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

Thinking about the learning

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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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

Thinking about the learning

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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Thinking about the teaching

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.

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

Thinking about the learning

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.

Thinking about the teaching

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).

Magnets

Wrong Track: Magnets attract metals

Right Lines: Magnets can attract or repel other magnets. Some metals and some non-metals can become magnetised.

Explaining how magnets work

Thinking about the learning

Pupils' everyday experiences of magnets are likely to focus on situations where the force of attraction is important: For example, in magnetic catches on cupboard doors. However, not all objects and not all metals are attracted to magnets.

Thinking about the teaching

The process of attraction can be explained in three simple stages:

1. An object needs to be within the field of influence of the magnet for the effect to be noticed.
2. The magnetic field influences the internal structure of the object. Atoms realign themselves. The object becomes a weak magnet by a process called induction. We now have two magnets.
3. There is a force of attraction between the two magnets.

Electric and magnetic forces

Wrong Track: The positive pole of this magnet attracts the negative pole of the other one.

Right Lines: Both magnetic and electric forces can attract and repel, but the the mechanisms to account for these interactions are different; in one case involving magnetic poles and in the other case involving electrically charged objects.

Explaining the difference

Thinking about the learning

The problem here is that pupils confuse the charge story of electric forces with the pole story of magnetic forces. This is hardly surprising since opposites attract and likes repel is a common mantra of the science classroom.

The same pattern of attraction and repulsion is found both for magnets and for electrically charged objects. Two like poles (two norths or two souths) repel each other, whilst opposite poles, a north near a south, attract. However, you should take care not to mix up attraction and repulsion between poles with attraction and repulsion between opposite or similar electrically charged objects.

Here are some children working in this area:

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Thinking about the teaching

The field concept can be used to describe the action-at-a-distance effect in both electric and magnetic situations. Repulsive and attractive forces are also evident in both cases.

However, we interpret electric forces to be the result of the separation of positively and negatively charged particles. Magnets, on the other hand, are not charged. The force between magnets can be accounted for in terms of the effect of an alignment of the atoms within the magnet. Each atom might be considered to be a mini magnet. When these atoms line up together their combined effect is strong enough to reach out beyond their immediate location – the magnetic field, a force field, exists around the magnet.

Gravity and magnetism

Wrong Track: Gravity is a magnetic force that attracts things to the Earth.

Right Lines: The Earth's gravity and the Earth's magnetic field are independent of each other.

Distinguishing between the two forces

Thinking about the learning

Mixing up gravity and magnetism is a common confusion. Both magnetic and gravitational forces act at a distance and both are in evidence at the Earth's surface.

Thinking about the teaching

One simple distinction to make between gravity and magnetism is that gravitational forces always attract and never repel. The force due to gravity follows from the interaction between the Earth's mass and any other body that happens to be nearby (the effect extends far beyond the Earth to the Moon and beyond).

Unlike gravity, magnetism doesn't work for anthign with mass. Magnetic forces only occur between specific materials (mainly iron and certain iron alloys). This alone is a strong argument that magnetism is not responsible for gravitational forces.