Levers and moments: how do they work?

A lever is a stiff bar that turns on a fixed point, called the pivot. A moment is the turning effect of a force, and it equals the force times its distance from the pivot. The further from the pivot a push lands, the harder it turns the lever. That is why a small push can lift a heavy load. Apply the push far from the pivot, place the load close to it, and the push makes the larger moment, so the lever turns and raises the load.

A lever is a stiff bar that turns on a fixed point, called the pivot, and a moment is the turning effect of a force, equal to the force times its distance from the pivot. Slide a crowbar under the edge of a paving slab and let it rest on a brick. The brick is the pivot. Your hands press down on the handle about 60 centimetres from the brick; the slab's edge sits about 5 centimetres from it on the other side. Your push makes a moment that turns the crowbar toward the slab, and the slab's weight makes a moment that resists the turn; whichever moment is larger sets which way the crowbar turns. Your push acts at twelve times the slab's distance, so the crowbar pushes up on the slab's edge with about twelve times your force, and a slab you could never lift by hand moves up. Your end of the crowbar sweeps down through a long arc while the slab edge rises a few millimetres, so the work matches. Your smaller force times its long sweep equals the slab's larger force times its tiny rise. That trade of distance for force is what a lever does, and each end gets one side of it: the end nearer the pivot gets more force, the end further out gets more movement. Engineers call the turning effect a moment, and mechanics call the same quantity torque, measured the same way.

A wheelbarrow is a lever you have pushed a hundred times. The wheel's axle is the pivot. The load sits in the tray, close to the wheel; your hands hold the handles, far behind it. Your lift on the handles makes a moment about the axle, turning the barrow up; the load's weight makes a moment turning it back down. Your hands sit maybe three times further from the axle than the load, so balancing it takes a third of its weight: you hold sixty kilos of sand up with twenty kilos of lift. Your hands also travel a long arc upward while the tray barely leaves the ground. The smaller lift acts over the longer distance, and the heavier load moves through the shorter one: that is the lever's trade.