Answer: a) 4.7 mi/hr. b) 86,500 lbs. mi²/Hr²
Explanation:
As in any collision, under the assumption that no external forces exist during the very small collision time, momentum must be conserved.
If the collision is fully inelastic, both masses continue coupled each other as a single mass, with a single speed.
So, we can write the following:
p₁ = p₂ ⇒m₁.v₁ + m₂.v₂ = (m₁ + m₂). vf
Replacing by the values, and solving for vf, we get:
vf = (200,000 lbs. 4 mi/hr + 100,000 lbs. 6 mi/hr) / 300,000 lbs = 4.7 mi/hr
If the track is horizontal, this means that thre is no change in gravitational potential energy, so any loss of energy must be kinetic energy.
Before the collision, the total kinetic energy of the system was the following:
K₁ = 1/2 (m₁.v₁² + m₂.v₂²) = 3,400,000 lbs. mi² / hr²
After the collision, total kinetic energy is as follows:
K₂ = 1/2 ((m₁ + m₂) vf²) = 3,313,500 lbs. mi²/hr²
So we have an Energy loss, equal to the difference between initial kinetic energy and final kinetic energy, as follows:
DE = K₁ - K₂ = 86,500 lbs. mi² / hr²
This loss is due to the impact, and is represented by the work done by friction forces (internal) during the impact.
and 
, and using the relationship mentioned above, we can find the mass m of the particle:
