What a scale measures

On a smooth flight, you can sit comfortably without feeling how fast the aircraft is travelling. During takeoff, you feel the seat pressing against your back as it speeds you up.

That pressure comes from the seat pushing on your body. When you stand on the ground, a supporting force acts through your feet instead.

A room and a rocket

Imagine two small windowless rooms: one standing on Earth, the other aboard a rocket accelerating through deep space. Adjust the rocket’s acceleration and the rooms can feel alike. A person presses against the floor, and a released ball falls towards it.

This illustrates the equivalence principle. The match holds for experiments over a small enough region and a short enough time. A larger experiment can detect variations in Earth’s gravity that the simple rocket example does not reproduce.

Why a falling lift feels weightless

A scale measures how hard it supports you. In a stationary lift, the support balances the gravitational force in Newton’s description. If the lift accelerates upwards, the scale must push harder and its reading rises.

If the lift and everything inside fall freely together, no supporting force is needed between your feet and the scale. Its reading becomes zero.

The scale reading is often called apparent weight. You may also see “weight” used for the gravitational force itself. Here, feeling weightless means that no floor, seat or other support is pressing on you.

Astronauts float for the same reason. The station, its floor and its crew are all falling around Earth. Gravity at the station’s height remains close to 90% of its surface value.

The next lesson compares this loss of support with the changing speed of a falling object. A scale and an observer on the ground measure different things, and both readings help us understand the motion.