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A disk of mass 10 gm is kept horizontaly in air by firing bullets of mass 5 g each. each at the rate of 10/s. If the bullet rebounds with which the bullets are fired is

$(a)\;49\; cm/s \quad (b)\;98\; cm/s \quad (c)\;147\; cm/s \quad(d)\;196\; cm/s$

mass of disc=$\large \frac{10}{1000}$ kg
Let speed of bullet $=v$
change in momentum per second $=\; 2 mvn$
where n is number of bullet, fired per second, m -mass of bullet
Therefore $Force$ applied by the bullets on the disk = $2mvn$
$\frac{10}{1000}$$\times g=.2 \times 10 \times \large \frac{5}{1000}$$v$
$v= 98 cm/s$
Hence b is the correct answer.
edited May 26, 2014

+1 vote