(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A spacecraft S of mass m moves in a straight line towards the centre of the earth. The earth

is modelled as a fixed sphere of radius R. When S is at a distance x from the centre of the

earth, the force exerted by the earth on S is directed towards the centre of the earth and has

magnitude k/x^{2}, where k is a constant.

(a) Show that k = mgR^{2.}

(2)

Given that S starts from rest when its distance from the centre of the earth is 2R, and that

air resistance can be ignored,

(b) find the speed of S as it crashes into the surface of the earth.

3. The attempt at a solution

I just have a qucick doubt on part B , I do get how to the question except the

markscheme states [itex]a=-\frac{k}{x^2}[/itex] where is the minus sign comming from , surely S is moving towards the earth so isnt it in the same direction as the force?

Thanks

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# Homework Help: Gravitational force quick doubt

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