Question

A bullet with mass 0.005 g is fired from a rifle of mass 5kg. The bullet has a speed of 400 m/s towards the right . HOw do I explain why the riffle jumps backwards as the bullet is fired? What is the momenetum of the bullet?

Answer 1

So,we consider that we don't know in which direction the riffle will move,and let,after the firing it will move with velocity `v`

As,during the process of firing,no external force is acting on the bullet-riffle system,we can say,linear momentum will be conserved during this process,

So, initial linear momentum = `0`(as velocity=`0` initialy)

Final momentum = `(0.005/1000) * 400 + 5*v`

So,we can say,

`0= 0.002 +5v`

or, `v= -0.0004 ms^-1`

The negative sign inidicates the riffle will move in a direction opposite to that of the bullet's velocity with the above value of velocity calculated.(i.e towards left)

And,momentum of the bullet is `0.005/1000 *400 = 0.002 Kg ms^-1`