Geometrica
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Hi everyone!
I just don't get part b), please help!

The Earth may be considered to be a uniform sphere of radius 6380 km with its mass of 5.98 × 1024 kg concentrated at its centre.

A mass of 1.00 kg on the Equator rotates about the axis of the Earth with a period of 8.62 × 104 s.

(a) Calculate, to three significant figures,

i) the gravitational force FG of attraction between the mass and the Earth,
=9.80 N

ii) the centripetal force FC on the 1.00 kg mass,
=0.0339 N

iii) the difference in magnitude of the forces.
=FG – FC = 9.77 N

Now what I don't get is why FG and FC are not added for part b). Don't both forces act in the same direction?

b) By reference to your answers in (a), suggest, with a reason, a value for the acceleration of free fall at the Equator.
=9.77 ms-2

Any help appreciated!
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Joinedup
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Well the word difference is a clue that the examiner wants you to subtract. mathematically finding the difference means subtracting.

The centrepetal force isn't a force that's created by objects in circular motion... it's the force that causes the object to be in circular motion instead of flying off on a straight line path into infinity. the object doesn't fly off in a straight line because it's stuck to the earths surface by gravity (mg) but the contact force is less than mg in magnitude because part of mg is being used to keep the object in circular motion.
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Geometrica
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(Original post by Joinedup)
Well the word difference is a clue that the examiner wants you to subtract. mathematically finding the difference means subtracting.

The centrepetal force isn't a force that's created by objects in circular motion... it's the force that causes the object to be in circular motion instead of flying off on a straight line path into infinity. the object doesn't fly off in a straight line because it's stuck to the earths surface by gravity (mg) but the contact force is less than mg in magnitude because part of mg is being used to keep the object in circular motion.
Oh, right! Thank you for explaining that, it was really bugging me
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