Elevators have a strange way of messing with your sense of gravity. The moment an elevator lurches upward, you feel it in your feet. For a second, the floor presses harder than usual. When the elevator slows down, the pressure decreases and you feel lighter for a short time.
If you stand on a scale inside an elevator going up, the number jumps. When it slows to a stop, the number drops. On the way down, the opposite happens.
Sign up for our newsletter
Sign up for our weekly Life’s Little Mysteries newsletter to get the latest mysteries before they hit the web.
The short answer is that you can feel the heaviest at two points: when the lift starts to move upwards (accelerates upwards) and when it brakes at the very end of a descent (brakes downwards). But the explanation depends on what “weight” actually means and what your body can feel.
“The word ‘weight’ in physics has different meanings,” Miguel Moralesa physics professor at the University of Washington in Seattle, told LiveScience. In physics, weight can refer to at least three related ideas: your mass (how much matter you’re made of), the force of gravity pulling on you, or how hard the scale below you pushes up, Morales explained.
“When you just stand still, everyone can be the same,” Morales said. “But as soon as the elevator starts to speed up or slow down, you get three different responses. That’s just physics.”
Your mass never changes, no matter what the elevator does. Gravity near the Earth’s surface also remains essentially the same between the bottom and top of a building. What changes is the third definition: how hard the scale pushes up. That upward pressure is what a scale actually measures.
Looking at gravity
This difference reveals something counterintuitive: “You can’t feel gravity. You never could,” Jason Barnesa physics professor at the University of Idaho, told LiveScience.
Barnes pointed to astronauts aboard the International Space Station. “The actual gravity of the Earth up there is almost the same as here,” he said. — But they don’t feel it.
It’s not because gravity disappears in orbit. At the station’s altitude (about 250 miles, or 400 kilometers, above our planet), Earth’s gravitational pull is still about 90% as strong as it is on the surface. The difference is that astronauts and the space station are in continuous free fall towards Earth.
The drive moves sideways at more than 17,000 mph (27,300 km/h). As it falls, the earth bends away beneath it. Instead of hitting the ground, it keeps missing it. The result is a constant fall around the planet.
Because the astronauts and the space station fall together at the same speed, the floor never has to push them up. And that upward pressure is what we actually feel as weight (also called the normal force).
On Earth, the ground constantly keeps you from falling by pushing you upwards. In orbit, there is no such pressure. The astronauts are still under the influence of gravity, but nothing prevents them from falling. Without the floor pushing upwards, they feel weightless.
Why do lifts make you feel heavier or lighter?
A lift briefly changes how hard the floor pushes you back. As the elevator begins to rise, it must also accelerate you upward. “Starting to go up, that’s when you feel heavier,” Barnes said. “The elevator pushes back harder than normal to accelerate you upwards.”
In a typical building elevator, the extra acceleration can be about 1 meter per second squared. That’s about one-tenth of Earth’s gravity. For someone who normally weighs 68 kilos, it will briefly add about 10% to the weight reading. Instead of 150 pounds, the scale may show around 165 pounds (75 kg).
Morales described the same effect from the perspective of scale. “Gravity hasn’t changed at all,” he said. “But now, for you to gain speed, something has to push you harder than gravity. So your weight on the scale will go up.”
When the elevator reaches a steady speed, the acceleration stops. Gravity and the upward push balance again and the scale returns to normal reading, even though you are still moving.
At the top, when the elevator slows to a stop, the opposite happens. Even though you are still moving upward, the elevator must accelerate slightly downward to slow you down.
Gravity has not changed. But because the elevator now accelerates downward, the floor doesn’t have to push up as hard to control your movement. With less upward pressure (normal force), the scale reading decreases.
“You kind of feel like you’re getting a little light,” Morales said.
The same pattern repeats on the way down. When the elevator accelerates downward, you feel lighter because the floor pushes up less than usual. But as it nears the bottom and slows down to a stop, the acceleration tilts back up, making you feel heavy again.
This everyday experience turns out to be linked to one of the most important ideas in modern physics.
“It’s an effect that Einstein first noticed when he was developing general relativity,” Barnes said. This insight, known as the equivalence principle, helped Einstein to understand gravity not as a force but as a consequence of acceleration and curvature of space-time himself.
