What if gravity disappeared?

When you drop a ball, it falls down. Every time!

That is because of gravity. Gravity is a pull. What if the pull just stopped?

Drop a ball and it falls. Jump and you come back down. Pour juice and it flows into the cup. All of this happens because of gravity, the invisible pull between Earth and everything on it.

So what would happen to a ball if gravity disappeared? In this lab you can switch gravity off in a model and see.

Gravity is the attraction between masses. Near Earth's surface it gives every object the same downward acceleration, about 9.81 meters per second squared, whatever its mass (as long as air resistance is small).

This lab asks a simple question with a revealing answer: if that acceleration were zero, how would a falling or thrown object move?

On Earth, things fall down when you let go.

They go faster and faster as they fall, until they hit the ground.

On Earth, a dropped ball does not fall at a steady speed. It speeds up all the way down, gaining about 9.8 meters per second of speed every second. Without air in the way, a ball dropped from 10 meters reaches the ground in about one and a half seconds.

If you throw a ball up, gravity slows it down, stops it at the top, and pulls it back.

With air resistance neglected, an object dropped from rest falls a distance h in a time t = √(2h/g). From 10 meters that is about 1.43 seconds, reaching about 14 m/s. A ball thrown upward decelerates at g, stops at its highest point, and returns, all following v = v₀ − g·t.

In real air, drag grows with speed, so light, broad objects such as feathers reach a terminal speed quickly; the model can switch air resistance on to show this.

In our pretend lab, Olly turns gravity off.

Now the ball does not fall! If it was still, it stays still. If you push it, it keeps going.

In the model, set gravity to zero. A ball that was resting in the air just stays there. A ball that was moving keeps moving in a straight line at the same speed, because nothing is pulling it down or slowing it.

This is a pretend experiment. We can't really switch gravity off, but a model lets us see what the rules of motion say would happen.

With g = 0 and air resistance off, the model's equations reduce to constant velocity: x = x₀ + v·t. An object at rest remains at rest; a thrown object continues in a straight line at its launch speed. That is Newton's first law, which gravity normally hides from us.

This is a thought experiment carried out with a model. Gravity cannot be switched off, and the model does not describe everything that would happen to a whole planet without it.

Things only speed up, slow down or turn when something pushes or pulls them.

Gravity is the pull that makes things fall. No pull, no falling!

Objects change how they move only when a force pushes or pulls them. On Earth, gravity is the force that pulls everything down, so dropped things speed up as they fall.

Take gravity away and there is no downward pull, so nothing speeds up or curves down. That is why a still ball floats and a moving ball keeps going.

Newton's second law, F = m·a, links force and acceleration. Gravity's force on a mass m is m·g, giving every object the same acceleration g. Remove it and, with no other force, the acceleration is zero and velocity stays constant.

The model's limits: it treats the ball as a point, uses a uniform g, and its air resistance is a simple quadratic drag. It says nothing about air, oceans or Earth itself, which gravity holds together; those effects are ideas, not model results.

Astronauts float in their space station.

But gravity has not stopped! The station and the astronauts are falling around Earth together, so they float.

Astronauts on the International Space Station float, but not because there is no gravity up there. At the station's height, Earth's gravity is still about 90 percent as strong as on the ground.

They float because the station and everyone inside it are falling around Earth together, so fast that they keep missing the ground. Falling together, they seem weightless. Scientists call this free fall, or microgravity.

NASA notes that at the space station's altitude, about 200 to 250 miles up, gravity is roughly 90 percent of its surface value. The crew appear weightless because they are in free fall: the station and everything in it accelerate toward Earth at the same rate while moving sideways fast enough to stay in orbit.

So 'zero gravity' in orbit is really shared free fall. Our model's g = 0 world is different: nothing is pulling at all, and nothing would curve into an orbit.