Every lever consists of three elements: a fulcrum (pivot point), an effort (input force), and a load (output force). What distinguishes the three classes is simply the order these elements appear along the beam.

Class 1: Fulcrum in the middle. The classic seesaw. Effort and load sit on opposite sides of the pivot. Examples: crowbar, claw hammer pulling a nail, scissors. You can trade force for distance or vice versa depending on where you place the fulcrum. A wrecking bar with the fulcrum 5 cm from the nail and your hand 50 cm away gives you a mechanical advantage (MA) of 10:1 — push with 20 kg of force, generate 200 kg at the nail.

Class 2: Load in the middle. The fulcrum is at one end, effort at the other, and the load sits between them. Examples: wheelbarrow, nutcracker, bottle opener. These always multiply force (MA > 1) because the effort arm is always longer than the load arm. A wheelbarrow with handles 1.2 m from the wheel axle and the load centered 0.4 m from the axle gives MA = 1.2 / 0.4 = 3. You lift 20 kg to raise 60 kg of gravel.

Class 3: Effort in the middle. Fulcrum at one end, load at the other, effort applied between them. Examples: tweezers, fishing rod, your forearm (bicep inserts close to the elbow, hand holds the weight far away). These always have MA < 1 — you sacrifice force to gain speed and range of motion. Your bicep pulls with roughly 7× the force of whatever you're holding because the insertion point is about 1/7th of the way along the forearm.

The governing equation is identical for all three:

MA = effort arm length ÷ load arm length

Rule of thumb for design: Need to amplify force? Use Class 1 or Class 2. Need to amplify speed or range of motion? Use Class 3. This is why hydraulic excavator booms (Class 2 at the bucket) crush rock, while your arm (Class 3) can throw a ball at 90 mph.

Practical calculation: You're designing a foot-operated trash can lid. The pedal hinge (fulcrum) is at the back, the lid linkage (load) connects 8 cm from the hinge, and the front of the pedal where the foot presses is 24 cm from the hinge. That's Class 2, MA = 24/8 = 3. A 1 kg lid needs only 0.33 kg of foot pressure — light enough to feel effortless.