On a frictionless horizontal air table, puck A (with mass 0.254 kg ) is moving toward puck B (with mass 0.367 kg ), which is initially at rest. After the collision, puck A has velocity 0.123 m/s to the left, and puck B has velocity 0.651 m/s to the right.

What was the speed vAi of puck A before the collision?

Calculate ΔK, the change in the total kinetic energy of the system that occurs during the collision.

According to the law of conservation of linear momentum, the total momentum of both pucks won't be changed regardless of their interaction if no external forces are acting on the system.

Being [tex]m_a[/tex] and [tex]m_b[/tex] the masses of pucks a and b respectively, the initial momentum of the system is

[tex]M_1=m_av_a+m_bv_b[/tex]

Since b is initially at rest

[tex]M_1=m_av_a[/tex]

After the collision and being [tex]v'_a[/tex] and [tex]v'_b[/tex] the respective velocities, the total momentum is

[tex]M_2=m_av'_a+m_bv'_b[/tex]

Both momentums are equal, thus

[tex]m_av_a=m_av'_a+m_bv'_b[/tex]

Solving for [tex]v_a[/tex]

[tex]v_a=\frac{m_av'_a+m_bv'_b}{m_a}[/tex]

[tex]v_a=\frac{0.254Kg\times (-0.123 m/s)+0.367Kg (0.651m/s)}{0.254Kg}[/tex]

[tex]v_a=0.8176 m/s[/tex]

The initial kinetic energy can be found as (provided puck b is at rest)

[tex]K_1=\frac{1}{2}m_av_a^2[/tex]

[tex]K_1=\frac{1}{2}(0.254Kg) (0.8176m/s)^2=0.0849 J[/tex]

The final kinetic energy is

[tex]K_2=\frac{1}{2}m_av_a'^2+\frac{1}{2}m_bv_b'^2[/tex]

[tex]K_2=\frac{1}{2}0.254Kg (-0.123m/s)^2+\frac{1}{2}0.367Kg (0.651m/s)^2=0.07969 J[/tex]

The change of kinetic energy is

[tex]\Delta K=0.07969 J - 0.0849 J = -0.00521 J[/tex]