Why do many students find the concept of levers so difficult? Maybe the problem is terminology; with each part of a lever system having a variety of alternative names. For example, fulcrum, pivot and joint are actually the same thing in terms of levers systems. I would always suggest that you stick with a single term for each part of a lever system, which means that all your notes, diagrams and presentations need to have a common format. Maybe the problem is that understanding how lever systems move is closely linked to A-level physics and mathematics in terms of the mechanics of moments of a force. Just remember that it was only a few years ago that the A-level specification required students to make mathematical calculations about lever systems that also involved trigonometry!
You could introduce lever systems through studying movements in our bodies or through practical/hands on demonstrations.
In simple terms, all our joints work as lever systems, with our muscles applying force to move another part of the body. When our biceps contracts it moves our hand. The way that works is through a lever system. When we try to ‘lever things up’, we wedge a rigid object under the thing to be moved and then apply a force to move it.
A lever is a “bar or some other rigid structure, hinged at one point, and to which forces are applied at two other points”. Hence lever systems have four parts.
The lever itself is some rigid object. In us that is our bones. When our biceps moves our forearm, the lever is the bones of the forearm (radius and ulna). You could also provide an object such as a broom handle to represent the lever itself.
Lever systems also have a pivot point. This is more properly called a fulcrum, and in our biceps/forearm example, the fulcrum is the elbow joint. When demonstrating using the broom handle, you could either grab hold of the broom handle, and where you grab the handle becomes the fulcrum, of you could place an object under the handle and this object is then the pivot or fulcrum point.
Levers systems also have a weight that needs to be moved. This is called the resistance (some people call it the load). In our biceps/forearm example, the resistance is the weight of the hand. With the broom, the mass of the broom on one side of the fulcrum is the weight.
Finally, lever systems will have some method of applying a force or effort to the system to move the resistance. In our biceps/forearm example, the biceps muscle can apply a force to move the hand. With the broom, the force that we apply to make the lever system work is the effort.
Try to get students to rethink levers as a bar with three other parts. These other parts, the fulcrum, resistance and effort, attach to/affect the bar at some point along the length of the bar. Either let the students play, or explain to them that there are only three ways in which these three parts can be arranged. Only one part can be in the middle.
When the fulcrum is in the middle, we have a first class lever system. The resistance and effort can be at either end of the lever; their positioning makes no difference to the type of lever system.
When the resistance is in the middle we have a second class lever system, and when the effort is in the middle, it is a third class lever system.
I like to use the acronym, 1 – 2 – 3, F – R – E, to help students remember the sequence of parts in a lever system.
1 – 2 – 3, F – R – E; so in a first class lever system, F, the fulcrum is in the centre. In a third class lever system, 1 – 2 – 3, F – R – E; E, the effort is in the centre.
The typical AQA examination question invariably requires the candidate to draw and label the lever system operating at a named joint. When drawing lever systems, candidates need to keep their diagrams as simple as possible. It is usual to represent the different parts of a lever system as follows:
The lever itself is a straight line; the fulcrum is usually a triangle; the resistance is usually shown as a square; the effort is drawn as an arrow. There is no need for candidates to produce detailed anatomical diagrams of joints
AQA examination questions generally require candidates to identify and draw the type of lever system operating at a specific joint. The phrasing used is invariably the same:
‘When running, the knee joint works as a lever system. Name, sketch and label the lever system operating at the knee during running.’ (May 2011, Ques 2 (c)). Such questions are often worth 3 marks. One for naming the lever system, one for getting the correct order and one for naming the different parts.
Remembering which lever system operates at which joint is not as difficult as first appears, because the AQA specification limits the movements that need to be understood to seven, several of which involve the same joint actions. In humans, virtually all our joints are third class levers – 1 – 2 – 3, F – R – E; so in a third class lever the effort is in the middle. The biceps/forearm example we used earlier if a third class lever system. The fulcrum is the actual elbow joint at one end of the lever (the bones of the forearm), the resistance is the weight of the hand, and the effort is supplied by the biceps muscle which attaches a short distance along the forearm from the elbow joint.
The only exceptions that we need to worry about are the first class lever system that operates at the elbow and the second class lever system that operates at the ankle.
Get students to consider the action of flexion at the elbow. During this movement, the force is provided by the triceps which attaches to the very end if the forearm (olecranon process), the resistance is again the weight of the hand, and the fulcrum is the elbow joint, which is anatomically in the middle of the other two parts of the lever system. So all throwing actions, such as shot putt involve a first class lever system operating at the elbow joint.
Now get them to think about plantar flexion at the ankle. During this movement the gastrocnemius contracts to supply the effort. This muscle attaches to the heel of the foot through the Achilles tendon. The fulcrum as we point our toes is the ball of the foot. So the foot is the lever, with one end having the effort and the other end having the fulcrum. In the middle is the weight of the body acting as the resistance – 1 – 2 – 3, F – R – E; the resistance is in the middle and this is a second class lever system.
In all classes of lever, there are two other parts of the lever to consider. The effort arm is the name given to the shortest perpendicular distance between the fulcrum and the point of application of the effort; whilst the resistance arm is the name of the shortest distance between the fulcrum and the resistance.
In the first and third class lever systems that operate in our bodies, the resistance arm is longer than the effort arm (get students to estimate distances on their limbs). When this is the case, only a small amount of movement where the force is applied produces a very large movement at the resistance end. When the resistance arm is longer than the effort arm, as in first and third class levers, we get rapid movements over a large range of movement at the resistance end. But because the effort arm is short, we are unable to apply a great deal of force during these movements.
In second class levers, the effort arm is longer than the resistance arm. When this is the case, only a relatively small force is required to cause the lever to work. For this reason, second class levers are especially good at moving heavy resistances, but suffer in terms of speed of movement and range of movement.
You could demonstrate the different effects of lever systems by comparing the actions of the biceps and the gastrocnemius. Most students will agree that these muscles are essentially the same size and can therefore exert roughly the same force. But their effects are very different.
The biceps (third class lever system) moves the hand quickly and through a large of movement (demonstrate), but cannot exert a very large force (how much can students bicep curl?).
The gastrocnemius (second class lever system) moves the foot relatively slowly; through a much more limited range of movement, but can exert large forces – it is easy to lift your own body weight and students can usually more than that!
The vast majority of levers in your body are third class levers, and are better suited to rapid movements over a large range of movement, because of the length of the resistance arm (the distance from the joint to the end of the bones forming the lever), is large as compared to the length of the effort arm (the distance from the joint to the muscle attachment).
Hence, we get the advantage of a large range of movement at the resistance end of our levers, for only a short amount of movement at the force end. However, because the effort arm is so short in these lever systems, we are unable to move as much resistance as a second class lever system would provide. In turn, we gain the disadvantage of being only able to move limited resistances. This is known as our mechanical disadvantage.