Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• subvert the notion that you get something done for nothing
• draw on the work done in identifying forces acting on objects
• make the compensating quantities, and the pattern of compensation, explicit
• use a wide range of everyday examples

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

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.

Describing the forces

Wrong Track: I push down this end and the lever pushes up at the other end.

Right Lines: At this end I exert a force downward on the lever, at the other end the rock exerts a force downwards on the lever. Here the pivot exerts a force upwards on the lever. The resultant force is zero.

Identifying the forces

Thinking about the learning

A central part of coming to understand levers is to correctly identify the forces; what they act on and where; what exerts them and in which direction. That is quite a lot of things to get right, all represented by drawing a few arrows on a diagram.

Thinking about the teaching

When you first introduce pupils to the forces acting on and around a simple lever:

Choose a simple example to look at, maybe a simple see-saw arrangement where the lever is initially horizontal and the forces acting are vertical. Pupils usually find the effort force straightforward to understand:

• The downward push of your hand on the lever.

However, there is often some confusion between the pair of forces acting at the other end of the lever:

• The downward force of the load on the lever
• The upward force of the lever on the load

These forces provide a good example of a pair of forces, replacing an interaction, each one acting on a different object (a Newton's third law pair of forces): the force of the load on the lever is equal the force of the lever on the load.

As you are interested in the forces acting on the lever, you need to look at:

• The effort force, acting on the lever
• The downward force exerted by the load, acting on the lever

It's the same old story, emphasised in the SPT: Forces topic. To avoid errors and keep things simple, focus on one object at a time, isolating it from the environment and drawing only the forces acting on that object.

A show of strength

A nice starter activity is to boast to the class that you have the strength to lift one of the pupils with one hand! No trouble! Ask them to think about how this might be possible, then demonstrate with a suitable lever.

A long wooden plank over a round wood log (or something similar) works very well. The lever needs to be arranged with the load close to the pivot point. Here the lever is acting as a force multiplier: You push down with a small effort through a large distance to lift the relatively large load a small distance.

Drawing forces

In drawing in the forces around a lever, encourage the pupils to think through a checklist (episode 01 of SPT: Forces topic) before placing each arrow:

Teacher: What provides the force?

Lois: My hand (the effort).

Teacher: What is the force acting on?

Sarah: The lever.

Teacher: Where does the force act?

Eloise: At one end of the lever.

Teacher: What is the strength of the force?

Zac: XX newtons.

Teacher: How long shall we make the arrow to represent this force?

Ibrahim: Shorter than the force on the load.

Teacher: In which direction does the force act?

Penel: Downwards.

Length and distance

Thinking about the teaching

If I apply a force to the end of a lever there are two measurements, both in metres, which I might pay attention to in describing the event:

• The length, L₁, from the pivot to the line of action of the force
• The distance, D₁, that the force moves as the lever pivots

Teacher Tip: Keep the lengths (L₁, L₂) from the pivot to the line of action of the forces separate from the distances moved by those forces as the lever spins (D₁, D₂).

This will help you think about either moments or energy.

To analyse the action of the lever in terms of moments:

• You'll need to consider the size of the force acting and the length from the line of action of the force to the pivot (L₁).

To analyse the action of the lever in terms of energy:

• You'll need to consider the size of the force acting and the distance moved by that force (D₁).

Not just bigger forces

Wrong Track: The force on this side is bigger so it must go down.

Right Lines: To predict what will happen with a lever you need to consider both the force and the distance on either side of the pivot.

Strength of force and length from the pivot

Thinking about the learning

With levers you must pay attention to both the size of the forces acting and the perpendicular lengths from the pivot (the length from the pivot to the line of action of the force).

Thinking about the teaching

So that pupils take length and force equally seriously make sure that both are emphasised (marked and labelled clearly) in any diagrams that you draw of levers.

Balanced moments

Wrong Track: The force times distance on this side equals the force times distance on that, so I'll just need to push a bit harder on this side to raise the load.

Right Lines: Once the lever is turning at a steady rate, the clockwise moment (force times length) on the lever is exactly equal to the anti-clockwise moment.

Steady motion from balanced moments

Thinking about the learning

A subtle point here takes us back to the earlier work on balanced forces (see episode 05 of the SPT: Forces topic). In other words, if the forces acting on an object are balanced, then it keeps moving with steady motion. The same rule applies to the lever: When the clockwise turning effect (or moment) balances the anti-clockwise turning effect, the lever continues to move around at a steady rate.

Thinking about the teaching

By far the best way of dealing with this learning challenge is to simply assume from the start that the levers you deal with move at a steady rate and to focus on the balancing of moments.

Note (for your own interest but not the pupils) that we're ignoring the part of the action of the lever where it is accelerated into motion (for that fraction of a second) and focusing on the part where it is then turning at a steady rate.

The wrong lever

Wrong Track: They must make those big, long screwdrivers like that so that they can turn screws more easily.

Right Lines: The length of the screwdriver can't help you with the turning effect. The only way of increasing the turning effect is to have a thicker handle (thereby increasing the length from your hand to the pivot point or axis).

Different uses for the same tool

Thinking about the learning

Household tools, such as screwdrivers, are often used in ways which were not intended in their design. Think about the line of action of the force both to be clear and to avoid nasty mistakes.

Thinking about the teaching

The screwdriver provides a good example to really challenge the pupils' understandings of levers. The key point is that a longer screwdriver does not provide a bigger turning effect. Screwdrivers are only made longer to provide easier access to difficult screws.

In describing the action of a screwdriver you need really clear diagrams to hand – or better still a large version of the tool itself as well as the diagram.

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

Try these

• relating the action of the lever to conservation of energy
• using separate terms, and separate shorthand, for distances moved and lengths to the pivot
• drawing forces consistently, and as you did during the forces topic

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 a single example, which is liable to misinterpretation
• reducing the topic to a series of algorithmic calculations
• moving too quickly from the physical to the abstract and mathematical

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.