And today's question.

How did Newton quantify gravity?

What we already know

Gravity is a weak force, Newton guessed (correctly) that that same force that made the apple fall from the tree kept the moon in orbit around the Earth


  • The best way to think about this is that you are just falling inside an airplane that is also falling
  • So the net acceleration due to gravity is zero.
  • What about your mass? Has this changed?

So when you 'weigh 50kg' is this correct, scientifically speaking?

'Weight' is a Force, measured in newtons (N)


So 'weight' on Earth is proportional to mass, and most scales use this proportionality to give your 'mass' instead of your 'weight' 

If F=ma, what is the 'a' in this formula?

a is acceleration due to gravity, given the symbol g



g=9.8 ms^{-2}
g=9.8ms2g=9.8 ms^{-2}

Newtons law of Universal Gravitation


So 'g' can be different on different planets and stars - more mass, higher acceleration

Year 8 Gravity and the four forces of nature

By Gavin King

Year 8 Gravity and the four forces of nature

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