eli5 If you have a large tank of water on a wheeled stand, and you poked a hole in the side, it should roll forward to conserve momentum. But how does the water transfer that motion to the tank?
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Gravity is constantly pulling the water down. We call this force the “static pressure”. You can think of this like the “potential energy” of the water in elementary physics. When you open that container that potential energy is converted into kinetic energy as the force of gravity pulling the water down pushes it out of the opening.
Of course, newtons laws of motion apply and in order or it to be pushed out it must also push against the tank with an equal amount of force.
The water have static pressure due to the force of gravity acting on its mass. The pressure just means it is pushing out in all directions. In fact you measure pressure as force over area. So the water is pushing the front of the tank and the rear of the tank with the exact same pressure and because they are the same area that means the same force. The net force is therefore zero. However when you poke a hole in the tank you remove some of the area that the pressure acts on. So now there is more force on the side with the larger area then the side with the smaller area due to the hole. A net force is therefore acting in this direction.
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