崔屁屁 asked in 科學其他:科學 · 1 decade ago

A rocket exhausts fuel with a

A rocket exhausts fuel with a velocity of 1500m/s, relative to the rocket. It starts from rest in outer space with fuel comprising 80 per cent of the total mass. When all the fuel has been exhausted its speed is:

A. 3600m/s

B. 2400m/s

C. 1200m/s

D. 880m/s

E. 400m/s

ans: B

1 Answer

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  • LI
    Lv 7
    1 decade ago
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    設以rocket速度方向為正:

    A rocket exhausts fuel with a velocity of 1500m /s, relative to the rocket.

    →V(fuel/rocket)=V(fuel)-V(rocket)= -1500 (因fuel遠離rocket)

    →V(fuel) = V(rocket)-1500

    動量守恆:

    V(fuel)*( 0.8M )+ V(rocket)*( 0.2M ) =0 (因為It starts from rest.)

    0.8[V(rocket)-1500]+ 0.2V(rocket) =0

    V(rocket) = 1200 (m/s)→C

    答案B應該不合理

    若rocket速度為2400,則fuel速度為2400-1500=900

    →兩者速度同方向

    動量不會為0(因為It starts from rest.)

    如果思考錯了

    請各路高手指正

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