In many free body diagrams of the chapter Newton's laws of motion , I see that the acceleration of an object going downwards is given in the upward direction and for an object going upward it's acceleration is denoted downwards.I am not at all getting this concept though I am aware of pseudo forces and tension . Please help me with this concept.

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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