Answer: 1.9 m/s
Explanation:
The question should be:
If a 20 kg green fish swimming at 2 m/s swallows a 1 kg orange fish at rest, in what direction, and how fast will the green fish swim after eating the orange fish?
Ok, here we have conservation of momentum.
At the beginning, the total momentum is equal to the sum between the momentum of the green fish and the momentum of the orange fish.
Where the momentum is written as:
P = m*v
m = mass
v = velocity.
The momentum of the green fish is:
Pg = 20kg*2m/s = 40 kg*m/s.
The momentum of the orange fish is:
Po = 1kg*0m/s = 0
The total initial momentum is:
Pi = Pg + Po = 40 kg*m/s.
After the green fish eats the orange fish, we do not have an orange fish anymore, and the mass of the green fish will be equal to it's initial mass, plus the mass of the fish that it ate, this will be:
M = 20kg + 1kg = 21kg.
Then the momentum will be:
Pf = 21kg*V
Where V is the final velocity.
For conservation of momentum, the initial momentum is equal to the final momentum, then:
Pi = Pf
40 kg*m/s = 21kg*V
(40/21) m/s = 1.9 m/s = V
The fish's final velocity is 1.9 m/s
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