper's head: the bag; the rest of the shopper. Simpler is better – so always focus on just one object. And even then we've only shown a few of the forces acting on the objects. The idea is to avoid hedgehog-like diagrams, with pointy bits heading off in all directions. Like rolled up hedgehogs, they're hard to handle.
The second is better for just this reason, as we have reduced the shopper and his bag to a single object. Why do we say
better? Simply because the model (and it is a model) is something that will allow us to make predictions.
An alternative way to make it simpler is by concentrating on some parts of the situation. The rule is always, always to focus on a single object at a time. Just one.
Get those spectacles on
- Look carefully at one object.
- Put your forces spectacles on to see it in a new way
- Draw in the forces.
Keeping it simple: modelling
The world seen through forces spectacles can be very complex. There are many more forces acting in the situation than those shown. (Think of the forces exerted by the muscles in the strained forearm and in the aching fingers.) To help you make sense of the complex world of forces, there is a simple strategy: focus on an object and its interactions with its environment – both local and remote. So first isolate one object from its environment. Here, let's choose the hand. Then identify the forces acting on it by considering the interactions of that object with the environment.
Here's what we do:
- Identify the object.
- Isolate it from the environment.
- Identify the forces acting by thinking about interactions with the environment.
In moving between steps 2 and 3 we're building a model of the situation. How do we know what to include in the model? By paying attention to the situation.
Here two elements of the local environment are both stretched – the forearm and the bag. They're warped, and we'll see that these kinds of distortion lead to a tension or compression force.
Making a model
Now consider the forces acting on the bag.
The process of simplifying a complex situation by concentrating on one part is an example of scientific modelling. An even simpler sketch of the situation might reduce the bag and its contents to a
point, as shown in the additional step here.
Let's summarise the three stages in this modelling process:
- Focus on one object of interest.
- Isolate this object from its environment, drawing it as simply as possible.
- Look at the world through
forces spectacles, enabling the identification of forces by considering interactions between object and environment.
The key idea is that we're dealing with the world one object at a time – no more.
Using arrows to represent forces
By now you'll have noticed that we use arrows to represent forces. These force arrows (or force vectors as they are referred to in physics books) are very helpful because they can be used to represent the two essential features of any force:
- The direction in which the force acts is shown by the direction of the arrow.
- The size of the force is shown by the length of the arrow (the longer the arrow the bigger the force).
A third convention to be consistent about in drawing force diagrams determines the positioning of force arrows:
- Each arrow is drawn so that it starts from the point where the force acts.
It'll also be helpful to always draw the forces in a particular style (there get to be a lot of arrows on diagrams in physics, not all of them forces). You might also choose colour to show the kind of interaction that the force replaces.
An essential skill is to be able to add the arrows as needed – and not any more than are needed. There are clues in the interactions between the object and its environment that'll help you identify the forces. Later we'll see what those clues are – more on that in episodes 2 and 3.
Adding arrows to a simple situation
Think about the forces acting on a book that is sitting on a table.
The upward push of the table on the book acts on the lower surface of the book and the force arrow is drawn from that lower surface.
The downward pull of the Earth (the gravitational pull of the Earth) on the book is taken as acting through the centre of the book, and the force arrow is drawn from that central point.
Being able to draw arrows to describe forces is an important skill. The activities in the Teaching Approaches section are designed to help pupils practise identifying forces though the use of cut-out card arrows, using these arrows to show forces in a force diagram.
Equilibrium - a question of balance
You saw a situation where the hand was pulled down by a single force, the tension in the shopping bag. It is quite possible for an object to be acted upon by more than one force. A team of nine husky dogs pulling an Arctic sledge or a commuter being squeezed into a busy rush-hour train by a bunch of people pushing on all sides are examples where many forces act on one object. If these forces all push in the same direction the result will be a rapid change – perhaps the Arctic sledge will race away or the commuter will be shoved along the carriage.
However, it could be that all of the forces acting on the object balance each other out. The object is then said to be in equilibrium. All of the forces acting on an object which is in equilibrium add to zero. In other words, one force acting upwards is balanced by another force acting downwards; one force acting to the left is balanced by a similar force acting to the right, and so on. All the forces that are present add up to produce no overall force at all. Here's another way of saying this.
Teacher: Objects in equilibrium have no unbalanced forces acting on them. The resultant force is zero.
Most objects are in equilibrium
Look around you and notice how most things are doing nothing. The coffee cup just sits on the table, the picture hangs from the wall, the shirt lies on the chair. The cup, picture and shirt all have forces acting on them but the result for each one is a balanced or equilibrium state. There are no unbalanced forces acting on each one of these objects. The resultant force is zero.
Lessons from this first look at forces
Helpful approaches: a summary
When describing a situation using forces, aim to answer these questions:
- What exerts the force?
- What kind of force is acting?
- What is the force acting on?
When drawing force diagrams, a helpful process is to:
- Imagine you are looking through
- Focus on one object of interest.
- Reduce this object of interest to its simplest representation. This can often be just a point.
- Only then add the force arrows.
In many cases all the forces acting on an object will balance. The net effect is than nothing changes. We describe such objects as
being in equilibrium.