The strong man decides to try a different test. This time he stands in the stree

Question

The strong man decides to try a different test. This time he stands in the street and pushes against the car as hard as he can when it drives towards him (do not try this at home; this is a thought experiment!). Although he is pushed back, this time he is able to stop the car after a few seconds. Why was he able to stop the car this time? A big 8 kg fish swimming at 3 m/s opens its mouth and swallows a small 2 kg fish swimming away from it at 1 m/s. Presume water resistance is negligible. Show your calculations. What is the momentum of the two fish system after the smaller fish has been swallowed? What is the speed of the two fish system after the smaller fish has been swallowed?

Explanation / Answer

( A)

Momentum of big fish P = MV = ( 8 kg ) ( 3 m/s ) = 24 kg. m/s

Momentum of small fish p = mv = ( 2 kg ) ( 1 m/s ) = 2 kg. m/s

So, the total momentum of the system of two fishes

P + p = 24 kg.m/s + 2 kg.m/s = 26 kg .m/s

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Use Conservation of Momentum

Momentum Before = Momentum After

Momentum of two fish = ( total mass of the fish ) ( Speed of the two fish after the smaller fish has been swallowed )

26 kg .m/s = ( 8 + 2 ) * v

Solve for velocity v

v = 2.6 m/s

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