Which of the following operates strictly on the principle of conservation of linear momentum?
A. Helicopter
B. Rocket
C. Hovercraft
D. Aeroplane
Answer: Option B
Solution (By JKSSB Mock Tests)
A rocket works on the principle of conservation of linear momentum. As the exhaust gases are expelled downward at high velocity, they carry downward momentum. To conserve the total momentum of the system (which is initially zero), the rocket must acquire an equal and opposite (upward) momentum, propelling it forward.
Explanation:
At highest point velocity is zero, but acceleration due to gravity is still g (9.8 m/s²) downward. This causes it to fall back. Acceleration constant throughout free fall.
Two slabs of equal thickness and cross-sectional area have thermal conductivities K1 and K2. If they are joined in series, the equivalent thermal conductivity of the combination is:
Explanation:
For slabs in series, the rate of heat flow (H) is the same. Total resistance R_eq = R1 + R2. Since thermal resistance R = L / (KA), and total length is 2L: (2L) / (K_eq * A) = (L / K1A) + (L / K2A). Canceling L/A yields 2/K_eq = 1/K1 + 1/K2. Solving gives K_eq = 2K1K2 / (K1 + K2).
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