Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• make explicit connection to the work on levers

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:

• be systematic in your use of terms
• adopt helpful conventions
• relate the action of the machines to fundamental physics
• restrict the term pressure to fluids

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.

Pressure vs. force

Wrong Track: The pressure of the water pushes directly on my ear drums.

Right Lines: The pressure of the water gives rise to a force which acts over the exposed area of my ear drums.

Pressure and forces are distinct

Thinking about the learning

Pressure and force are two distinct kinds of things. The words tend to be used interchangeably in everyday contexts but in science they have quite specific and different meanings.

Thinking about the teaching

Aim to develop the picture of pressure as existing throughout a fluid and giving rise to forces in all directions due to the bombardment of the fluid molecules. Forces act on objects in particular directions; a pressure simply exists throughout a fluid.

Talking about pressure and forces

When talking about pressure and forces be careful to distinguish between them. It is helpful to refer to:

• The pressure at a point in a liquid (pressure is a scalar – you cannot have a direction associated with it).
• Molecules bombarding a surface resulting in a force.
• The force acting on an object (forces must always act on something in a specific direction; they are vector quantities).
• The force exerted by one thing and acting on another.
• The force acting on a surface due to the pressure of the fluid.

Teacher Tip: … pressure in a fluid…

Teacher Tip: … force acting on…

Teacher Tip: … force exerted by…

Pistons and forces

Wrong Track: I push down on this piston and the force goes around the bend and pushes up on the other piston.

Right Lines: The hydraulic fluid is squeezed. So there are more collisions on the bottoms of both pistons. These extra collisions provide forces. The larger the piston the more collisions there are and the bigger the force. The pistons generate forces proportional to their area.

The force does not travel

Thinking about the learning

Some pupils develop a picture of forces being carried, by the fluid, from cylinder to cylinder in a hydraulic machine.

Thinking about the teaching

You need to be careful not to suggest that the force slides around the bend from one piston to another. Instead make the connection via the pressure in the fluid.

Pressure from particles

Wrong Track: How can there be pressure inside gases? They're too thin, they're not like water.

Right Lines: The air around us has a pressure just like water. This air pressure is due to the bombardment of air molecules. We don't notice air pressure until air is removed from one side of a surface (remove air from a Coke can and the air on the outside crushes the walls inwards).

Starting with liquids

Thinking about the learning

Pupils will readily accept the idea of pressure existing in liquids because it is a familiar phenomenon. However they may have problems in accepting that gases, such as the air around them, exert pressures.

Thinking about the teaching

Given that pupils are more familiar with pressure in liquids we recommend that you start with examples involving liquids and then move on to gases.

Pressure vs. suction

Wrong Track: If you pull the air out of the can with that pump, the pump sucks the walls of the can in. It's all to do with suction.

Right Lines: When the air is removed from inside the can the forces on the outside, from the air pressure, collapse the walls inwards. This air pressure is due to the bombardment of air molecules. We don't notice air pressure until air is removed from one side of a surface (remove air from a coke can and the air on the outside crushes the walls inwards).

What's doing the sucking?

Thinking about the learning

Even when you emphasise the air pressure explanation for familiar phenomena, problems can arise.

Thinking about the teaching

This kind of thinking can be a big problem. Pupils need to make the move from thinking about the can being sucked in from the inside to accepting that the can collapses because of the air pressure on the outside. A useful starting point in challenging suction ideas is to ask:

Teacher: What is doing the sucking inside the can? There's nothing there. All of the air had been taken out.

A common reply to this is that the vacuum does the sucking. In which case we return to the question, how can nothing do the sucking? At this point you need to re-emphasise the pressure explanation.

Snowshoes

Wrong Track: Wearing snowshoes decreases the pressure on the snow.

Right Lines: Pressure is a property of liquids and gases, but not solids. The snowshoes help you to walk on the snow because they reduce the stress applied to the snow.

Talking about stress

Thinking about the learning

Almost any example that has one solid resting on another will tend to lead to thinking leading along the Wrong Tracks.

Thinking about the teaching

Using pressure ideas to explain the behaviour of solids is so commonplace that you really need to decide as a science department on the approach that you will take (otherwise it will certainly be confusing for pupils). There is no doubt at all that the approach which we have set out here is the sensible and correct one from a physics point of view.

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

Try these

• relating pressure to the action of fluids
• using hydraulic machines as a context

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

• suggesting that pressure is a vector
• conflating stress and pressure
• using examples where solids are squeezed to introduce pressure

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.