Sixth Class · Science · 2 minutes
Levers: how a small push lifts a big load
Effort × distance = load × distance.
Key terms
- Lever
- A rigid bar that turns on a pivot.
- Fulcrum
- The pivot a lever turns on.
- Effort
- The push you give to move a load.
- Load
- The weight that the lever moves.
- First class lever
- A lever with the fulcrum in the middle, like a crowbar or scissors.
- Second and third class levers
- In a second class lever the load is in the middle, as in a wheelbarrow; in a third class lever the effort is in the middle, as in tweezers.
Transcript
Could a small child lift an adult right off the ground? With muscles alone, no way. But with a lever, they can!
A lever is a rigid bar that turns on a pivot, called the fulcrum. The push you give is the effort. The weight it moves is the load.
Here's the secret. A one hundred newton load, one metre from the fulcrum, is balanced by just twenty five newtons, four metres away. Effort times distance equals load times distance. Four times the distance, a quarter of the effort.
Watch this seesaw. The adult is twice as heavy, so their side goes down. Now slide the fulcrum towards the adult. Closer, closer... balanced! One step more, and the child lifts the adult!
That's a first class lever: the fulcrum sits in the middle. Crowbars and scissors work the same way.
In a second class lever, the load is in the middle. Like a wheelbarrow: the wheel is the fulcrum, and you lift the handles. Nutcrackers, too.
In a third class lever, the effort is in the middle. Tweezers, a fishing rod, even your forearm! Your elbow is the fulcrum, and your muscle pulls close to it. More effort, but a fast, far reach.
More than two thousand years ago, Archimedes is said to have boasted: give me a place to stand, and I will move the Earth! He'd need a very long lever.
So: a lever is a rigid bar turning on a fulcrum. Effort times distance balances load times distance. And where the fulcrum, load and effort sit tells you the class.
Sources
Every fact in the video, and where it comes from.
- 1vedantu.com/physics/types-of-lever
- A lever is a rigid bar that turns about a fixed pivot point called the fulcrum; the force applied is the effort and the object moved is the load.
- First class lever: fulcrum between effort and load, e.g. see-saw, crowbar, scissors.
- 2physics.weber.edu/carroll/Archimedes/lever.htm
- Law of the lever (principle of moments): effort x effort distance = load x load distance at balance. 100 N at 1 m is balanced by 25 N at 4 m (100 N m each side).
- Seesaw example: 300 N child 2 m from the pivot balances a 600 N adult 1 m away (600 N m each side); plank weight ignored.
- 3unacademy.com/content/question-answer/physics/what-are-examp…
- Second class lever: load between fulcrum and effort, e.g. wheelbarrow (wheel/axle is the fulcrum), nutcracker.
- 4revisiondojo.com/student-faq/difference-between-first-second-th…
- Third class lever: effort between fulcrum and load, e.g. tweezers, fishing rod, the human forearm (elbow = fulcrum, biceps effort attaches near the elbow, load in the hand). Needs more effort than the load but moves the load further and faster.
- 5en.wikiquote.org/wiki/Archimedes
- 'Give me a place to stand and I will move the Earth' is attributed to Archimedes (3rd century BC) by Pappus of Alexandria, about AD 340; the attribution is second-hand.
- 6britannica.com/biography/Archimedes
- Archimedes of Syracuse lived c. 287 - c. 212 BC (about 2,250 years ago).
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