KVPY 2019 SA Exam Previous Papers

Question : 67 of 80

Marks: +1, -0

In an hour–glass approximately 100 grains of sand fall per second (starting from rest), and it takes 2 sec. for each sand particle to reach the bottom of the hour–glass. If the average mass of each sand particle is 0.2 g then the average force exerted by the falling sand on the bottom of the hour–glass is close to :

0.4 N
0.8 N
1.2 N
1.6 N

Validate

Solution:

Velocity of sand particle just before striking the bottom is v = u + at
v = 0 + 10 × 2 = 20 m/s

p_i

= (0.2 × 10

^{–3}

) × 20

p_\f = 0

|∆p| = 4 × 10^{–3 }

k–m/s

\f_\avg={|∆p| }/{ ∆t }×n

={4×10^{-3}×100}/1

= 0.4 N

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