Why The Sun Will Never Fall


The Sunday Nation


23 January 2005


What force keeps the Earth moving around the Sun? Why doesn’t it stop? And how does the Sun float in space with nothing to support it and prevent it from falling? These questions can be answered if think about a simple experiment:

Everyone knows that the harder you throw an object the further away it will land.  If you fling a stone horizontally at 80km/h, it will hit the ground just over 12.5 metres away. If the speed is doubled (160km/h), the landing distance also doubles to 25m.

Now suppose you mounted a powerful cannon on top of mount Kenya aiming horizontally. Bearing in mind that the Earth is round, is it possible to shoot the cannonball so fast that it goes round the planet and comes back to the mountain from the opposite side? Yes! Theoretically, if it were fired at an initial speed of 29,000km/h, the cannonball would never land. At this speed, it would orbit the earth and return to the starting point in 83 minutes.

In reality, however, the cannonball slows down as it flies through the air. It would therefore land somewhere halfway around the Earth. But if the experiment were repeated from a very high altitude - where there is virtually no air at all - the ball would go round the planet.

This is how satellites manage to stay up is space for many years without ever needing fuel. In the same way, the Earth keeps moving round the Sun in empty space. It does not need to be pushed by anything because there is nothing slowing it down.

Going back to the stone throwing experiment, what if we aim vertically upwards? The results are also well known - the harder you throw, the higher the stone will reach. 80km/h will achieve a height of 25m. But in this case, when the initial speed is doubled, the maximum height does not double - 160km/h will achieve almost 100m.

The maximum height increases as the square of the initial velocity. Thus when the speed is doubled, the highest distance increases by a factor of four (two squared) - 25 x 4 = 100m. For this reason, it is easy to throw objects to great distances in the sky.

But there is another complication: the force of gravity decreases as one goes further away from the Earth. At 2,700km in the sky, the gravitational pull is half that on the ground. And every 2,700km after that, the force decreases by half. Therefore, if an object gains height at a rate greater than the decrease in gravity, it would never stop moving. For this to happen, the object must start off at a minimum of 40,000km/h. At the speed, it would never fall back to Earth.

Finally, how does the sun float in space without falling? The answer to that lies in another question: where would it fall? Nowhere! Since there is no place to fall then it does not fall.

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