Gravity

Gravity is the force that attracts everything with mass. To escape the gravitational pull of another object, you need to achieve escape velocity. To have a stable orbit of an object, you need orbital velocity.

Gravity weakens or strengthens

Gravitational pull weakens over distance. When the distance is twice further from the centre, for example, you are standing 6,371 km from the Earth’s surface, you will experience gravity being weakened 4 times. While you fall at an acceleration of 9.8 m/s2 on the surface, you accelerate for 2.45 m/s2 from that distance. Gravitational pull strengthens over distance, too. If you are halfway through the centre of the Earth, you will fall 4 times faster than on the ground.

How Gravity works

Gravity also keeps things in the Universe in orbit, and it is the only connection to celestial bodies’ locations. Gravity is why the Moon orbits the Earth, why Earth can have its atmosphere, oceans and layers in place, why tides occur, why planets orbit the Sun, why stars can exist… That’s a lot of things, also life is thanks to gravity. Gravity is so important, and it is one of the four fundamental forces in the Universe.

Free-fall

And why can’t astronauts in the ISS feel Earth is pulling on it? That is because are actually in a free-fall state where they can’t feel gravity because gravity is the only force acting on them. There is no force to counteract with it so we can’t feel gravity. This phenomenon is called weightlessness or microgravity.

Not only skydiving can bring you into a free-fall state, orbiting can too. Actually, Moon is falling to Earth, Earth is falling to Sun, and Sun is falling into Sagittarius A*! They fall around, not fall into, is because they have reached a stable speed. Do you remember orbital velocity? That’s the case!

Extreme Examples

Neutron stars and black holes are extremely dense, so they have tremendously strong gravitational pulls. A neutron star’s gravitational pull can be billions of times stronger than Earth’s and their escape velocity is a third to half the speed of light! Black Holes are more extreme. Their escape velocity exceeds the speed of light, which nothing can escape when they go into the event horizon, the boundary of the black hole. You could escape in a safe distance from the event horizon, so this is the phenomenon of weakening escape velocity.

Tidal forces and Roche Limit

Gravitational pull is also life-threatening if it gets so strong, especially the tidal forces, which gravity makes an object to become an ovoid, or in extreme cases like a black hole, they will cause spaghettification in an unsafe distance.

There is also a limit called Roche limit, which when a celestial body reaches its limit, its own gravity cannot withstand the tidal forces acting on it, causing them to break into small pieces, and possibly form a ring around it. In fact, Saturn’s rings might form because a moon of it reaches the Roche limit.

Gravitational lensing

There is also a phenomenon called gravitational lensing, which happens by bending light using gravity. Some exoplanets are discovered by microscopic gravitational lensing caused by the planet itself.

Curvature of spacetime

Gravity is also the curvature of spacetime, which is why the time you feel on Earth, or near a black hole, is actually slower than what you feel in space with almost no attractions. Gravitational waves occur when something like two black holes merge, and it sends curvature of spacetime throughout the Universe.