A ball with a mass of #6 kg# moving at #3 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?

2 Answers
Jan 13, 2016

Momentum is conserved, so if the first ball stops from the collision, then the second ball will have the same momentum as the first ball did.

Explanation:

The momentum of the firs ball is:

#p = mv #
#p= 6kg * 3m/s#
#p=18 kgm/s#

If that ball stops, then all the momentum is transferred to the second ball,

#p = mv #
#18 kgm/s = 8kgm/s*v#

Solve for #v#

Jan 13, 2016

(6 X 3)/8 = 2.25 m per sec in the same direction

Explanation:

Use law of conservation of momentum.
Momentum = mass X Velocity
Ball 1 has mass #m_1# and initial velocity #v_i^1#
Ball 2 has mass #m_2# and initial velocity #v_i^2#
Initial momentum = #m_1# X #v_i^1# + #m_2# X #v_i^2#
Ball 1 has final velocity #v_f^1#
Ball 2 has final velocity #v_f^2#
Final momentum = #m_1# X #v_f^1# + #m_2# X #v_f^2#
Initial momentum = Final momentum
#m_1# X #v_i^1# + #m_2# X #v_i^2 = m_1# X #v_f^1# + #m_2# X #v_f^2#
As initial velocity of Ball 2 is 0 before collision and velocity of Ball 1 is 0 after collision.
#m_1# X #v_i^1# = #m_2# X #v_f^2#