# How Does Changing Drag Coefficient Affect Terminal Velocity in Downhill Skiing?

• dprimedx
In summary, the conversation discusses the factors that affect a skier's speed while downhill skiing, including air drag and kinetic friction. The terminal speed is determined to be 59 m/s based on given variables. The question asks for the corresponding change in terminal speed if the drag coefficient is adjusted, and the answer can be found by differentiating the quadratic drag force formula with respect to C. The constant 'k' is also mentioned in the conversation.
dprimedx
Information:
"In downhill speed skiing a skier is retarded by both the air drag force on the body and the kinetic frictional force on the skis. Suppose the slope angle is θ = 31.5°, the snow is dry snow with a coefficient of kinetic friction μk = 0.0400, the mass of the skier and equipment is m = 79.5 kg, the cross-sectional area of the (tucked) skier is A = 1.30 m2, the drag coefficient is C = 0.150, and the air density is 1.20 kg/m3."

Questions:
What is the terminal speed? Ok I got this part and came up with a velocity of 59 m/s.

"If a skier can vary C by a slight amount dC by adjusting, say, the hand positions, what is the corresponding variation in the terminal speed? (dvt/dC)"
This doesn't make any sense to me. I checked around online and all I can find is that dvt/dc=(-1/2)(k)(c^(-3/2)), but no one explains how they derived this. Also, the program I'm using seems to want an actual number for an answer but it doesn't tell me how much c is changing by. I also am unsure as to what 'k' is in the previous equation. Could someone explain what it is?

Can anyone help explain this to me? I'm not looking for the exact answer for the question, just an explanation as to how to arrive at it.

APhysicsstudent
I am assuming you got the first part right using the quadratic drag force formula to find the terminal velocity . If you differentiate it with respect to the drag coefficient, C, you'll get your answer, and discover what the constant 'k' is.

I can help explain the concepts and equations involved in this problem. The first thing to understand is that terminal velocity is the maximum velocity that an object can reach when falling through a fluid, in this case air. It occurs when the drag force on the object is equal to the force of gravity pulling it down. In other words, the net force on the object is zero, so it stops accelerating and reaches a constant velocity.

Now, in this situation, the skier is experiencing both air drag force and kinetic frictional force, both of which are acting in the opposite direction of their motion. The air drag force is dependent on the air density, the velocity of the skier, and the cross-sectional area of the skier. The kinetic frictional force is dependent on the coefficient of friction, the mass of the skier, and the slope angle. By plugging in the given values into the equations for these forces, we can calculate the terminal velocity of the skier, which you have correctly determined to be 59 m/s.

Now, for the second part of the question, we are asked to consider how a slight change in the drag coefficient, C, would affect the terminal velocity. The equation you found, dvt/dc=(-1/2)(k)(c^(-3/2)), is known as the sensitivity equation. It shows the relationship between the change in the terminal velocity (dvt) and the change in the drag coefficient (dc). The "k" in this equation is a constant that represents the combined effects of the air density, cross-sectional area, and mass of the skier. It is not a specific value, but rather a coefficient that allows us to calculate how much the terminal velocity will change for a given change in the drag coefficient. This equation is derived from the fundamental principles of fluid mechanics and is beyond the scope of this explanation, but you can find more information on it in textbooks or online resources.

So, to answer the question, the corresponding variation in the terminal speed would be given by the sensitivity equation, which depends on the value of "k" and the amount by which the drag coefficient is changed. The program you are using may want a specific numerical value, so you may need to plug in some values for "k" and the change in the drag coefficient to get a numerical answer. I hope this helps to clarify the concepts and equations involved in this problem.

## 1. What is drag?

Drag is a force that opposes the motion of an object through a fluid, such as air or water. It is caused by the interaction between the object and the molecules of the fluid it is moving through.

## 2. How does an object's shape affect drag?

The shape of an object can greatly affect the amount of drag it experiences. Objects with a streamlined shape, such as an airplane or a dolphin, experience less drag than objects with a large surface area, such as a parachute or a flat sheet of paper.

## 3. What is terminal velocity?

Terminal velocity is the maximum velocity that an object can reach when falling through a fluid. It occurs when the drag force on the object is equal to the force of gravity pulling the object downward.

## 4. How is terminal velocity affected by the mass of an object?

The mass of an object does not have a significant effect on its terminal velocity. All objects, regardless of their mass, will eventually reach the same terminal velocity when falling through a fluid. However, heavier objects may take longer to reach this velocity due to the increased force of gravity pulling on them.

## 5. Can terminal velocity be exceeded?

In theory, terminal velocity can be exceeded if the force acting on the object is greater than the combined forces of gravity and drag. However, in most real-world scenarios, it is very difficult for an object to exceed its terminal velocity due to the increasing resistance of the fluid as the object accelerates.

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