Why do object placed in a river rotates at different position when object is in center of flowing river it just moves forward but when it is on either side it rotates and moves forward what's the reason behind this.
In a river the velocity of water has a maximum at centre and decreases to zero at the bank.
Consider an object situated midway between middle and bank. It will be dragged by water at a velocity that is an average of velocities at its sides. Then the side facing the middle will see water moving downstream whereas the opposite side will see water moving upstream. So water's friction on both sides will cause a torque acting on body and making it rotate.