Question

What is the value of "G" and "g" on the moon 
solve it clearly and short because it is 2 marks question
also 
If the net external force acting on a body is 0 then the total momentum is ________
a) 0    b) variable    c) constant    d) information not sufficient

Answer 1

Gravitational force of attraction on the Moon is given by

$F_g = \frac{G\ M_m\ m }{d^2}\\ \\G = universal gravitational constant = 6.67 * 10^{-11} N-m^2/kg^2\\ M_m=mass\ of\ moon\\m=mass\ of\ a\ body\ on\ moon\\d=distance\ of\ mass\ m\ from\ center\ of\ moon\\\\G\ is\ same\ anywhere.\\ \\g_{moon}=\frac{GM_m}{R^2},\ \ R=Radius\ of\ Moon$

g on the Moon is found to be around 1/6th of g on the Earth.  So it is around 9.81/6 m/sec²
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If the net external force on a body is zero, it means the body has an acceleration of 0.  It means the velocity is constant (it can be zero also).  So linear momentum is a constant.over time.   It can be zero also.  

==> Force = rate of change in momentum. = 0.  SO momentum is a constant.

COrrect answer is   c) constant.