On this page
The fear with any lift is simple: what happens if the rope snaps? For most of history the honest answer was that the platform fell. Elisha Otis changed that answer. This post covers who he was, how his safety catch works according to his own patent, a worked example with Newton's second law, the mistakes students make in lift problems, and what you can take from his way of working.
Who was Elisha Otis?
The National Inventors Hall of Fame, which inducted him in 1988, gives his life as 3 August 1811 to 8 April 1861. He was born near Halifax, Vermont, and had several failed businesses before moving to Albany, New York, in 1845, where he worked as a master mechanic in a bedstead factory. There he invented a railway safety brake and devices for making bed rails.
By 1852 he was in Yonkers, setting up machinery for the bedstead firm of Maize & Burns. Josiah Maize needed a hoist to lift heavy equipment to an upper floor, and ordinary hoists were dangerous. The Hall of Fame puts it plainly: Otis "didn't invent the elevator"; he invented the brake.
What happened at the Crystal Palace in 1854?
According to the Hall of Fame, Otis demonstrated his device at the Crystal Palace Exposition in New York in 1854. He rode the platform up an open-sided shaft, and halfway up he had the hoisting cable cut with an axe. The platform held. The demonstration mattered because people could see the safety catch work, not just read a claim about it.
How does the safety catch work?
His US patent No. 31,128, granted on 15 January 1861 for an "Improvement in Hoisting Apparatus", explains the mechanism. Two upright posts carry racks with hook-shaped teeth that point upward. The moving platform carries two pawls on levers. Springs push the pawls towards the racks, and the lifting rope, pulling on a bar, holds them back.
In the patent's words, if the rope "should break" while hoisting, the pawls, "released from the pull of said rope, will immediately be thrown" into the racks by the springs. Because the teeth point up, the weight of the load pulls the pawls in deeper.
The clever part is that the safety system is powered by the failure. While the rope is under tension, the brake stays off. The moment tension disappears, the brake comes on by itself. The same patent also uses a counterweight, described as "a counterpoise for the platform", so the machine lifts only the difference in weight.
Otis died less than three months after the patent was granted.
Worked example: forces in a lift, before and after the rope breaks
Sample numbers: car plus load 500 kg, g = 9.8 m/s². Ignore friction and the mass of the rope.
Accelerating upward at 1.0 m/s². Newton's second law for the car: T − mg = ma, so T = m(g + a) = 500 × 10.8 = 5,400 N.
With a 400 kg counterweight, held still. The hoist only has to supply the difference: (500 − 400) × 9.8 = 980 N.
The rope breaks. Suppose the car falls freely for 0.10 m before the pawls catch. Its speed is v = √(2gh) = √(2 × 9.8 × 0.10) = 1.4 m/s.
The catch stops it in 0.05 m. Deceleration a = v²/(2d) = 1.96 / 0.10 = 19.6 m/s², which is 2g, directed upward.
Force on the racks. N − mg = ma, so N = m(g + a) = 500 × (9.8 + 19.6) = 14,700 N, three times the car's weight.
Halve the stopping distance and the deceleration doubles. That is the trade-off any safety brake makes: it must stop the car quickly, but not so suddenly that the force injures passengers or breaks the rack.
What mistakes do students make in lift problems?
Using T = mg when the lift accelerates. That only holds at constant velocity or at rest.
Getting the sign of deceleration wrong. Slowing a downward motion means acceleration upward.
Forgetting that free fall feels weightless. In step 3 a bathroom scale in the car would read zero. Our post on the weighing machine reading in a lift works through this.
Ignoring the counterweight. It changes the force the motor must supply, not the car's weight.
The JEE (Main) 2026 syllabus covers Newton's laws in Unit 3 and work and energy in Unit 4, which is all this example needs. Check the syllabus for your exam year. For a step-by-step method, see our guide to free-body diagrams and Newton's second law.
If drawing the forces is where you get stuck, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student take from Otis's way of working?
Design for failure. His brake does not depend on anyone noticing the rope has broken. The broken rope itself sets the brake.
Show, don't only tell. A public test with an axe convinced people faster than any description could.
Start from a real problem. He was solving his employer's need to lift heavy machinery safely.
Next step: redo steps 3 to 5 with a 0.20 m free fall and a 0.05 m stopping distance, and compare the force on the racks.
