Question

why is it saying that sum of all forces is zero when the boat is moving at a constant speed​

Answer 1

Answer:

Because a body will stay in state of motion or rest until and unless interrupted by some external force. (Newton's Law of Motion)

To understand better, let me cite an example. You must have seen space videos in which if an astronaut throws away an object both, the object and astronaut himself start moving in opposite directions. What happens is that he applies force on that object and that object on him simultaneously. When both depart they have velocities which satisfy the equation M1V1 + M2V2 = 0 : where M and V are mass and velocity respectively and numerals 1, 2 represent astronaut and the object. Now the kinetic energy is stored in both of them (1/2 mv^2) which will be in it until something stops it and that energy is dissipated in some form, be it friction or say in the resultant motion of the interrupting body.

Explanation:

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