Universe Guide

Gravity, the Force of Nature that Keeps us Grounded

Gravity is one of the four fundamental forces of nature, the others are Electromagnetic, Weak Nuclear and Strong Nuclear. Even though 'Weak Nuclear' can imply to some that it is the weakest of the bunch, Gravity is actually the weakest of the bunch. You can see how weak gravity is by just picking up an object off a desk. 5. At the start of the Universe, it is theorized that all four forces were equal, even part of the same force but as the Big Bang took off, the four forces split apart.

Every piece of matter has gravity, even the molecules that make up the human body has gravity. The gravity that is produced by the human body is so negligible, it is not registerable. The bigger an item is, the stronger the gravity field it has. the Sun has the greatest gravity field in followed by Jupiter, the biggest planet in the solar system.

Whilst most things are held down in the atmosphere by gravity, helium (he) is unaffected by gravity and when it is free, it escapes the Earths atmosphere to never return. There is a finite amount of helium which when the bulk of the helium goes, they'll be no helium balloons and no coolants for MRSI scans unless of course they come up with a replacement. Gravity is also the name of a film starring George Clooney and Sandra Bullock, trapped in space after a debris field destroys their space shuttle.

Same Height, Different Weight

If you had two different sized and weighted objects and you dropped them at the same height, both objects would land at the same time. You would probably think that the heavier one would fall quicker but its not true. You would need to factor in drag and air resistance when doing the experiment, you couldn't compare the dropping of a piece of paper with a pen as the paper has more size and affected by air. Physics Classroom

Why does the Earth not get pulled into the Sun?

The simple answer is that any object that is moving does not fall. If you've ever seen the motorcycle stunt 'Wall of Death' where cyclists ride a bike up a cylindrical wall and not fall down, that's because they are moving. Planes also use this principle which is why they can take off and fly. This is the same principle why the Earth is not sucked into the Sun. It is also the reason why stars aren't sucked into the supermassive black holes at the centres of galaxies, the stars in the galaxies orbit round the centre of the galaxy. If you had thought the Sun was static in its location, I hope I've enlightened you.

What is gravity like on the other planets such as Mars and Moon

The strength of an object in space's gravity is directly proportional to the size of the object. The bigger an object is, the stronger its gravity is. The Earth has a gravity that is 2.6 times the gravity that is on Mars. 1 or to look at it another way, the Mars has .375 the strength of gravity that Earth has. If you weighed yourself on Earth and you was 100 kilograms then on Mars, you would weigh 37.5 kilograms. The human body might struggle to adapt to the weak gravitational strength of the planet.

The Moon is a quarter the size of the Earth but its gravitational pull is about 16%, in other words, if you weighed yourself on earth, and you weighed 100 kilograms, on the moon you would weigh 16 kilograms. If you're curious about your weight on other planets, moons and some other astronomical object, Exploratorium has a brilliant page to help you answer the questions. When you watch television clips of people walking on the moon, you see them bouncing on the atmosphere, they are just walking, its because the gravitational pull is so low that they bounce. The Moon exerts a gravitational pull on the oceans and water which is why when there is a full moon, there is high tide.

The Earth is loosing its grip on the Moon, the Moon is currently moving away from Earth at a rate of 3.8 centimeters a year 4 and whilst at the moment, it is bigger enough to block out the Sun during an eclipse. In millions of years time, the Moon will not block it out as it does now. In years past, the Moon would have been bigger in the sky than it is now. Also as the Sun grows older, it will have used up a lot of its fuel and will grow, it will probably loose its gravitational power and the Earth will move from its celestial position.

The heavier you are in terms of relation to earth, the stronger you would need to be. If an alien came from a planet where their gravity was three times the strength of Earths, they would be much stronger than us, they would have no problem lifting things we couldn't. However if the alien stayed on our planet for long enough and didn't exercise, their muscle would probably waste away. Likewise, if we went to their planet, we would be much stronger than them if other planet was bigger than ours. If you ever see the film John Carter, John Carter is able to jump higher, further and lift heavier weights than the Barsoomians because Earth is bigger than Mars and therefore our muscles are stronger than theirs. Likewise, Clark Kent's birth planet Krypton would be many times bigger than our planet.

What are the effects of zero-gravity?

In the short term, there are no real effects but if you're going to be going up for a long time such as astronauts on the International Space Station, the effects can be serious. Without gravity, the muscles in your body begin to waste away as there's nothing to "motivate" the muscle. To compensate for zero-gravity, astronauts exercise every day to counter the damaging effects.

What is escape velocity?

Escape velocity is the speed at which an object such as a rocket needs to go to escape a planet or an objects gravitational pull. For the Earth, the escape velocity is a meagre 7 miles a second or about 25,000mph 2. It is meagre because compared to the Sun, you would need to be travelling at 1381600.8 mph to escape the Sun's pull. To escape the pull of a black hole would be impossible because they say not even light can escape the pull of a black hole. On the moon, you need less power to break free of the atmosphere, about 5320.73mph which was what the astronauts of the Apollo space missions had no trouble with.

Gravity on Super-Earths

There are Exoplanets out there that have been described as being Super-Earths, they are planets that are bigger than Earth but smaller than Neptune. It is assumed that rocky planets will never be as a large as a gas planet. A rocky planet has all its molecules tightly together whereas a gas planet is larger because its molecules are in a gas form. They could be rocky planets or gas planets. If for this section, we travelled to Olanis, the planet on the Exoplanets page and that planet was about say three times bigger than Earth, we wouldn't be able to land on the planet. Our puny human bodies wouldn't be able to cope with the planets gravity. We would have to live above the planet and send robots down to bring up supplies. Those robots would have to be able to deal with the gravity as well. Aliens from a super earth would be stronger than us so we would be advised not to get into a bar-fight with an alien.

Gravity on Spaceships

When we watch television of space ships like the Millennium Falcon or the U.S.S. Enterprise, we see people walking about the craft normally as they would do on Earth because the ships have their own gravity. However, in real-life, we have not yet discovered how to create an artificial gravitational field for use in space crafts. For a journey to Mars, the Red Planet which would take over a year, a year in weightlessness using current technology. The Tardis, Doctor Who's space ship is infinite in size and has a collapsing star as its energy source so that could have have gravity but even that needs a little imagination.

Discovery of Gravity

Gravity was first discovered by the British scientist Sir Isaac Newton who as the story goes saw an apple fall from the tree. This made him wonder why the apple falls, why does it not just hang there or go up. His most noted work is the Philosophiae Naturalis Principia Mathematica ("Mathematical Principles of Natural Philosophy") which in it, he lays the foundations for laws of motion and universal gravitation. His works would later be used by future scientists such as Albert Einstein who both would expand on his theories.

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