Question

A large fish has a mass of 25.0 kg and swims at 1.00 m/s toward and then swallows a smaller fish that is not moving. If the smaller fish has a mass of 1.00 kg.

a) What is the speed of the larger fish immediately after it finishes lunch?

b) Is this an elastic or inelastic collision?

c) Is energy conserved in this collision?

Answer 1

Consider the mass of the large fish as M and the small fish as m. Also consider that the initial velocity of the large fish as u and the final velocity as v.
Initial momentum of the large fish, Pi = Mu
= 25 x 1 = 25 kg m/s
Final momentum of the large + small fish, Pf = (M + m) v
= (25 + 1) v = 26 v

Using the conservation of momentum, Pi = Pf
25 = 26 v
v = 25/26
= 0.96 m/s

b)
Since the kinetic energy is not conserved, the collision is inelastic.

c)
Energy is not conserved as seen in part b.