Question

A bullet of mass 7g is fired into a block of metal weighing 7kg.The block is free to move.After the impact the velocity of the bullet and the block is 0.7 m/s.What is the initial velocity of the bullet? Class 11

Answer 1

Look into the photo and understand the answer.

Answer 2

Answer:

Speed of the bullet before the impact is 700.7 m/s.

Explanation:

Conservation of linear momentum:

The conservation of linear momentum is defined as the initial momentum before the impact and the final momentum after the impact is conserved, if the net work done by the external forces on the system of masses should be zero.

Given,

• Mass of the bullet = $m_1\ =\ 7\ g\ =\ 0.007\ kg$
• Mass of the block = $m_2\ =\ 7\ kg$
• Final speed of the block and the bullet = $v\ =\ 0.7 m/s.$

Let u be the initial speed of the bullet after the impact. And the block is initially at rest.

From the conservation of linear momentum,

Initial momentum of the system of masses = final momentum of the system of masses.

$\therefore m_1u_1\ +\ m_2u_2\ =\ (m_1\ +\ m_2)v\\\Rightarrow m_1u\ +\ 0\ +\ (m_1\ +\ m_2)v\\\Rightarrow u\ =\ \dfrac{(m_1\ +\ m_2)v}{m_1}\\\Rightarrow u\ =\ \dfrac{(0.007\ +\ 7)0.7}{0.007}\\\Rightarrow u\ =\ 700.7\ m/s.$

Hence the initial speed of the bullet before the impact is 700.7 m/s.