A 60 kg man holding a 20 kg box rides on a skateboard at a speed of 7 m/s. He throws the box behind him, giving it a velocity of 5 m/s. with respect to the ground. What is his velocity after throwing the object?
A
8 m/s
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B
9 m/s
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C
10 m/s
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D
11 m/s
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E
12 m/s
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Solution
The correct option is E 11 m/s Using the law of conservation of momentum, Initialmomentum=Finalmomentum(m1+m2)v=m1v1+m2v2(60+20)7=60v1+20(−5)Thus,velocityoftheman=80×7+20×560m/s=11m/s