A 5000 kg rocket is set for vertical firing. The exhaust speed is 800 ms–1. To give an initial upward acceleration of 20 ms–2, the amount of gas ejected per second to supply the needed thrust will be (g = 10 ms–2) 

(1) 127.5 kg s–1

(2) 187.5 kg s–1

(3) 185.5 kg s–1

(4) 137.5 kg s–1

Concept Questions :-

Variable mass system
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If a person with a spring balance and a body hanging from it goes up and up  in an aeroplane, then the reading of the weight of the body as indicated by the spring balance will 

(1) Go on increasing

(2) Go on decreasing

(3) First increase and then decrease

(4) Remain the same

Concept Questions :-

Spring force
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A body is imparted motion from rest to move in a straight line. If it is then obstructed by an opposite force, then 

(1) The body may necessarily change direction

(2) The body is sure to slow down

(3) The body will necessarily continue to move in the same direction at the same speed

(4) None of these

Concept Questions :-

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A second's pendulum is mounted in a rocket. Its period of oscillation decreases when the rocket 

(1) Comes down with uniform acceleration

(2) Moves round the earth in a geostationary orbit

(3) Moves up with a uniform velocity

(4) Moves up with uniform acceleration

Concept Questions :-

Application of laws
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Two balls of masses m1 and m2 are separated from each other by a powder charge placed between them. The whole system is at rest on the ground. Suddenly the powder charge explodes and masses are pushed apart. The mass m1 travels a distance s1 and stops. If the coefficients of friction between the balls and ground are same, the mass m2 stops after travelling the distance 

(1) s2=m1m2s1

(2) s2=m2m1s1

(3) s2=m12m22s1

(4) s2=m22m12s1

Concept Questions :-

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A force vector applied on a mass is represented as F=6i^8j^+10k^ and accelerates with 1 m/s2. What will be the mass of the body 

(1) 102kg

(2) 210kg

(3) 10 kg

(4) 20 kg

Concept Questions :-

Application of laws
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A cricket ball of mass 250 g collides with a bat with velocity 10 m/s and returns with the same velocity within 0.01 second. The force acted on bat is 

(1) 25 N

(2) 50 N

(3) 250 N

(4) 500 N

Concept Questions :-

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A pendulum bob of mass 50 gm is suspended from the ceiling of an elevator. The tension in the string if the elevator goes up with uniform velocity is approximately 

(1) 0.30 N

(2) 0.40 N

(3) 0.42 N

(4) 0.50 N

Concept Questions :-

Application of laws
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The average force necessary to stop a bullet of mass 20 g moving with a speed of 250 m/s, as it penetrates into the wood for a distance of 12 cm is 

(1) 2.2 × 103 N

(2) 3.2 × 103 N

(3) 4.2 × 103 N

(4) 5.2 × 103 N

Concept Questions :-

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A man measures time period of a pendulum (T) in stationary lift. If the lift moves upward with acceleration g4, then new time period will be 

(1) 2T5

(2) 5T2

(3) 52T

(4) 25T

Concept Questions :-

Application of laws
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