01Levers

Ma01TA of the Machines topic
• 01 Lifting Jim (or Jenny)Ma01TAnugget01 Activity

Using levers for lifting

What the activity is for

This is a quick demonstration to show that levers can be used to generate large forces from small forces.

What to prepare

• a long wooden plank and a suitable pivot to go under the plank (maybe a small round log of wood or a strong wooden box, not too wide)

Safety note: Always ensure that if the pupil falls off the plank the distance is as small as possible and that there is no risk of the plank breaking.

What happens during this activity

The class arrives and is met by a seemingly impossible claim from the teacher:

Teacher: I've been working out in the gym! I'm so strong I bet that I can lift Jim (or Jenny) with one hand!

After the class has stopped laughing get them to consider how you might do it (keep the long wooden plank hidden for the moment).

Then… try it out! Volunteers please!

This starter activity always grabs the attention of pupils and opens up the way to thinking about how levers actually work. In addition, pupils often have stories to tell about how members of their family have used levers to move giant rocks in the garden and so on.

• 02 Close the door!Ma01TAnugget02 Activity

Closing the door

What the activity is for

This is a short demonstration to show the relationship between force, length from pivot to the line of action of the force, and turning effect in the context of closing a door.

What to prepare

• gather the class around the lab door

What happens during this activity

With suitable gravitas the teacher announces that today's first activity will involve the famous physics experiment of closing the lab door!

Invite one pupil to see if they can achieve the impossible and close the door. They do so! Can they do it with one finger? They can! Now ask them to push half way across the door. What about the size of the push needed? It's bigger. With one finger? Yes!

Now ask them to push on the door at a point very close to the hinges. Can they still close it with just one finger? Maybe just, but a much bigger push is needed.

It seems that to close the door you need to take account of both the size of the push and where you push:

• If you push a long way from the hinges, only a small force is needed.
• If you push close to the hinges, a much bigger force is needed.

So the scene is set for thinking about levers in general.

• 03 Model doorsMa01TAnugget03 Activity

Building model doors

What the activity is for

A quick experiment to get a feel for the differing effects of the action of a force applied at different lengths from the pivot.

To get some experience of making and manipulating a physical model.

To appreciate the care that must be taken in order to use the results from the model to make predictions about real behaviour, and to understand the system.

What to prepare

• small rectangles of aluminium sheet, approximately 10 centimetre by 4 centimetre (other rigid rectangles could be substituted – but do ensure that the edges are smooth)

What happens during this activity

Work in pairs. One pupil acts as the door hinge, gripping one end of the rectangle between finger and thumb. The other tries to open the door by pushing with a single finger.

You might ask the pair to model the following situations, recording their actions for a discussion by a simple plan view sketch.

• Opening a door by pushing near the hinge.
• Opening a door by pushing in the middle.
• Opening a door by pushing as far from the hinge as possible.

From these three, they should be able to say something about the relative difficulty of opening the door, provided they have designed their experiments with care (kept the same hinges). Asking what they have done to make their findings reliable should allow them to identify this as the significant factor.

We suggest you concentrate on the balance between the force their finger exerts on the door and lengths to the pivot as the two key factors to draw out.

Then you might ask them to model hinges that make the door harder to open. (Careful experimenters will find two ways to do this: By gripping harder, and by sliding the finger and thumb farther up the door. You might manage to make something of this, drawing out the effects of force and length for the hinge as well as for the finger doing the opening).

Teacher Tip: A digital camera and an interactive white-board could provide a large image for a chosen pair to explain their experiment to the class.

• 04 Keep it on the level!Ma01TAnugget04 Activity

Experiencing turning effects

What the activity is for

Here the pupils can get a direct feel for the turning effect of a force acting on a lever because they have to oppose that turning effect.

What to prepare

• a retort stand rod or length of stiff strong dowel, approximately 60 centimetre long
• an object that can be suspended from the rod (about 500 gram is suitable)

What happens during this activity

Work in pairs. Pupils hold the rod horizontal (or try to) as the object is gradually slid out towards the end of the rod. Pupils should be encouraged to describe what they feel, firstly in terms of what they had to do to keep the rod horizontal, and then in terms of the effect of the force provided by the mass.

Pat: The force stays the same, but I have to try much harder to keep the rod horizontal. This is because the same force has a larger turning effect. The force has a larger turning effect because it is farther from the pivot.

• 05 Labelling leversMa01TAnugget05 Activity

Spotting the lever

What the activity is for

Pupils are invited to label photographs of levers so that they can get better at finding the lines of action of the forces, the forces acting and the pivots. This analysis is essential if they are to be able to model things as levers.

What to prepare

• a collection of images of levers – some are provided for you: A door pushed, a spanner in use, a crow bar in use, a screwdriver, a helping hand
• this interactive object, running on a large display, or running on a number of computers so that two or three pupils can work around the computer at once

What happens during this activity

Work through a single, simple example with the pupils. Ask them what could be taken to be the lever and add it to the diagram. Ask them where to place the forces and how large these should be.

Ask them where the pivot is, and place this as well. Mark in the length from the pivot to the forces. Then invite them to work through some more on their own, if working with small groups around computers. Once agreed, they could print the results as a record of their thinking, or you could have a few groups come out to the front and run through their analysis on a large display.

You might extend the activity by asking pupils to bring in images or artefacts of their own to be scanned or captured by digital camera. Going on a lever hunt around the home shows just how many objects are designed using this simple principle and provides a nice way of linking the outside world with their learning in physics.

• 06 A collection of leversMa01TAnugget06 Activity

Experimenting with levers

What the activity is for

Here pupils get their hands on devices based on levers, and get to describe and analyse them as such. You might choose to do this as a circus, where each group tries everything, or as a series of presentations, where each group of pupils prepares one example then reports back, or as a series of demonstrations (perhaps less fruitful here).

What to prepare

A collection of levers, for example:

• a wheel brace with a mounted nut
• a large pair of kitchen scissors
• a helping hand for opening jam jars (with a jam jar)
• a hammer lifting a nail
• a bike brake lever, a spanner and mounted nut
• a screwdriver and mounted screw, a screwdriver and tin lid to be forced off
• an Allen key and mounted bolt, a mounted tap
• a mounted rotating door handle
(It depends what you can find easily! But do beware of compound levers, such as nail clippersâ€¦).

Per group:

• each group will also need a coloured perspex rod or piece of dowel, about 15 centimetre long

What happens during this activity

For each device we suggest the following series of actions:

• Find out what the device is for and model the action you'd expect to carry out for it (for some classes you might need to have some prepared cards for some devices).
• Use the coloured rod to show the axis around which everything spins. This will help you to find the pivot. You might find it helpful to keep this in place while you do the rest of the analysis.
• Identify the input and output forces.
• Decide which force moves farthest as you spin the lever.
• Make a connection between these movements and the lengths to the pivots.

Keeping a record of all this thinking will be too time consuming, but you might like to choose an example for the pupils to record in some detail. It might save a lot of time if you are able to provide them with a printed copy of a carefully chosen photograph of the example, complete with the coloured rod correctly positioned. You'll have to take the picture rather carefully, so that they can annotate a photocopy of it with the forces, lever and lengths.

• 07 Modelling leversMa01TAnugget07 Activity

Building models of levers

What the activity is for

You can use this model to bring out and keep to the fore the central qualitative relationships between the forces, lengths and distances related to levers.

What to prepare

• either one computer, connected to a large display, running the modelling program VnR
• or a collection of computers running the modelling program VnR, so that pupils can work in groups of two or three
• a plan to allow pupils to build models like these
• View clip

What happens during this activity

Either you or the pupils build the models shown. The reconstruction given gives the salient points, but you need not follow the sequence slavishly.

In particular, you can adapt the starting points, perhaps giving the pupils the variables and asking them to connect them together. They could then move on to a larger range of variables, with a different goal set by you.

In any case it is wise to set the pupils mileposts along the way to the final destination, and to get them to check their models as they go.

If you choose to build the model up with the class, you may find it a good strategy to have a volunteer to do the keyboarding and pointer driving for you, allowing you to concentrate on running the discussion with the class.

A good way to work is for you to build up one of the models, then allow the pupils to build, or at least be in the driving seat for, one or both of the others.

• 08 Lever calculationsMa01TAnugget08 Activity

Making calculations based on levers

What the activity is for

The purpose of this activity is to engage the pupils in making calculations based on levers. These can be hard to do, particularly since they involve four variables.

What to prepare

• two diagrams
• a simple example or two to calculate

What happens during this activity

Talk through what you are doing as you perform a simple calculation, following the logic of the diagram provided:

• Calculate one moment.
• Calculate the opposing moment.
• Compare these two moments.

Here is a second sequence, that reduces the algebraic demand:

• Calculate the change in one energy store.
• Calculate the change in the second energy store.
• Compare these two changes.

This sequence reduces the algebraic demand and in any case emphasises the physics – you cannot get out more than you put in – only go for a trade-off! If you reduce the force, then you'll gain in distance (we can be a bit careless here – similar triangles ensures that this can be either distance moved or length to pivot). We recommend that you are a bit inflexible here, insisting on a clear and clearly reported logic in the calculations. You might emphasise these calculations as a kind of argument. It is not only getting the right answer that matters, but how reliably the evidence supports the case. This is only obvious if you take the trouble to lay out the argument carefully.

• 09 How stable is that?Ma01TAnugget09 Activity

Toppling an animal

What the activity is for

You can use this activity to get pupils to relate moments to the stability of objects.

What to prepare

• a collection of model animals
• a set of one-litre squash bottles with screw tops, marked so that they can easily be filled to 1/4 full, 1/2 full, 3/4 full
• one litre plastic measuring jugs, filled with water
• boards with protractors marked on every 15 degrees, for one quadrant only
• blocks or bench clamps to hold the boards vertical
• a coloured perspex or wooden rod, about 15 centimetre long
• this interactive