physics problem?

Question

two fishes "A" whose mass is 5kg and "B" whose mass is 7.5kg, collide while swimming in opposite directions. Their speeds are A=2m/s and B=0.5m/s. After the completely inelastic collision, what are their speeds? What equations would be used to figure this question out?

Answer 1

Since the collision is INELASTIC, some of the kinetic energy is lost -- how much depends on other factors. It sounds like the bigger fish ate the smaller fish and while the big fish might slow down a little in the process, it would continue (we assume) in the same direction it had been going. Since KE = 0.5 mv^2, if you apply it to the two fish, you find that the smaller fish has a greater KE than the larger one by over a factor of ten. This would show that the big fish would actuall be jolted backwards by the force of the speedier small fish, but because this is an inelastic collision, some of that energy is lost to heat or other internal conversion, so the final velocities can't be calculated with the information you've provided.

Answer 2

Yes indeed, during inelastic collisions the conservation of kinetic energy is in some way violated, but they obey the conservation of momentum:

m1v1 + m2v2 = (mi + m2) vf (if completely ineslatic both fishes are stuck together after the collision)

where : vf = final velocity

(5 Kg) (2 m/s) + (7.5 Kg) (0.5 m/s) = (5 Kg + 7.5 Kg) vf

10 + 3.75 = 12.5 vf

we finally get:

vf = 1.1 m/s

That´s it.

Good luck!