1. Levers
D. Crowley, 2008

2. Levers To know how levers work Saturday, April 06, 2019
Archimedes – “give me a lever long enough, and a fulcrum on which to place it, and I shall move the world”

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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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