Obviously, we can make rockets to go very fast, but what is a reasonable top spe
ID: 1453208 • Letter: O
Question
Obviously, we can make rockets to go very fast, but what is a reasonable top speed? Assume that a rocket is fired from rest at a space station in deep space, where gravity is negligible. Assume that c = 3.00 × 108 m/s.
A)
If the rocket ejects gas at a relative speed of 2000 m/s and you want the rocket's speed eventually to be 2.00×103 c, where c is the speed of light, what fraction of the initial mass of the rocket and fuel is not fuel?
Express your answer using three significant figures.
B)
What is this fraction if the final speed is to be 3100 m/s ?
Express your answer using three significant figures.
Explanation / Answer
A. v - vo = vx*ln (mo/m)
vo = 0 ,fired from rest
v/vx = ln (mo/m)
mo/m = e^(v/vx)
m/mo = e^(-v/vx)
v = 2*10^-3*c = 6*10^5 m/sec
vx = 2000 m/sec
m/mo = e^(-6*10^5/2000)
m/mo = 5.148*10^-131
B. now v = 3100 m/sec
vx = 2000 m/sec
m/mo = e^(-3100/2000)
m/mo = 0.212
21.2 % of the total initial mass is not fuel, 78.8% is fuel.
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