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

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

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

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

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

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

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

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 60

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 distance

Constant

Constant load mass

Variable effort mass

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

300N

300N

Work done load = 300 x 2 = 600NmWork done effort = 300 x 2 = 600Nm

300N

300N

2m 10m

Work done load = 300 x 2 = 600NmWork done effort = 300 x 10 = 3000Nm

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• Not enough effort to move load (rock)