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Question

# A surface is hit elastically and normally by n balls per unit time. All the balls have same mass m and moves with the same velocity u. The force on the surface is :

A
1/2 mnu2
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B
mnu2
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C
2 mnu2
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D
2 mnu
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Solution

## The correct option is A 2 mnuHere, a surface is hit elastically and normally by n balls per unit time. The mass of n balls is mn and velocity of each ball is u. The momentum of balls before collision is mnu and after collision is −mnuNow, the force exerted by the balls on the surface is F=ΔptF=mnu−(−mnu)1=2mnu

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