⚡Physics & Applied Math · 5 min read

Force, Work & Power

The three physical quantities that describe how elevators move loads

The EIAT tests basic physics concepts that underpin all mechanical systems. You need the formulas, units, and the ability to apply them to elevator scenarios. These three quantities are directly related, and questions often ask you to solve for one when given the other two.

Force

Force is a push or pull on an object. F = m × a (mass × acceleration). In elevator calculations, the primary force is gravity: weight in pounds already accounts for this — a 150 lb person exerts 150 lbs of downward force.

Net force determines acceleration. If the upward force equals the weight, the elevator moves at constant speed (net force = zero). If upward force exceeds weight, the car accelerates upward.

Unit: pounds (lbs) in imperial, Newtons (N) in metric. The EIAT uses imperial units.

Work

Work = Force × Distance (W = F × d). Work is done only when a force causes movement — if nothing moves, no work is done in the physics sense regardless of how much force is applied.

Unit: foot-pounds (ft·lbs). To lift a 500 lb load 10 ft requires 500 × 10 = 5,000 ft·lbs of work. Note: the work required is the same whether you lift it slowly or quickly — power changes, not work.

Power

Power is the rate of doing work. P = Work ÷ Time = F × velocity. More power is needed to do the same work in less time.

Unit: horsepower (HP). 1 HP = 550 ft·lbs per second = 33,000 ft·lbs per minute.

Elevator motor size calculation: Lift force (lbs) × speed (FPM) ÷ 33,000 = HP required.

Example: Lifting 2,200 lbs at 100 FPM: HP = (2,200 × 100) ÷ 33,000 = 6.67 HP. Accounting for counterweight and efficiency, actual motor HP is lower, but this is the calculation approach.

Torque

Torque is rotational force — the twisting force around an axis. T = Force × moment arm (radius). Unit: ft·lbs.

Gears multiply torque. A 5:1 gear reduction multiplies torque by 5 (before friction losses): 50 ft·lbs input → 250 ft·lbs output, at one-fifth the speed.

This is why elevator machines need high-torque, low-speed output: the drive sheave must exert enormous rope force at low RPM to move a heavy car smoothly.

Pressure

Pressure = Force ÷ Area. Unit: PSI (pounds per square inch).

Critical for hydraulic elevators: the pump builds pressure in the oil. That same pressure acts on the jack piston. Force on piston = Pressure × Piston area.

Example: System pressure is 400 PSI, piston area = 30 in². Force = 400 × 30 = 12,000 lbs — enough to lift a fully loaded elevator car.

Pascal's law: pressure applied to an enclosed fluid transmits equally in all directions throughout the fluid. This is what makes hydraulic force multiplication possible.

Key Takeaways

✓1 HP = 33,000 ft·lb/min. Memorize this for motor size problems.
✓HP = (lbs × FPM) ÷ 33,000. This is the elevator motor formula.
✓Work = Force × Distance. Doubling either doubles the work.
✓Power is about time — same work, less time = more power needed.
✓Pressure × Area = Force. Larger piston = more force at the same pressure.

Key Terms

Force

A push or pull. In elevators: weight in lbs. F = m × a.

Work

Force applied over a distance. W = F × d. Unit: ft·lbs.

Power

Rate of doing work. P = W ÷ t. 1 HP = 33,000 ft·lb/min.

Torque

Rotational force. T = Force × moment arm. Unit: ft·lbs.

Pressure

Force per unit area. P = F ÷ A. Unit: PSI.

Horsepower (HP)

Unit of power. 1 HP = 550 ft·lb/sec = 33,000 ft·lb/min.

FPM

Feet per minute — elevator speed unit.

Pascal's Law

Pressure in an enclosed fluid transmits equally in all directions.

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