Levers To know how levers work Archimedes – “give me a lever long enough, and a fulcrum on which to place it, and I shall move the world”

Levers Levers are simple machines – they can make work easier by increasing the size of a force

Body Levers The diagram shows the human arm in two positions (straight arm and arm lifted) The human arm works as a system of levers: – Long levers are helpful A lever moves at a pivot The effort is supplied when a muscle contracts Straight arm Arm lifted

Body Levers In the arm, where is the pivot? In the straightening of the arm, which muscle is contracted? In the lifting of the arm, which muscles in contracted? The biceps and triceps are antagonistic muscles, as they work against each other – what does this mean? Straight arm Arm lifted

Body Levers The pivot is at the middle, where the arm bends The triceps contract when straightening the arm The biceps contract when lifting the arm The biceps pulls up the arm, but then is stuck… until the contracting triceps pull it back – antagonistic pair Straight arm Arm lifted

Experiment Your task is to plan an experiment to investigate how changing the length of lever affects the force needed to lift an object Is it better to have a short / long lever? pivot

Plan Write a plan for your experiment – remember you want to find how changing the length of lever affects how much force you need to lift an object Explain how you will make your experiment a fair test, and how you will get accurate results pivot

Experiment Set up your apparatus as shown: - 70cm distance 30cm distance Put the load at the end of the ruler (0cm) Put some masses on the opposite side, on the 45cm mark – keep adding masses until the ruler just balances, and record the number of masses needed Now move the masses into the 60cm mark and the 90cm mark, adding or removing masses as necessary until the ruler just balances

Distance from pivot (cm) Number of masses needed Results Record your results in the table (how many masses were needed to just balance the ruler) Mark on ruler (cm) Distance from pivot (cm) Number of masses needed 45 15 60 30 90

Conclusion Write down your conclusions for the experiment Remember, the load stayed the same (the object you were lifting) as well as the distance between the load and pivot Only the force, and distance between the force and pivot changed Variable effort mass Constant load mass Variable distance Constant

Work done = force x distance Levers Levers are simple machines – they can make work easier by increasing the size of a force A longer lever makes lifting the load easier Work done = force x distance If the forces are the same, but the lever is a longer distance on the effort side, then you will lift the load: - 2m 2m 2m 10m 300N 300N 300N 300N Work done load = 300 x 2 = 600Nm Work done effort = 300 x 2 = 600Nm Work done load = 300 x 2 = 600Nm Work done effort = 300 x 10 = 3000Nm

work done = force x distance Look at the following levers – work out which lever will lift the load work done = force x distance A: - Effort = 2 x 800 = 1600Nm Load = 2 x 4000 = 8000Nm Not enough effort to move load (rock) B: - Effort = 3.5 x 800 = 2800Nm Load = 0.5 x 4000 = 2000Nm More effort than load (rock) – rock moves! C: - Effort = 1.5 x 800 = 1200Nm Load = 2.5 x 4000 = 10’000Nm