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M3: Help particles moving towards/away from Earth watch

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    For solving Newton's law of gravitation to solve problems involving a particle moving away from/towards the Earth, i don't get which way the +/- signs go when working out the speed of a rocket at a certain distance from the earth.

    Eg, for question 3b from M3 June 2007 (http://www.freeexampapers.com/get_pa...M3+2007-06.pdf), if i say +ve is the direction from the rocket towards the earth, and the rocket is moving towards the earth, then using newtons second law, isn't the force acting upon the rocket and the acceleration experienced by it both positive? i know that is wrong, but can someone please explain why one is positive and one is negative? thanks!
 
 
 
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Updated: March 23, 2011
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