|MadSci Network: Physics|
About 300 years ago a very insightful man named Issac Newton realized objects behave in certain ways. He wrote these ways down, and today his ideas are known as the Newtonian Laws of motion.
One of the things he wrote down is the law of inertia. It says an object at rest stays at rest unless another force acts upon it, and an object in motion stays in motion unless a force acts on it. A very tiny force can deflect a large object, if that force is constant and has enough time to affect it. Gravity is the weakest of the universal forces, but it holds the moon in orbit simply because it never takes a rest.
The water in a swimming pool is at rest.
Newton's second law of motion states the force of an object is equal to its mass multiplied by its acceleration (Force = mass X acceleration). The more massive an object is, and the faster it moves, the harder it hits.
Your hand has much more force when you slap the surface of the water than when you gently push through it. You're disturbing the inertia of the water suddenly instead of gradually.
Newton's third law of motion says for every action, there is an equal and opposite reaction. When you gently and gradually disturb the inertia of the water by pushing your hand through, the water gently and gradually disturbs the inertia of your hand. But when you forcefully affect the inertia of the water, it forcefully affects the inertia of your hand.
The density of the water hasn't changed. It feels harder when you slap it, but it isn't. It's resisting your hand with more force than it did when you pushed through it, and that's why it stings.
I hope this answers your question,
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