The three classes of lever
Use a crowbar, scissors, a wheelbarrow, a nutcracker, tongs and a broom. Find the fulcrum, the load and the effort on each, see which is in the middle, and work out how much each one helps.
Core concept
Levers are grouped by what is in the middle: the fulcrum (class 1), the load (class 2) or the effort (class 3); mechanical advantage = load ÷ effort = effort arm ÷ load arm
Topic: Simple machines
Every lever: A bar that turns on a fixed point to lift heavy loads with less effort, like a see-saw. All key words has three points: the fulcrum: The fixed point a lever turns on. All key words it turns about (F), the load it moves (L) and the effort you put in (E). What is in the middle decides its class. Class 1 levers, like a crowbar and scissors, have the fulcrum in the middle. Class 2 levers, like a wheelbarrow and a nutcracker, have the load in the middle. Class 3 levers, like tongs and a broom, have the effort in the middle. The lever balances when effort × effort arm = load × load arm. The mechanical advantage (MA) is load ÷ effort: more than 1 means the lever makes the job easier.
What you will learn
- Find the fulcrum, load and effort on everyday levers.
- Classify levers as class 1, 2 or 3.
- Calculate the effort and the mechanical advantage of a lever.
- Explain why class 3 levers are useful even though their MA is less than 1.
Before you start
- Know the principle of moments: a lever balances when the turning effects on each side are equal.
Materials
- A crowbar (or a ruler and an eraser), scissors, a nutcracker, tongs, a broom, a wheelbarrow
Equipment
- A spring balance to measure the effort
Procedure
- Lift a 100 N load with a crowbar and with scissors, and record both.
- Use a wheelbarrow and a nutcracker, and record both.
- Use tongs and a broom, and record both.
- Put a 200 N load in the wheelbarrow and record.
How this simulation is simplified
- The levers are light and turn without friction about their fulcrum.
- Arm lengths are typical for each tool.