A ball with a mass of #6 kg# moving at #9 ms^-1# hits a still ball with a mass of #4 kg#. If the first ball stops moving, how fast is the second ball moving?

1 Answer
Jan 14, 2016

The total momentum before the collision, #54 kg ms^-1#, will be the same as the total momentum after it. When divided by the mass of the second ball this yields a velocity of #13.5 ms^-1#.

Explanation:

Momentum is the product of the mass of an object and its velocity, and it is conserved : the total momentum before the collision is the same as the total momentum after it.

#p=mv#

Before the collision, the #4 kg# ball has a velocity of #0 ms^-1# and therefore also has momentum of #0 kgms^-1#. That means all the momentum is carried by the #6 kg# ball.

#p=mv=6*9=54 kgms^-1#

After the collision the #6 kg# is stationary so its momentum is zero, and all the momentum is carried by the #4 kg# ball.

#p = mv#
#v = p/m = 54/4 = 13.5 ms^-1#