Expert: Jayendra Upadhye - 10/15/2007

Question
Hi Jayen, I have a doubt in rocket propulsion. I have studied and heared that earth's gravity is a weak force when compared to other forces. My question is, if gravity is a weak force, that why a rocket needs that much fuel to propell itself to outer space. Is this effect happens as per Einstein relativity equation. As the speed increases, mass also increases along with it.
Please clarify my doubts. Thanking you in expectation.
Cheers
Mohan

Answer
Hi,
How old are you Mohan?
The reason i ask is that if you are at inter college level, it would mean you need serious brushing up to do, and if you are at ssc level, you have courage to ask such a question and expose your lack of understanding.
Very few people have that courage.
So I commend you for doing so.

Fundamentals first.
Strong and Weak are relative terms.

If rockets have to toil so much, imagine what people would have to do to work against the strong forces (Nuclear foces that bind protons together within a nucleus despite their strong electrical repulsion to each other at such a close range)!

Rockets designed currently travel so slowly, that einstein's mass increase does not come in at all.

what is 11.665 km / sec (earth's escape velocity) against 300,000 km/sec which is the speed of light?

Rockets have to toild due to 2 things:-
1 - As they have to carry all their fuel supply with them, they have to lift the "payload" + weight of all the fuel they will ever use, all at the same time!
[Often fuel weight far exceeds the weight of the "pay load"!!]

2 - Though gravity is a weak force, it extends over huge distances!
Gravitation has what people call "staying power / stamina!! It tirelessly ceaselessly does its work!
Eventually causing even demise of great stars...(black holes)..
A shell / missile fired directly vertically at speed less than 11.665 km/sec, will eventually stop, reverse its direction and head earth wards, hitting it with a great speed. (may not be same speed due to bidirectional dissipative loss processes such as fricton.

Rockets work on a law of diminishing returns..the greater the  weight of the payload, the higher the weight of the fuel, the more is fuel burnt per meter rise for the payload.

This is what happens to stars too. The more massive the star, the faster it has to burn its hydrogen to sustain its great dead weight from falling inwards! Blue giants die in millions of years, pale small stars last billions of years!

So what is considered a weak force, is actually "very strong" when humans are considered.

Hope that clears your thoughts!
Jayen