Friction
Forces and Motion

A sequence to develop ideas about friction: grip, slip, drag

Classroom Activity for 5-11 Supporting Physics Teaching

Frictional forces are a group of forces that affect movement. They can set things moving (as when you expect your shoe to grip the floor: think about when it does not – icy patches wreak havoc with our expectations of getting going) and these are grip forces. Another set of frictional forces are slip forces: these slow things down as one surface moves past another surface (and both are the surfaces of solids, not fluids). This activity is an exploration of these two kinds of grip forces, to encourage children to think through the detailed mechanisms that underpin these forces and therefore to be able to reason about changes that well increase or decrease the grip or slip force. One reason for focusing on mechanism is that it provides a field guide to help children identify where to expect forces. Another is that this particular focus enables a physical richness to the explanations that afford exploration, rather than simply associating the label friction with the phenomenon that you're looking at.

Forces are best represented with labelled arrows, of different lengths to show different values, and we'd suggest that you make extensive use of these as physical labels on things, as this makes it clear that forces are acting on objects, with some effect on the motion of the object. That's where all this discussion about forces is heading in the the long term.

There is a third member of the frictional group of forces, that is not dealt with here: drag. Drag is the force exerted by a fluid (a liquid or a gas) as the object moves through the fluid, and you may well meet particular examples described as air resistance or water resistance. The mechanisms underpinning these interactions are complicated – even more complicated than grip and slip. And even more so if it's something like a boat that moves at the interface between two fluids (water and air). So here we'd suggest being very clear in your own mind that you're only dealing with frictional forces between solids.

Meeting reality: valuable experiences

  • Trying to lift slippery objects
  • Starting and stopping moving on more or less slippery surfaces

A sequence for developing the idea

The idea of friction is often rather vague, and used as a word to avoid explaining, rather than to thinking in some detail about the mechanisms at work. This sequence encourages you to be much more explicit, so allowing children access to a much richer understanding. Such detailed and careful thinking and representing provides them with a starting point for asking much more interesting and much more fruitful questions.

Teacher Tip: The ideas are developed in the Physics Narrative.

Practice labelling things isolated from their environment, with forces to represent interactions with the environment, using cardboard arrows.

Discussion about instances


Explore children's starting ideas about forces by discussing everyday situations.

Cardboard arrows


Helping children to identify forces and give them a language to describe forces. Exploring pupil's ideas – and making them explicit – is a central feature of this introductory work.

Looking through forces spectacles


Set an everyday task that depends on frictional forces – for example lifting a tin.

Lifting a tin


Introduce the grip and slip force arrows with a pair scrubbing brushes. Relate surface roughness to the ordering of the grip and slip arrows.

Visualising what happens at surfaces for grip and slip


Pull on the slippy and grippy puppets pressed on everyday surfaces.

Ordering slip or grip forces


Describe the experiences of the pull by labelling with slip or grip force arrows of different lengths.

Everyday grip and slip


Pulling with a standard force

Getting a feeling for a one newton force


Stop something moving over with grippy and slippy puppets.

Braking uses slip forces


Messages from research and practice: specific tripwires for this idea

Children have plenty of experience of pushing and pulling things to change their location. But they have rather fewer experiences of acting to change the motion of things, because frictional forces are so pervasive. Give something a shove, and it might move a bit, but it will then very likely come to a halt: the location has been changed, but the motion has not been (except in the short term). There are two significant challenges here. The first is that once you've analysed the world carefully enough, force changes motion, and not location. The second is that children need to generalise their idea of agency, so that they can imagine the world as a place where many different things in the environment can exert forces, and not just human pushes and pulls.

Prior ideas to re-activate and build on

  • Forces are represented by arrows
  • Forces are like children's pushes and pulls

A key element of making ideas about frictional forces worth teaching is to go well beyond labelling something as to do with friction: drilling down into the physical mechanisms, imagining the interactions between the thing and its environment that provide these frictional forces. It's a good plan to expect to work towards some detailed descriptions of how things are, through sketches, words and physical experiences, rather than stopping as soon as the word friction is reached.

Teacher: Well, here we have a stone sliding across ice. It stops after a while…

Abi: I know, it's friction, miss.

Teacher: Yes, but what's rubbing against what: where do we need to put the arrow?

Dave: The bottom of the stone's rubbing on the ice.

Teacher: And the arrow?

Alex: It's a slip arrow, the slip force.

Teacher: A long arrow, for a big force?

Abi: No, no, no. It's a short one.

Geoff: Because the ice is smooth.

Abi: So it's only a small slip force, and the stone takes a long time to stop.

In many cases the explicit link to the reason for expecting there to be a frictional force there at all, and the resultant effect on motion, are best drawn out, so that any difficulties can be made explicit.

The challenges and difficulties are explored in the Teaching and Learning Issues strand – here is an essential selection:

Forces are a part of the imagined world of the physicist and so you cannot see them, however powerful the telescope or microscope. You do have to re-describe the lived-in world in a new and rather special way.

I can't see a force – so it isn't there


The conflation of force and motion is well documented in the research. There are good reasons to believe children have experiences in the lived-in world that reinforce these kinds of confusions.

Force equals motion: no motion equals no force


Force equals motion: motion equals force


Air seems very thin, and so it seems more or less unbelievable that it could exert a significant force on moving objects.

Air resistance – how can it work?


Teacher Tip: Avoid arguing from the effect to the implied cause, so don't use: The stone takes a long time to slow down, so the slip force must have been small.

Teacher Tip: These challenges and some suggestions for working with them are more fully explained in the Teaching and Learning Issues.

Representing and reasoning: doing physics

Re-describing the world in terms of forces acting on objects needs imagination. Since forces cannot be seen, even with the most powerful microscope or telescope, and are not obviously just there, it is worth emphasising this imaginative aspect. Thoughtful and precise drawing of diagrams representing different forces using arrows is, at the same time, both fruitful and requiring significant work.

Using ideas about friction is all about seeing the world through forces spectacles.

Looking through forces spectacles


In general we're concerned with identifying and labelling a force.

Let's talk forces


In the case of frictional forces, a force in which is exerted by one surface on another.

Friction


Children have a number of different ideas about forces. Research shows that many are both persistent and incorrect, particularly with respect to the effect on motion. This connection is particularly important for friction, which sometimes reduces motion, and sometimes enables it. We suggest that it's important to distinguish between the two, with a label for each:

  • Grip for getting things going, where there is no movement between the surfaces, and nothing gets warmed.
  • Slip for slowing things down, where there is warming.

These labels link well with everyday ideas of surfaces being slippy (there is very likely to be movement between the surfaces) or grippy(it's much less likely that there is movement).

To identify the forces you need to pay attention to the surfaces between which there might be movements.

The focus here is on one surface sliding over another, and not on moving through fluids, which results in drag. Drag is a much more complicated member of the forces which are grouped together under the frictional label, and is best dealt with separately (perhaps when you're working on air resistance or water resistance).

Teacher Tip: Keep in your mind that drag is rather different from grip or slip, even though all three are grouped as frictional forces.

This activity provides some physical experiences, that provide a hands-on experience to get children's minds-on in exploring the act of seeing the world through forces spectacles. This re-imagining, seeing through (special) forces spectacles is really important, as forces are not the kind of things you can see with conventional sight, even with a magnifying glass.

Friction


Teacher Tip: Grip and Slip, or even friction are always forces, so it's a really good idea to have to hand a small collection of force arrows, of three different sizes, to label different situations in front of you, or perhaps invite children to do so.

The physics ideas are explored in more detail in the Physics Narrative – here is an essential selection:

You have to re-imagine the world to re-describe it with forces: the forces spectacles are one way to identify and label this process of modelling.

Looking through forces spectacles


It is possible to build rather complicated descriptions rather easily. The idea in physics is to keep things simple.

Keeping it simple: modelling


If you'd like a quick reminder, or prefer your arguments graphically, you might also want to check:

An interactive diagram showing the process of looking through forces spectacles.

Re-imagining the world


An interactive diagram showing the process of re-imagining a held bag of shopping as seen through forces spectacles.

Modelling a simple situation


An interactive diagram showing how you can re-describe pushing a box in terms of grip and slip forces.

Pushing, grip and slip


Teacher Tip: Remember the key long term learning outcome here is that the force acting RightArrow change in motion. Here you're making a start by identifying the forces acting on an object. Much later children will need to learn how to combine these forces into a single force.

The main ideas to have to hand at all times

  • Grip accounts both for getting going and for preventing movement
  • Slip accounts for slowing down, when surfaces move past each other
  • Drag accounts for slowing down, when moving through a fluid

Teacher Tip: Find out more from the Physics Narrative.

Building this into other activities

Thinking about the difficulties of moving one surface over another provides plenty of opportunity for connections with other areas. There are many practical implications.

  • Transportation over land was much easier once stopped just dragging things(and started using rollers, wheels, ball bearings, lubricants)
  • Woven cloth is held together by the grip force exerted by warp on weft and by the grip force exerted by weft on warp
  • Knots rely on grip forces
  • Grip forces are reduced by lubrication: cooking oil on the hands reduces the chance of lifting that tin

You could invite children to write an invented story about the differing adventures of the puppet people slippy and grippy in their own world. Varying the world or how slippy or grippy they are allows many different ideas to be explored. Such counter-factual explorations are particularly good for exploring children's ideas.

Friction
is a special case of Force
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