Question

A ball with a mass of `12 kg` moving at `7 m/s` hits a still ball with a mass of `15 kg`. If the first ball stops moving, how fast is the second ball moving?

Answer 1

The second ball is moving at `5.6 \ "m/s"`.

Explanation:

We use the law of conservation of momentum, which states that,

`m_1u_1+m_2u_2=m_1v_1+m_2v_2`

• `m_1,m_2` are the masses of the two balls

• `u_1,u_2` are the initial velocities of the two balls

• `v_1,v_2` are the final velocities of the two balls

And so, we have,

`12 \ "kg"*7 \ "m/s"+15 \ "kg"*0 \ "m/s"=12 \ "kg"*0 \ "m/s"+15 \ "kg"*v_2`

`84 \ "kg m/s"=15 \ "kg"*v_2`

`v_2=(84color(red)cancelcolor(black)"kg""m/s")/(15color(red)cancelcolor(black)"kg")`

`=5.6 \ "m/s"`