The velocity of the rocket is 7.8 m/s
Explanation:
We can solve the problem by using the law of conservation of momentum: in fact, in absence of external forces, the total momentum of the rocket+fuel system must be conserved.
Before the launch, the total momentum of the system is zero, since the rocket and the fuel are at rest:
After the launch, the total momentum is
where
M = 4.0 kg is the mass of the rocket
V is the velocity of the rocket
m = 50.0 g = 0.050 kg is the mass of the fuel ejected
v = -625 m/s is the velocity of the fuel (taking "backward" as negative direction)
Since the total momentum is conserved, we have
So we can solve the equation to find V, the velocity of the rocket:
And the positive sign means the rocket moves forward.
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