A ball with a mass of #7# #kg# moving at #14# #ms^-1# hits a still ball with a mass of #15# #kg#. If the first ball stops moving, how fast is the second ball moving?

1 Answer
David G.
Mar 13, 2016

Momentum is conserved, so the momentum after the collision is the same as the total momentum before. This means the final velocity of the #15# #kg# ball is #6.5# #ms^-1#.

Explanation:

Let's call the #7# #kg# ball '1' and the #15# #kg# ball '2'.

Momentum before the collision:

#p=m_1v_1+m_2v_2=7*14+15*0=98# #kgms^-1#

Momentum after the collision:

#p=m_1v_1+m_2v_2=7*0+15*v_2#

But conservation tells is this is equal to the total momentum before, so:

#7*0+15*v_2=98#

Rearranging, #v_2=98/15=6.5# #ms^-1#

(PS Hi, fellow Queenslander! I taught at UQ in St Lucia for 6 years, but now teach at Griffith on the Gold Coast)

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