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Question :
76. A model rocket sits the launch pad until its : 2102131

76. A model rocket sits on the launch pad until its fuel is ignited, blasting the rocket upward. During the short time of blast-off, as the ignited fuel goes down, the rocket goes up because:

A. the fuel pushes on the ground.

B. air friction pushes on the escaping fuel.

C. the downward force of gravity is less than the downward momentum of the fuel.

D. of none of the above reasons.

77. At liftoff, the engines of the Saturn V rocket consumed 13 000 kg/s of fuel and exhausted the combustion products at 2 900 m/s. What was the total upward force (thrust) provided by the engines?

A. 3.77 ´ 10^{7} N

B. 7.54 ´ 10^{7 }N

C. 1.47 ´ 10^{8} N

D. 2.95 ´ 10^{8 }N

78. Neglecting gravity, doubling the exhaust velocity from a single stage rocket initially at rest changes the final velocity attainable by what factor? Assume all other variables, such as the mass of the rocket and the mass of the fuel, do not change.

A. The final velocity stays the same.

B. The final velocity doubles.

C. The final velocity increases by a factor of 0.693.

D. The final velocity increases by a factor of 0.310.

79. Neglecting gravity, doubling the exhaust velocity from a single stage rocket initially at rest changes the final kinetic energy of the burnout stage by what factor? Assume all other variables, such as the mass of the rocket and the mass of the fuel, do not change.

A. It is the same.

B. It doubles.

C. It quadruples.

D. It increases by a factor of 1.693.

80. A rocket of total mass M and with burnout mass 0.20 M attains a speed of 3 200 m/s after starting from rest in deep space. What is the exhaust velocity of the rocket?

A. 1 000 m/s

B. 2 000 m/s

C. 3 000 m/s

D. 4 000 m/s