How does gravity work?

The force of gravity is the weakest interaction that exists in nature and is what causes two bodies to attract each other. We've all experienced many times dropping an object we're holding. What happens then is that it falls and hits the ground. You've surely heard of Isaac Newton's apple , the story about the English physicist who sat under a tree and watched an apple fall to the grass. It is said that he then wondered why the apple fell, but the Moon didn't. And from there, he began his research into gravity.

What we knew, what Newton and Galileo identified, is that gravity causes acceleration. The object that was initially still in your hand falls with acceleration when you release your grip. As I mentioned at the beginning, the force of gravity is what causes two objects to attract each other. And that attraction, or that force, depends on two things: in this case, the mass of the Earth and the mass of the object your hand releases.

It's important to know that acceleration is independent of the mass of the falling object. If I want to accelerate something very large, I have to apply a lot of force; if I have to accelerate something smaller, I have to apply less force, but the acceleration is the same. This is what Galileo demonstrated when he climbed the Tower of Pisa and dropped metal balls of different masses to show that the descent time is independent of the mass of the falling object because the acceleration caused by gravity is the same. If you're wondering why he didn't drop feathers or tree leaves, the explanation is that he didn't do so so he wouldn't have to worry about the additional effect of friction.

In reality, when something falls, both move: the Earth moves, and the falling object moves. The thing is, the gravity exerted by the small object falling from your hand on the immense Earth is very small, so we don't notice it.

Things change when we compare two things with similar masses, such as the Moon orbiting the Earth. Why doesn't the Moon fall? The explanation is that when it formed in the solar system , the Moon wasn't stationary; it was spinning at a certain speed. This spin is what keeps it from falling. But it's the force of gravity that's making it spin. It's like a cowboy's lasso at a rodeo: when he spins it, the lasso doesn't fall. The Earth's gravity on the Moon has a similar effect.

What I've explained to you so far is the physics discovered by Galileo and Newton. But when Einstein appears on the scene, things change. Einstein's gravity also modifies time because, according to his theory, gravity is an effect of the curvature of space-time. As he demonstrated, when the gravitational field is very strong, time passes more slowly.

If we think of an object falling from your hand when you release it, it appears to be falling in a straight line, but in reality, it isn't. If we were able to see the small changes in the force of gravity, we would see that small deviations occur alongside them. Although they appear to us to be straight lines, in a curved space-time like ours, they aren't exactly so. They are straight lines adapted to that geometry, like when you draw a straight line on a sphere or a cylinder.

Another topic that might interest you is the relationship between gravity and weight. Your weight is exactly the force with which the Earth attracts you. If you were on the Moon, your weight would be less because, as our satellite has a much smaller mass than our planet, so is the force of gravity it exerts. And that's why astronauts, who have the same mass everywhere, weigh less on the Moon than on Earth, because the two celestial bodies do have different masses. You can imagine that the particles that make up both bodies pull on the astronaut's mass, attracting it. On the Moon, there are fewer particles exerting that attraction than on Earth, so the force of gravity is less.

Ruth Lazkoz holds a PhD in physics and is a professor at the University of the Basque Country.

Coordination and writing: Victoria Toro .

Question sent via email by Francisco Gómez .

Nosotras Respondemos is a weekly science consultation, sponsored by the L'Oréal-Unesco 'For Women in Science' program and Bristol Myers Squibb , which answers readers' questions about science and technology. These questions are answered by scientists and technologists, members of AMIT (Association of Women Researchers and Technologists). Send your questions to [email protected] or via X #nosotrasrespondemos.