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?

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Answer 1 · 2018-04-21T12:26:34

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

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"#