When a bird flies, it pushes the air with force in the downward direction with its wings. In accordance with Newton’s law of motion, the air exerts equal and opposite reaction. According to the parallelogram law, the resultant of the reaction acts on the birds in the upward direction and helps the bird to fly upwards.

P = 3x and Q = 5x R = 35N θ = 120°

R = P2+R2+2PQcosθ−−−−−−−−−−−−−−−−√

35 = 7x or x = 35/7 = 5

P = 15N Q = 25N.

While walking, a person presses the ground with his feet slightly slanted in the backward direction. The ground exerts upon him an equal and opposite reaction. Its horizontal component enables the person to move forward while the vertical component balances this weight.