# 02Contact forces

Fo02TL of the Forces topic
• ## 01 Things you'll need to decide on as you planFo02TLnugget01 Decisions

### Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• convince children that inanimate things can push, just like they can
• develop the idea that a mechanism underpins the interaction that is replaced by force
• convince children that air can exert 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:

• relate floating and sinking to forces, not to rules about displaced fluids
• relate friction to the mechanisms of friction
• focus on the physical reasons for placing arrows
• justify omitting arrows from some diagrams

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.

• ## 02 How can the floor make a force?Fo02TLnugget02 Challenge

### How can a floor push?

Wrong Track: How can the floor push up on Papa Tombola? It's just there for him to stand on.

Right Lines: Any body can provide a force that supports.

### A floor in terms of atoms

A floor seems to just be. How such a passive object can provide an upward force is indeed a puzzle.

A helpful way to think about this force is to imagine the atoms in the body. At a microscopic level the body is being distorted. The upward force is provided by the spring-like bonds between atoms.

Here are some children's ideas on contact forces:

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• ## 03 Why do some things float and others sink?Fo02TLnugget03 Challenge

### Heavy or dense?

Wrong Track: Well that's easy, heavy things sink and light things float.

Right Lines: Floating and sinking depend on the density of the material.

### Explaining density

For equivalent volumes of substances the statement heavy things sink and light things float is close to being true. However, pupils faced with a heavy block of wood, perhaps too heavy for them to lift, and a lightweight paper clip, will make the wrong predictions. They will predict that floating and sinking depends on the gravity force and not on an intrinsic property of the material, which is the density.

Children struggle to make sense of the ideas of floating and sinking:

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Tangled up in this misunderstanding is the issue of density and also the language we use to compare things. What do larger or bigger mean? To say that a block of metal sinks in water because the mass is greater is only part of the explanation.

Its mass is greater than the mass of the same volume of water. Density is the key concept behind these ideas and it is not an easy concept. It involves the relationship between two quantities, mass and volume. Each of these is hard enough to understand on its own. Here we are not only expecting such an understanding but also an appreciation of how the two ideas combine.

The critical observation is to see what happens to two blocks. One should be a low density material such as wood, but with a large mass (i.e. a large lump of wood.) The other should be a metallic object with a smaller mass than the wood. A coin would do. Holding the two objects will give pupils a clear and immediate sense of a difference in mass. Following a prediction about which will float, the two objects can be lowered into a sink or bowl full of water.

For materials, it is some feature of the material, as opposed to the object itself, that determines if it will float or sink. Mass is a characteristic of a particular object. Volume is also a characteristic of a particular object. Density is an intrinsic property of a material.

For deciding about objects, again the target concept is the compound property of density, not the contributing concepts of mass or volume alone.

• ## 04 How can you tell if there is friction?Fo02TLnugget04 Challenge

### Friction and movement

Wrong Track: You only get friction when things move.

Right Lines: Friction exists between all surfaces, moving and tending to move.

### Friction with and without movement

This misconception appears in two forms. The first holds that friction happens only when things move (slipping). This idea is based on the conception that friction is about two surfaces rubbing together, and rubbing together is an action requiring motion (slipping). The second is that friction happens only when things don't move (gripping). This is based on the notion that, once moving, an object has overcome friction. Either way the ideas show a limited view of friction.

Thinking about how children go about explaining this difficult idea can help to suggest some approaches.

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Once again this conceptual challenge can't be resolved by showing friction as a concrete entity to pupils. You can't see something called friction. Through forces spectacles we can draw arrows to show where the friction forces are acting and perhaps this is a good starting place. To show that frictional forces act on stationary objects that are trying to move we can set up many simple situations where a force is acting on a heavy object with the result that it remains in equilibrium. Why isn't it moving even though a force is acting on it? The force which maintains the equilibrium is a grip force, a frictional force.

Some teachers find it helpful to distinguish between slip and grip to make both facets of frictional forces exerted by solids explicit.

Try the same demonstration on an object on wheels or a slippery surface and the result will show that the same external force will indeed have an effect. As with most of these force situations, drawing out pupils' own ideas, often through discussion, is a critical part of any teaching and learning activity.

• ## 05 Air resistance – how can it work?Fo02TLnugget05 Challenge

### Air getting in the way

Wrong Track: Air is so light, how can it be strong enough to exert a force?

Right Lines: Air is not nothing. It has a mass and it does get in the way of moving objects.

### Particles colliding

For many learners air is seen as nothing. Comparing the mass of air in a small domestic room to the mass of a child might help to undermine this distinction (both can be 40 kg).

It is invisible and so doesn't really amount to much, if anything. It certainly is not the same sort of thing as a car engine, which we can see is strong enough to exert a force. The fact that air pressure at the surface of the Earth is about 100,000 newton / metre 2 of surface is also not at all obvious.

There are many cases where we need air to get in the way. Parachutes rely on air colliding with the large area of the canopy. Wind generators move because air exerts a force on the blades. Sail boats depend on a force from moving air. Although air does have a low density, if we can bump into enough of it (via a large area like a sail) and if the air is moving fast enough it will exert a considerable force. The moving air is not in itself a force. The air doesn't carry a force simply by moving. However, when the air collides with a surface, both the air and the surface experience a force. A force from the surface acts on the air to slow it down. There is a force in the opposite direction acting on the surface. It is this force on the surface which drives the sail boat through the water.

Motion is relative. Air colliding with a surface happens when either the moving air hits the surface (e.g. a sail boat) or the moving surface hits the air (e.g. a parachute or a sprint cyclist).

• ## 06 Thinking about actions to takeFo02TLnugget06 Suggestions

### 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
• Building in explicitly the steps from physical situation to the forces acting on the object
• Modelling how you come to expect a warp force to be acting
• Discussing the placement of arrows
• Focusing on the physical reasons for expecting a force to be acting – e.g. the bombardment by particles
• Sharing tangible examples of frictional forces
• Arranging for children to experience drag forces in air

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

• Using friction as a blanket term, without reference to its physical origins
• Treating contact forces exerted by inanimate objects as obvious
• Stating, without sharing the appropriate experiences that give the statements meaning
• Being drawn into discussing the details of the drag forces

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.

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