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Q1: Initially the angular momentum of the water/can is zero. As the water escapes, the momentum of the moving water and the momentum of the moving can cancel each other out so that the total momentum of the system is zero. Q4: More examples of Newton's third law: When a ball is kicked, the toe pushes the ball forward (action) and the ball pushes back on the toe (reaction). The force of Earth's gravity pulls down on objects (action) and these objects pull up on the Earth with an equal but opposite force (reaction). This reaction force goes unnoticed because the mass accelerated by this force (the Earth) is huge, making the acceleration itself incredibly small. When water shoots out of a fire hose in one direction, the hose itself is pushed in the opposite direction. When a paintball is fired out of a gun in one direction, the gun recoils in the opposite direction.

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Explanation:

Q1:

In order to find the total momentum of the system, we first consider the initial stage when the can is at rest. The angular momentum of the water and the can is 0.

When the can starts moving, the water in the can also starts escaping. Therefore we can say that the momentum of escaping water and momentum of moving can cancel each other out.

Hence the net  momentum or total momentum of the system is equal to 0.

Q4:

Examples of Newton's third law:

1. When a player kicks a football, it moves in the forward direction which is considered as the action. At the same time, the ball pushes back the toe which is considered as a reaction. Both forces are equal in magnitude put opposite in the direction.

2. The Earth's gravitational force is a huge example of Newtons third law. Consider a case of chandelier hanging from the roof. The Earth's gravitational force pulls the chandelier down which is an action. whereas, the same time, the tension in the string attached to roof also pulls the chandelier with equal amount of energy. This is the reaction force.

3. Consider another case of water shooting out of a fire hose. The water moves out of the hose in one direction, which is the action. The force with which the waters moves out pf the hose, pushes the hose in backward direction which is the reaction. In all the examples you can observes that both action and reaction forces are acting in opposite direction but are equal in magniture.