Final answer:
In physics, the velocity of two masses after a collision can be determined by using the conservation of momentum. The combined velocity will typically be lower than the velocity of the moving object before the collision due to the increase in total mass.
Step-by-step explanation:
The question involves the concept of conservation of momentum from physics, specifically related to collision theory. When the 30-g fish moving at 80 cm/s is eaten by the 45-g fish, the velocity of the fish after the 'collision', which in this case could be considered an inelastic collision as they combine to be one mass, will be different from their initial velocities. According to the law of conservation of momentum, the total momentum before and after the collision must be equal, assuming no external forces are acting on the system.
Momentum is calculated using the formula p = mv, where p is momentum, m is mass, and v is velocity. For the total system
By setting the total momentum before equal to the total momentum after and solving for the final velocity, you will find how the velocity of the combined fish changes. Generally, the resulting velocity will be less than the velocity of the smaller fish before it was eaten because the total mass has increased.