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How gravity works Essay

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“Gravity is a force of attraction that exists between any two masses, any two bodies, any two particles. Gravity is not just the attraction between objects and the Earth. It is the attraction that exists between all objects”. (NASA gravity 2000) Gravity is a very essential part of life on earth. We are in the exact position in the sun’s gravitational field to keep us in orbit and keep life ongoing on our planet. Gravity is a downward force on any body of mass, the larger the mass of the body, the stronger the gravitational force. On earth the force is 9.8 N/Kg (Newton’s per Kilogram) roughly, depending on where you are on earth.


The original idea of gravity was discovered by Sir Isaac Newton (Jacob 1999). It is said that Newton was sitting under an apple tree and an apple fell from the tree striking him on the head. (What probably happened was that he watched an apple fall from a tree.) He concluded that there must be a force pulling all objects towards the ground, such as the apple and himself, and because the apple started at rest, there must be acceleration due to that force. Later with his invention of the telescope, he discovered that larger bodies have a stronger gravitational pull. It had been previously proven that the earth and other planets orbited around the sun. Because the sun was the largest body in the solar system, Newton concluded that larger bodies had larger and stronger gravitational fields.

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As seen above, gravity is a force affecting 2 bodies. This is not totally exact because it does not factor in air resistance and friction. Even though gravitational acceleration is constant, air resistance affects free fall. For instance, if you took a sky diver and a ball of lead that weighed the same, and dropped them from an airplane at the same time, the sky diver could spread his arms, which would cause resistance and slow his fall. The lead ball would reach the ground first. If the skydiver streamlined himself and dove head first, he would hit the ground first. It all depends on air resistance.

Gravity has a lot to do with the mass of the object that produces it(i.e. Earth’s affect on Newton’s apple). The larger the object (Earth) the bigger the force.

Weight is another thing affected by gravity. The weight of an object is different on different planets. Weight is a factor of mass. The gravity of the moon is about 1/6 that of the Earths. ( Earths is 9.8 kg/N the Moons is1.6 kg/N) (NASA Gravity) To determine the weight of someone, you would multiply their mass by the gravity of wherever they are. So, someone with a mass of 15kg would weigh 147kg on Earth and 24kg on the moon. This explains weightlessness in space. In space there is no gravity, so you have no weight.


At any time on earth there are 2 high tides and 2 low tides, each directly across from each other, as shown in the diagram above. In this diagram,(from Physics for a modern world 1986), points 1 and 2 are at high tide and points 3 and 4 are at low tide. The force of the moon on the earth complies with normal laws of gravity. While the moon pulls strongly upwards (away from the earth) on the water closest to itself, it pulls the water on the far side of the earth downwards (towards the earth). The earth however, has a MUCH stronger force of gravity which keeps the ocean from falling off (Gravity and Tides 2000). So from this we might expect that the oceans would simply bulge up on the Moonward side of the Earth.

But this is not the full story. Both the Moon and Earth exert forces on each other as well. The downward force exerted by the Earth on the Moon pulls the Moon toward it, causing it to fall inward. But, because the Earth is always moving sideways, the Moon will never fall on us. As the Earth moves sideways, or away from the moon, the Moon is falling towards Earth, this keeps them relatively the same distance apart at all times. In other words, the moon orbits around the earth. But although we usually talk about the Moon orbiting the Earth,

“it (the moon) also exerts a gravitational force on us (earth) which in turn orbits about the Moon. In fact, both orbit about their common center of mass, which is within the Earth but not at its center. This rotation causes an upward centrifugal force on the oceans, which is proportional to the rotational speed and the distance from the center of mass. This force is greater on the far side then the near side” (Gravity and Tides 2000)

When you add this to the moons gravitational force, they equal. This causes both sides of the ocean to bulge as illustrated in the diagram above causing 2 high tides, about 12h and 25 minutes apart.

Relatively the same idea applies to the effects of the Sun on Earth, though the centrifugal force on the near side of earth is downward, not upward. As it turns out, the force of the sun is about half the force of the moon. We get the high spring tides when the Earth, Sun and Moon are in a straight line and their forces are combined. After 7 days when they are at right angles to each other they cancel each others forces giving the low neep tides.

A very important factor about tides that is not included in the above, is the height of the ocean floor. Different heights can cause funneling of the tidal flow, heaping the tides up in shallow water and causing even more complex effects where flows meet.


Gravity is an essential to life. It is also one of the four key physical forces which are, Gravity, Electromagnitivity, Strong Nuclear Force and Weak Nuclear Force (Physics for a Modern World 1986). It controls a lot of things here on earth like tides and weight. Without gravity, life would be very different. How would you eat food from a plate? How would you pee in a toilet? The significance of the helium ballon would cease to exist. And if you flip your pancakes would you ever see them again? All in all, gravity is very important to life and hats off to Newton for discovering it.

(Some Pictures are Missing in this… See original Document)


Gravitation, an Encarta Encyclopedia Article Titled Gravitation

(December 18 2000)


December 18 2000

Hirsch, A (1986) Physics For a Modern World

Canada, John Wiley & Sons Canada Limited

Gravity and Tides

(December 19 2000)


December 19 2000

Nasa Gravity

(Decemer 17 2000)


December 17 2000

Jacob, M C. (1999). Newton, Sir Isaac. In World Book multimedia Encyclopedia [CD-ROM]. Sandiago California, United States of America IBM/World Book, Inc

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