Ask A Scientist© Astronomy Archive

  
>
>name       rk
>  >  status     other
>  >  age        50s
>  >  EMAIL
>  >  Question - How does it explain that we have to do only 190,000 miles
>  > on way to moon at speeds like 225,000 miles an hour or so and beyond that
>  > moon's graVITY PULLS IN.pLEASE EXPLAIN BASED ON NEWTON'S LAW OF GRAVITY.
>
>The book by ABC science editor says,we have only to worry about first
>190,000 miles,rest 50,000 miles are easy to cover since Moon's gravity pulls
>in the mooncraft.I am asking How Newton's laws come in this;how the relation
>m1xm2/r2 plays in this.Thanks.RK VARMA,PE.
>
>Newton's Law of gravity is F =G mM/R^2 , where G is the gravitational 
>constant, m and M are the masses of the bodies being considered, and R is 
>the distance separating the masses m and M.
>
>Gravity obeys the law of superposition, i.e. the total effect of gravity 
>on a body, say M, is the sum of contributions from all the other masses in 
>the Universe. Of course, practically, the distance R is so large that only 
>a few bodies have an effect on M. So the net force of gravity on a 
>mooncraft M is the resultant of
>of two major forces: Fearth in one direction and Fmoon in the opposite 
>direction. All the other planets, stars, etc. have only a minor effect on M.
>
>Once the mooncraft reaches a certain distance from the earth, Fmoon > 
>Fearth so the tug of the earth is smaller than that of the moon, so the 
>craft can just "coast" the rest of the way. In fact, it will accelerate as 
>the craft approaches the moon in a fashion analogous to what happens to a 
>rock when it is thrown up in the air. It goes up so far then falls back 
>(coasts) to earth.
>
>This gravitational attraction is used to "slingshot" a rocket around the 
>moon and off into outer space.
>
>  Vince Calder
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