ZIMSEC O Level · Design and Technology · Form 2, Form 3
Levers
A lever is a rigid bar that turns on a pivot (fulcrum).
- Class 1: fulcrum between effort and load (see-saw, scissors, crowbar).
- Class 2: load between fulcrum and effort (wheelbarrow, nutcracker, bottle opener).
- Class 3: effort between fulcrum and load (tweezers, fishing rod, the human forearm).
Mechanical advantage = load ÷ effort.
Other mechanisms
- Gears: change speed, direction or force. Gear ratio = teeth on driven gear ÷ teeth on driver gear.
- Pulleys and belts, chain and sprocket (bicycles), cams (change rotary to reciprocating motion), cranks, screw threads and linkages.
Worked example: A driver gear with 20 teeth turns a driven gear with 60 teeth. Find the gear ratio and the output speed if the driver turns at 90 rpm.
Gear ratio = 60 ÷ 20 = 3 : 1
Output speed = 90 ÷ 3 = 30 rpm (slower, but with more turning force)
Structures
Frame structures (bridges, towers, roof trusses) and shell structures (cans, car bodies, egg shells).
Forces: tension (pulling), compression (pushing), bending, torsion (twisting) and shear. Triangulation makes frames rigid.
Exam tip: To identify lever class, find what is in the middle: Fulcrum (1), Load (2), Effort (3) – “FLE 123”.
Check yourself
- Which class of lever is a wheelbarrow?
- A 15-tooth gear drives a 45-tooth gear. Find the gear ratio.
- Why are triangles used in frame structures?
Show answers
- Class 2.
- 3 : 1
- Triangles cannot change shape without changing the length of a side, so they make frames rigid.
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