Question

why a boatman pushes the river bank backward with a long bamboo pole on launching his boat into water

Answer 1

So that the river bank applies equal and opposite force on the boat and push it in the water.

Answer 2

Answer: When the boatman pushes the river with a bamboo pole, the river bank offers an equal and opposite force. This force helps the boat to move into the water.

Hope this helps✌️