Angular momentum(L)is the cross product of r and linear momentum.If external force is zero,then linear momentum is a constant,thereby angular momentum is constant.My doubt is that there are cases where torque external is not zero even if external force is zero for example couple.torque is rate of change of angular momentum.since torque is non zero that means angular momentum changes.So which is true and why?

Three identical balls are placed on a frictionless horizontal surface touching each other. They stick to each other because of adhesive. Another ball of same radius and mass m is placed over the void created by the three balls. Find the forces applied by the balls kept on the floor to each other if the system remains in equilibrium.

A car loaded with water having total mass 1000 kg moves on a straight horizontal Road starting from rest under the action of a force of 100 Newton. The water spills throw a small hole in the bottom at a rate of 0.1 KG per second the velocity of cart after 300 seconds is nearly.

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