Why does water get 'hard' on impact?

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    Hard Impact Water
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Discussion Overview

The discussion revolves around the phenomenon of water behaving differently upon impact compared to gentle immersion, exploring the reasons behind the perceived hardness of water during high-speed entry. It touches on concepts of fluid dynamics, drag forces, and surface tension.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the rapid distribution of forces across water molecules contributes to the hardness experienced upon impact.
  • Another participant notes that resistance force in water is proportional to the speed cubed, indicating that increased speed significantly amplifies the force experienced.
  • A participant provides a drag equation to quantify the force experienced when entering water at high speeds, including specific values for density and area.
  • There is a suggestion that the drag equation is also applicable to cars, with a clarification that the equation is valid under turbulent flow conditions.
  • Discussion includes a note that the drag equation may not accurately describe the dynamics of hitting the water's surface, as different equations apply under varying conditions of fluid thickness and flow.
  • One participant introduces the concept of surface tension as a relevant factor in the discussion.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the factors contributing to the hardness of water on impact, with no consensus reached on the primary explanation or the applicability of the drag equation in this context.

Contextual Notes

The discussion highlights the complexity of fluid dynamics, with participants noting different conditions under which drag equations apply, and the potential limitations of these models in describing high-speed impacts with water.

Ask1122
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Hi all, have been wondering, why is it that when you put your hand in water, it's soft and there is hardly any resistance, but when you jump into it like from 100m above, you will most probably kill yourself?

Is it because there is not enough time for the forces to distribute across the water molecules at the time of impact?
 
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How much resistance force (friction force) an object moving through a medium experiences depends upon the speed (obviously- no speed, no force!) and experimentation has shown that the resistance to motion in water is roughly proportional to the speed cubed. That is, if you go through water at twice the speed, you feel eight times the force.
 
It is due to drag, the resistance to a body traveling through a fluid.

[itex]F_{drag} = \frac{k_{drag}\rho v^{2}A}{2}[/itex]

k_drag — drag constant (k/1)
ρ — mass density of medium (kg/m^3)
v — velocity relative to medium (m/s)
A — cross-sectional normal area (m^2)

Let's say that water has a mass density of 1000 kg/m^3, the impact velocity is 20 m/s, the normal area at impact (of two feet) is ~0.03 m^2, and the drag constant is 0.9. This results in an instantaneous force of 5.4 kN.
 
Thanks for the reply guys!
I suppose that drag equation are used in drag for cars as well?
 
Ask1122 said:
I suppose that drag equation are used in drag for cars as well?
Yes, that's the drag equation for cars ( k is normally written as Cd for aerodynamic drag)

The drag equation not really correct for hitting the surface of water, the equation is valid for what we call turbulent flow - when you are going fast through a fluid and the air or water is swirling past you. There is a different equation for when you are going slowly through a thicker fluid - like oil flowing through an engine

Notice that it depends on Cd (which is why makers quote it for sports cars) - how streamlined the shape is.
but also A (the front area) - which is why fast cars are small.
And V^2, so as you go twice as fast the drag increases by 4 times - this is what really limits the speed of cars.
 
it's called surface tension
 

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