Question

A ball with a mass of `3 kg` moving at `4 m/s` hits a still ball with a mass of `8 kg`. If the first ball stops moving, how fast is the second ball moving?

Answer 1

`3/2 or 1.5m/s`

Explanation:

P=mv

Momentum stay the same before and after a collision

`(3)(4) = (3)(0)+(8)(v)`

`v=12/8=3/2`

Answer 2

`1.5 \ "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 objects

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

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

So here, we got,

`3 \ "kg"*4 \ "m/s"+8 \ "kg"*0 \ "m/s"=3 \ "kg"*0 \ "m/s"+8 \ "kg"*v_2`

`v_2=(12color(red)cancelcolor(black)"kg""m/s")/(8color(red)cancelcolor(black)"kg")`

`=1.5 \ "m/s"`