System of Particles and Rotational Motion Test

Result

System of Particles and Rotational Motion Test - 11

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Question 1
1 / -0

A jet airplane travelling at the speed of 500 kmh^–1ejects its products of combustion at the speed of 1500 km h^–1 relative to the jet plane. What is the speed of the combustion with respect to an observer on ground?

A
-1000 km/h

B
-2550 km/h

C
-1250 km/h

D
-2000 km/h

Solution

Speed of the jet airplane, v_jet = 500 km/h Relative speed of its products of combustion with respect to the plane, v_smoke = – 1500 km/h

Speed of its products of combustion with respect to the ground = v′_smoke

Relative speed of its products of combustion with respect to the airplane, v_smoke = v′_smoke – v_jet

=> – 1500 = v′_smoke – 500

=> v′_smoke = – 1000 km/h

The negative sign indicates that the direction of its products of combustion is opposite to the direction of motion of the jet airplane.
Question 2
1 / -0

If a gymnast sitting on a rotating stool with his arms outstretched, suddenly lowers his hands

A
the angular momentum increases

B
the angular velocity stays constant

C
his moment of inertia decreases

D
the angular velocity decreases

Solution

When gymnast lowers his hand the distance of mass from rotational axis decrease. Hence his moment of inertia decreases and angular velocity increase to conserve angular momentum.