Why does water get 'hard' on impact?

In summary, the resistance to motion in water increases with speed and is dependent on the drag equation, which takes into account factors such as drag constant, mass density, velocity, and cross-sectional area. This is why jumping into water at high speeds can result in significant force and potential harm. The drag equation is also used in calculating drag for cars, with factors such as Cd (streamlining), A (front area), and V^2 (velocity) affecting the force. Additionally, there is a different equation for slower, thicker fluids such as oil. Surface tension also plays a role in water resistance.
  • #1
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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  • #2
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.
 
  • #3
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.
 
  • #4
Thanks for the reply guys!
I suppose that drag equation are used in drag for cars as well?
 
  • #5
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.
 
  • #6
it's called surface tension
 

Related to Why does water get 'hard' on impact?

1. Why does water become hard when it is hit?

Water becomes hard on impact because of the phenomenon known as water hammer. When an object hits the surface of the water, it creates a sudden disturbance, causing the water molecules to rapidly move and collide with each other. This rapid movement and collision causes the water to feel hard to the touch.

2. Is the hardness of water on impact the same as the hardness of a solid object?

No, the hardness of water on impact is not the same as the hardness of a solid object. Water is a liquid and does not have a fixed shape or volume like a solid object. The hardness of water on impact is only temporary and will return to its original state once the disturbance is gone.

3. Does the temperature of the water affect its hardness on impact?

Yes, the temperature of the water can affect its hardness on impact. When water is colder, the molecules are closer together, making it feel harder on impact. On the other hand, warmer water has more space between its molecules, making it feel less hard on impact.

4. Why does water feel harder when it is hit at a higher velocity?

Water feels harder when it is hit at a higher velocity because the force of the impact is greater. When an object hits the water at a higher velocity, it creates a larger disturbance, causing the water molecules to move and collide with more force, making it feel harder to the touch.

5. Can the hardness of water on impact cause damage?

Yes, the hardness of water on impact can cause damage. If an object hits the water with enough force, it can cause injury or damage to the object or surface it is hitting. Additionally, repeated impacts on the same spot can cause erosion or wear on the surface of the water, such as in the case of a waterfall.

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