Do you weigh more when an elevator goes up or when it comes down?

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, that pressure eases, leaving you briefly lighter.

If you stand on a scale inside an elevator going up, the number jumps. When it slows to a stop, the number dips. On the way down, the opposite happens.

So what's really going on? Do you weigh more when the elevator goes up or when it goes down?

The short answer is that you can feel heaviest at two points: when the elevator starts moving up (accelerating upward) and when it's slowing down at the very end of a downward trip (decelerating downward). But the explanation depends on what "weight" actually means and what your body can feel.

"The word 'weight' in physics has different meanings," Miguel Morales, a physics professor at the University of Washington in Seattle, told Live Science. In physics, weight can refer to at least three related ideas: your mass (how much matter you're made of), the gravitational force pulling on you, or how hard the scale beneath you is pushing up, Morales explained.

"When you're just standing still, those can all be the same thing," Morales said. "But as soon as the elevator starts to speed up or slow down, you get three different answers. It's just physics."

Your mass never changes, no matter what the elevator does. Gravity near Earth's surface also stays essentially the same between the bottom and top of a building. What does change is the third definition: how hard the scale pushes upward. That upward push is what a scale actually measures.

Looking at gravity

This distinction reveals something counterintuitive: "You can't feel gravity. You never could," Jason Barnes, a physics professor at the University of Idaho, told Live Science.

Barnes pointed to astronauts aboard the International Space Station. "The actual gravity of the Earth up there is almost the exact same as here," he said. "But they don't feel it."

That'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 at the surface. The difference is that astronauts and the space station are in continuous free fall toward Earth.

The station is moving sideways at more than 17,000 mph (27,300 km/h). As it falls, Earth curves 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 are falling together at the same rate, the floor never needs to push up on them. And that upward push is what we actually feel as weight (also called the normal force).

On Earth, the ground constantly prevents you from falling by pushing upward against you. In orbit, there's no such push. The astronauts are still under the influence of gravity, but nothing is stopping them from falling. Without the floor pressing upward, they feel weightless.

Why do elevators make you feel heavier or lighter?

An elevator briefly changes how hard the floor pushes back on you. When the elevator starts rising, it must accelerate you upward, too. "To start going up, that's when you feel heavier," Barnes said. "The elevator pushes back harder than normal in order to accelerate you upward."

In a typical building elevator, that extra acceleration might be about 1 meter per second squared. That is roughly one-tenth of Earth's gravity. For someone who normally weighs 150 pounds (68 kilograms), that would briefly add about 10% to the scale reading. Instead of 150 pounds, the scale might show around 165 pounds (75 kg).

Morales described the same effect from the scale's perspective. "The force of gravity hasn't changed at all," he said. "But now, in order for you to be speeding up, something's got to be pushing you harder than gravity. And so your weight on the scale will go up."

Once the elevator reaches a steady speed, the acceleration stops. Gravity and the upward push balance again, and the scale returns to its normal reading, even though you're still moving.

At the top, when the elevator slows to a stop, the opposite happens. Even though you're still moving upward, the elevator must accelerate downward slightly to slow you down.

The force of gravity hasn't changed. But because the elevator is now accelerating downward, the floor doesn't need to push up as hard to control your motion. With less upward push (normal force), the scale reading drops.

"You kind of feel yourself get 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 approaches the bottom and slows to a stop, the acceleration flips upward again, making you feel heavy again.

This everyday experience turns out to be connected to one of the most important ideas in modern physics.

"It is an effect that Einstein first noted when he was developing general relativity," Barnes said. That insight, known as the equivalence principle, helped Einstein understand gravity not as a force but as a consequence of acceleration and the curvature of space-time itself.