Question

A 300 g bird flying along at 6.0 m/s sees a 12 g insect heading straight toward it with a speed of 33 m/s (as measured by an observer on the ground, not by the bird). The bird opens its mouth wide and enjoys a nice lunch.

What is the bird's speed immediately after swallowing?

Express your answer to two significant figures and include the appropriate units.

Answer 1

Initial momentum of the bird, Pb = Mv
Where M is the mass of the bird and v, its initial velocity.
Pb = 300 x 6 = 1800 g m/s

Initial momentum of the insect, Pi = m (-V)             [Negative sign indicates that it is opposite to v]
Where m is the mass of the insect and V is its velocity
Pi = - 12 x 33 = - 396 g m/s

Total initial momentum = Pb + Pi
= 1800 - 396
= 1404 g m/s

Consider that the final velocity of the bird after it swallows the insect is u.
Final mass = m + M
= 300 + 12
= 312 g
Final momentum = 312 u

Using the conservation of momentum, initial momentum = final momentum
1404 = 312 u
u = 1404/312
= 4.5 m/s