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Energy, electricity and mechanics

Levers and block and tackle

Force needed to lift a load with a lever or a block and tackle, based on mechanical advantage and friction.

m

Distance from the fulcrum to the point where you apply the force.

m

Distance from the fulcrum to the load.

kg
%

Friction losses at the fulcrum or in the pulleys. Typical: lever 90-95 %, block and tackle with plain pulleys 80-90 %.

Result

Force needed
— N
Force needed
— kgf
Real mechanical advantage
—
Rope to pull in per metre of lift
— m

How it is calculated

Lever: ideal mechanical advantage MA = effort arm ÷ load arm. Block and tackle: the ideal mechanical advantage is the number of rope parts supporting the load.

Ideal force (no friction): F = load × 9.81 ÷ MA. Real force with friction: F_real = F ÷ efficiency. The real mechanical advantage achieved is MA × efficiency.

In a block and tackle, for every metre the load rises you have to pull in as many metres of rope as there are parts supporting it.

Keep in mind

  • 1 kgf ≈ 9.81 N: the weight of a 1 kg mass at the Earth's surface.
  • Always use ropes, pulleys and anchors with a wide safety factor (usually 5:1 or more) against the working load, because shock loads while hoisting multiply the real force.
  • Never stand underneath a suspended load.

See also

More tools in “Energy, electricity and mechanics”