# 03Finding forces

Mf03TL of the Motion and force topic
• ## 01 How can the floor make a force?Mf03TLnugget01 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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• ## 02 How can you tell if there is friction?Mf03TLnugget02 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 children. 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 pupil's own ideas, often through discussion, is a critical part of any teaching and learning activity.

• ## 03 Air resistance – how can it work?Mf03TLnugget03 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: this again suggests that it doesn't really amount to much, if anything. It certainly is not the same sort of thing as a car engine, which we can rather easily imagine exerting a force.

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).

• ## 04 What can magnets do?Mf03TLnugget04 Challenge

### 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

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.

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.
• ## 05 Differentiating between electric and magnetic forcesMf03TLnugget05 Challenge

### 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 electric charges.

### Explaining the difference

The basic problem here is that children confuse the charges story of electric forces with the poles 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 electric charges. 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 electric charges.

Here are some children working in this area:

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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.

• ## 06 Differentiating between gravity and magnetismMf03TLnugget06 Challenge

### 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.