(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

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Gravitational force quick doubt

**Physics Forums | Science Articles, Homework Help, Discussion**