Simple Machines
Primary 6 · Science and Elementary Technology
Simple Machines
Lesson 2 of 5
15 min
Let's begin
At break time, Keza and her big cousin Byiringiro get on the seesaw. Byiringiro sits down and Keza shoots up into the air, stuck. Everyone laughs.
Then Byiringiro slides closer to the middle and, as if by magic, the seesaw balances. He did not get lighter. So what changed?
A lever is a strong bar that turns on a fixed point. It has four parts:
A lever balances when:
load × its distance from the fulcrum = effort × its distance from the fulcrum
Worked example
Byiringiro has a mass of 40 kg and sits 1 m from the middle. Keza has a mass of 20 kg. How far from the middle must she sit to balance him?
Keza must sit 2 m from the middle. Half the mass needs twice the distance. This is why a long crowbar handle lets a small push lift a heavy rock: the longer your side, the less effort you need.
Levers are sorted into three classes by what sits in the middle.
| Class | What is in the middle? | Examples |
|---|---|---|
| First class | The fulcrum is between the effort and the load. | Seesaw, crowbar, claw hammer pulling a nail, pliers, scissors, a beam balance. |
| Second class | The load is between the fulcrum and the effort. | Wheelbarrow (the wheel is the fulcrum), bottle opener, nutcracker, a door. |
| Third class | The effort is between the fulcrum and the load. | Tweezers, kitchen tongs joined at one end, a broom, a fishing rod, your forearm lifting a cup. |
First and second class levers usually help you lift heavy loads with less effort. Third class levers need more effort, but they make the load move further and faster, like the end of a broom sweeping across the floor.
If your school has computers, open the Balancing Act simulation. Put bricks on both sides of the plank and move them nearer to or further from the middle until the plank balances. Check the balancing rule each time: mass × distance should be the same on both sides.
Tip
What is in the middle? Say "1, 2, 3 → F, L, E".
Class 1: Fulcrum in the middle. Class 2: Load in the middle. Class 3: Effort in the middle.
Try each one first, then tap Show answer to check.
A bottle opener rests on the top of the cap, lifts the edge of the cap, and you pull up the handle. What is in the middle? Which class is it?
Answer: The load (the edge of the cap) is in the middle, between the fulcrum and your effort. It is a second-class lever.
Which class of lever is a pair of tweezers?
Answer: Third class. Your fingers (the effort) press in the middle, between the joined end (the fulcrum) and the tips (the load).
A 30 kg child sits 2 m from the middle of a seesaw. How far from the middle must a 60 kg adult sit to balance?
Answer: 30 × 2 = 60, and 60 × 1 = 60, so the adult sits 1 m from the middle.
Why is it easier to pull a nail out with a long claw hammer than a short one?
Answer: The longer handle puts your effort further from the fulcrum, so you need less force.
What you learned today
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
Quick check
Match each class of lever to what sits in the middle.
Tap one on the left, then its partner on the right.
