Teachers Notes

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Of all the forces in nature, Gravity is the strangest (and is the force that causes Physicists the greatest problem when trying to find a unified theory of Physics linking General Relativity to Quantum Mechanics). It's also strange because it is so weak (as an example, you can lift up a feather with your hand and overcome the entire gravity of the Earth) and it only acts in one direction (towards the centre of the object with Gravity; more about this below).

 

At GCSE the important thing to understand is, the more massive an object is (i.e. the greater its mass), the greater the gravitational field strength (g). This is based on another law of Newton; the law of universal gravity.

 

Although beyond the scope of KS4, for teachers and students who want to study KS5 A Level physics the formula is as follows:

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F=Gm1m2/r  where:

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    F is the force between the two masses m1 and m2

    r is the distance between the two masses m1 and m2

    G is the gravitational constant

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As F=ma (Newton’s second law) where a, in this instance, is g (the gravitational field strength) and m is either m1 or m2 (depending on which mass you want to calculate g for) we end up with the formula:

g=Gmx/r   where x=the mass of the object being accelerated in the gravitational field of the other mass (in fact each mass has its own gravitational field strength so g, in this instance, is the net acceleration between the two masses).

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There is a great explanation here

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When discussing orbits we need to understanding centripetal acceleration but here is a summary of how we think students can best understand how objects orbit other objects in a gravitational field.

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If somebody fires a gun then there are two components of the bullets motion we need to consider; its horizontal motion and its vertical downward motion. Remember, because the horizontal motion and the vertical motion are perpendicular to one another, then the forces acting on them are independent of one another.

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Lets consider its downward motion first. As this is independent of the horizontal motion, irrespective of how fast the bullet is fired horizontally, it will fall to earth at the same rate (F=mg) which means it will hit the earth at the same time irrespective of how powerful the gun is firing the bullet horizontally.

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Now lets consider the bullets horizontal motion (and ignore friction). As the time it falls to earth is the same irrespective of the horizontal speed of the bullet then the faster the bullet travels horizontally, the further it will travel before it hits the earth. But imagine if you could fire the bullet fast enough so that it traveled all the way round the earth before it fell to ground. And then imagine you fire the bullet so fast that the bullet arrived back at the point you fired it but still at the same height as you fired it (making sure you have stepped out of the way of course!!!). In this example, the bullet is falling at the same rate in which it is travelling horizontally so the bullet is now in orbit. In essence the bullet is 'falling around the earth permanently' which is exactly how satellites and the moon 'fall' around the the earth and how the earth (and all the other planets) 'fall' around the sun.

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If you have any queries then please email us.

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