Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• draw on learners experience of, and ideas about, their own actions and relate these to force
• explore and expose children's ideas about forces
• draw out children's everyday ideas about motion and the forces required
• introduce children to a new way of seeing—with forces

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• connect interactions between objects with the idea of a force
• represent objects simply, particularly extended objects
• adopt consistent conventions about where the arrows are drawn
• develop a consistent graphical language for your arrows, representing a force ontology

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

The main challenges

The main teaching and learning challenges in introducing the idea of force are:

The use of familiar terms, such as force, in unfamiliar, specialist contexts The use of common sense ideas which may be incorrect from the scientific point of view The fact that you can't see forces – they require an abstract construction

The key is that we're after a simple, tractable description – a usable model. Make it simple enough and no simpler. The tendency is to add the not-needed and not-useful: often impairing complexity persists.

The model consists of one thing, and cunningly selected arrows, representing forces acting on that isolated thing.

To think about – who do you agree with here?

InsertCMR{MfCarOnDriveDIPThreadCMRL}

InsertCMR{MfRocketInGroundDIPThreadCMRL}

Placing arrows

An OHP transparency or a well crafted computer-displayed slide can be used to show how the forces might be drawn on such a diagram. One technique is to cut out mini-force arrows made from card and add them onto an OHT, moving them about as directed by pupils.

We hope you'll use this to draw together pupils' work on placing arrows.

Tables pushing

Wrong Track: How can the table push up on the book? It's not alive is it?

Right Lines: A force can be exerted by any object whether living, dead or never lived.

Identifying forces

Thinking about the learning

Pupils are often unable to associate a force with an inanimate object. People can apply a force to objects – they make a decision to lift a bag of shopping or to push a motor car. However what about a book resting on a table? How can the table be forcing the book? The table is not alive. This is indeed a tricky situation. There is no easy practical activity which can immediately unravel this dilemma. However, a useful way of thinking about and explaining support forces is presented at the start of episode 02.

Here are some children trying to identify forces. These clips are worth watching several times. Particularly interesting, at this stage, is to think about how you can put them in a position where they can work out how to identify the forces, rather than just telling them the answers.

download clip to view

A force is what is known in science as a vector quantity. To communicate the idea of forces, learners are expected to annotate diagrams using arrows. To build confidence in doing this it is important to offer learners an experience with a number of force-related situations. Invite learners to explore the language of forces, and experiment using force arrows. There are teaching activities here designed to support such learning.

download clip to view

Thinking about the teaching

Being able to see that a force is acting and also where a force is acting is a skill to be developed. Being able to identify, describe and label forces is part of being able to model the real world as a scientist might. Learners need to be given the key to a language which will help them to describe forces. Here are three guiding principles:

• The first is that in every situation they meet, the vertical force due to gravity is present. Many start their description with this force.
• Secondly, look carefully at the selected object. It will be interacting with local things in its environment, and perhaps with other things not so close. Each of these kinds of interaction will be replaced with a force arrow. Use the kinds of interactions to identify the force arrows.
• Finally, the phrases acts on and exerted by are critical to talking about forces. For example: a force exerted by that thing acts on this object.

Keep it simple

When selecting things from the lived-in world to be about objects on which forces might act it's a good idea to select simple objects. In particular asking for

Teacher: … the forces on the human…

or even

Teacher: … the forces on the bicycle…

requires quite sophisticated (and perhaps even implicit) modelling of the object before you can answer the question.

We think it's best to avoid things with internal structure that's essential to the forces that they exert.

It should be obviously easy to reduce the selected things to a small blob (technically a point mass – but that can come later (more in the SPT: Force and motion topic) without affecting its motion in any way. For cars being propelled by the action of their wheels on the ground, bicycles propelled by human riders and running people this is not the case.

Forces on stationary objects

Wrong Track: It's just stood there. There are no forces acting.

Right Lines: Stationary objects that appear to be affected by no force might have several forces acting on them which all add to zero.

Forces in equilibrium

Thinking about the learning

Early introductions to forces in equilibrium will be via static situations. However, rather than identifying forces which sum to zero, pupils often describe such static situations as having no forces. This is an understandable conclusion and one that is almost correct. In equilibrium there is no resultant force.

Thinking about the teaching

You can subvert this natural tendency by identifying the forces acting from looking at the physical situation – an approach that is built into this SPT: Forces topic. We'd suggest not using equilibrium as a means of identifying forces. However it may be useful, in tricky situations, as a backup, and as a means of nudging individuals towards looking again at the situation because you believe that there is another force yet to be identified.

No resultant force

When talking about stationary objects it is a good idea to refer to there being

Teacher: … no resultant force…

or even

Teacher: … there are several forces acting on the object but they added up to nothing … we'll say that the resultant is zero.

We think it's best to avoid saying:

Teacher: The forces have cancelled out.

As forces are still actually there, present in the model. And they have to be, as their presence is as a result of the interactions of the object with its environment in the physical world.

Moving and forces

Wrong Track: The ball has a force which keeps it moving through the air. If it's moving it must carry a force.

Right Lines: No constant force is needed to keep an object moving at a steady speed.

Giving forces to things

Thinking about the learning

That moving things need a force to keep them going is a very common misconception. When questioned, a child might typically argue:

Sandra: The trolley is carrying the force you gave it when you pushed it to start it off.

Objects moving with a steady speed are often labelled by pupils with force arrows in the direction of motion. A child might say:

Naz: It will stop when its force is used up.

The idea that a moving object carries a force, usually traced back to the force that was originally applied, is common sense and it is almost correct. A moving object might indeed have been forced to start its motion. However this force was involved in the initial action, the starting push. Once the pushing agent, perhaps a hand or an elastic band, is removed the force is no longer there. The object continues to move but does not carry a force. But the energy in a kinetic store does increase (see the SPT: Energy topic) as a result of the action of the force.

Here are some children talking about moving objects:

download clip to view

Thinking about the teaching

The big challenge here is to try to eliminate the problems associated with friction. Everyday experience tells us that to keep something moving we need to keep pushing it. This continued application of a force is required simply to overcome the retarding effect of friction. In a world without friction there would be no need to keep pushing. Objects, once in motion, would carry on moving. Pupils don't live in a world without friction. Their experience tells them you need to push to keep things moving.

Friction-free motion

You can try to use imagination at this point, for example:

Teacher: Imagine we were on a perfectly smooth surface, like a skating rink. One push and the book will carry on moving, sliding but never slowing.

This will be effective if you are a good story teller and the pupils can relate to the experience of an ice skating rink.

However you might also try some experiments with friction-free skaters. Balloon (hover) buggies, blocks on ball-bearings, dry ice pucks and air-tracks were all introduced to schools in order to demonstrate friction-free motion.

A final task for the imagination is to think about space vehicles in deep space (think Star Trek: Enterprise or Red Dwarf). Such vehicles, once started, will keep moving with no booster rockets blasting away. They just keep on the move, silently and effortlessly. There is no friction force in space to act on them, retarding their motion.

Force and energy

Wrong Track: That lorry on the motorway had lots of force, it had lots of energy before it crashed.

Right Lines: A moving object has a kinetic store of energy.

Distinguishing between force and energy

Thinking about the learning

This challenge arises from an imprecise use of specialist terms. A moving object does not have force. However it does have some energy in the kinetic store. The motion will stop when this energy is shifted elsewhere, for example once brakes have been applied (a thermal store is filled as the brakes and surrounding air are warmed). A free-wheeling vehicle will eventually stop once the frictional forces have emptied the energy from the kinetic store. In describing such behaviour as running out of force we can see pupils with almost the right idea but mixing the terms force and energy.

Thinking about the teaching

In helping pupils to untangle this web of language we ought to offer them a clear term which they can use to describe what a moving object has. The default word for many pupils is force. We know that this is incorrect. energy in the kinetic store is totally correct (although momentum might be an acceptable alternative term to use).

At high school an appropriate statement is therefore:

Teacher: A moving trolley has energy in the kinetic store. It will stop when this energy has been shifted from this kinetic store.

The energy shifted to the store as the motion started was through the action of a force (a push).

In their explanations, learners often argue that the force carried by a moving object is the very force which started it in motion. For example:

Abigail: It has all the force which the push gave it.

It is our job to separate the forcing action (the initial push to start the motion) from the energy shifted during the forcing process. Once the force stops acting, the object will be in a new state of motion, having been either slowed down or speeded up by the action of the force.

Resultant forces of zero—and motion

Wrong Track: If the forces all balance each other it can't be moving.

Right Lines: An object moving at a steady speed is one example of forces in equilibrium.

Explaining motion in equilibrium

Thinking about the learning

Objects that move at a steady speed in a straight line are often seen as having a force pushing them. The idea that there might be several forces, the sum of which is zero, is not at all obvious. It is certainly possible to have an equilibrium state which involves motion. A child sliding down a playground slide at a steady speed is being pulled by gravity and also being opposed by friction. These two forces balance and the child continues to move at a steady speed. With forces balanced, the child is in equilibrium but is still moving.

Children certainly have some interesting thoughts about equilibrium, and listening to what they have to say, and seeing what they do as they seek to explain situations involving equilibrium can be very instructive.

download clip to view

Thinking about the teaching

Here are two examples of moving objects in equilibrium. For each one, identify the most significant forces that are acting. A carefully drawn diagram will help you to show that these forces might add to zero.

A car moving at 70 mph along a straight motorway.

Emergency food supplies falling with a parachute.

Try deliberately building these challenging situations into your lessons. By creating a challenging situation you will offer learners an opportunity to talk to you and to each other about what they think is happening. It is through this discussion that you will be able to listen, explore and direct thinking about dynamic equilibrium.

There's a good chance you could improve your teaching if you were to:

Try these

• Building explicit connections between the actions of animate and inanimate environments on an object
• Being consistent in the drawing of force arrows
• Giving children a variety of representations to hand when asking for descriptions
• Arranging it so that the placing of arrows is provisional and open to debate
• Focusing on the physical reasons for expecting a force to be acting – e.g. the bombardment by particles
• Acting as if Newton's first law really is hard to believe
• Speaking, acting and drawing with exemplary precision, so children can apprentice their practice on yours
• Using a rich variety of examples

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• Relying too much on precise words by themselves
• Acting as if the placing of arrows is obvious and open to a simple inspection
• Referring to forces cancelling out
• Using complex objects on which forces might be acting (with internally moving parts – bicycles, cars, people.)

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.