Understanding Drag in Water: Calculation and Hypothetical Scenarios"

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In summary: It is possible that it may bounce slightly if the material is elastic, but it would not fly up into the air. In summary, the drag on an object falling into water can be calculated using the equation 0.5 * velocity^2 * water density * Drag Coefficient * Cross Sectional Area, with velocity measured in m/s and other units kept consistent. The force of drag will decrease as the object decelerates, so it is unlikely to be flung up or bounce significantly unless it is made of elastic material.
  • #1
aosome23
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Correct me if I'm wrong but to find drag for water you have to use this equation:
0.5 * velocity^2 * water density which is 1000kg/m^3 * Drag Coefficient * Cross Sectional Area
Velocity is supposed to be in m/s right? not km/h?


Okay so for example if there was an imaginary planet with no air and atmosphere, but only a large body of water(ocean). If a 1*1*1 meter cube was falling and it hit the ocean at 200m/s, the drag will be:
0.5 * 100m/s * 1000kg/m^3 * about 1 * 1m^2 = 50000
when it hits the water

If the cube is made out of really strong light weight material, wouldn't the cube get flung up? For example if the cube is only a gram. Is this even possible? When I think about it in my head, no matter how hard an object hits the surface of water, it would never bounce that much...

So anyways, i feel like I am misunderstanding something... Can someone help me?
 
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  • #2
aosome23 said:
Velocity is supposed to be in m/s right? not km/h?
The units do not matter, as long as you keep them consistent. If different quantities are given in different units, you'll have to convert some of them.

0.5 * 100m/s * 1000kg/m^3 * about 1 * 1m^2 = 50000
Don't forget units.

If the cube is made out of really strong light weight material, wouldn't the cube get flung up?
It would decelerate extremely fast and then come to a rest (approximately) - unless the cube itself is elastic, then it might bounce.
 
  • #3
mfb said:
It would decelerate extremely fast and then come to a rest (approximately) - unless the cube itself is elastic, then it might bounce.

But if the force applied to the cube is 50000N wouldn't the cube be flung up because the mass is really small? So since force is m * a, if a cube is 1kg, that means that the acceleration will be 50000m/s^2. Since that is way larger than gravity which is 9.8m/s^2, wouldn't it make the cube fly up in the air? Or am I not calculating this correctly?
 
  • #4
Force in what direction? You were asking about drag. That will be directed opposite to the velocity. In order to have a force upward to "fling" the cube upward the velocity must be downward. But as in order to go "upward" the velocity would have to change from downward to upward so must, at some point, be 0. As soon as that is true, there is no longer any drag force.
 
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  • #5
aosome23 said:
But if the force applied to the cube is 50000N wouldn't the cube be flung up because the mass is really small? So since force is m * a, if a cube is 1kg, that means that the acceleration will be 50000m/s^2. Since that is way larger than gravity which is 9.8m/s^2, wouldn't it make the cube fly up in the air? Or am I not calculating this correctly?
You are not thinking about this correctly. As soon as the cube starts to contact the water, it will decelerate very quickly, its velocity will drop rapidly, and the drag force will decrease very quickly. So the force on the cube won't stay anywhere close to 50000N for more than a minute interval of time.

Chet
 

1. What is drag and how does it affect water?

Drag is the force that opposes the motion of an object through a fluid, in this case water. It is caused by the friction between the object and the fluid. Drag affects water by creating resistance, making it harder for objects to move through the water.

2. How do you calculate drag in water?

The drag force in water can be calculated using the equation Fd = 0.5 * ρ * v^2 * Cd * A, where ρ is the density of the water, v is the velocity of the object, Cd is the drag coefficient, and A is the cross-sectional area of the object.

3. What factors affect the amount of drag in water?

The amount of drag in water is affected by several factors, including the shape and size of the object, the speed at which it is moving, the density and viscosity of the water, and the surface roughness of the object.

4. How can you reduce drag in water?

To reduce drag in water, you can use streamlined or aerodynamic shapes for objects, decrease the speed at which they are moving, or make the surface smoother. Other methods include using lubricants or adding fins to the object to redirect the flow of water.

5. Why is finding drag important for water-based activities?

Finding drag is important for water-based activities because it helps us understand the forces acting on objects in water and how they affect their movement. This knowledge can be used to improve the design and efficiency of watercraft, as well as to make predictions for safety and performance in activities such as swimming, surfing, and boating.

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