
#1
Feb2807, 04:36 PM

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1. The problem statement, all variables and given/known data
hello everyone, Im new to this forum but i really need some help doing a physics assignment. Im trying to use newtons second law and the equation for wind resistance(click link) to determine the the angle made by a specific velocity. The link to the assignment is http://www.instruction.greenriver.ed...Essay2201.pdf . I've tried two different approaches but im not sure if those are correct. First, I did draw my free body diagrams and broke them up into components of x and y axis: Here is what I got x(axis)=(wind resistance)(m)(g)sin(theta) y(axis)=(Force of tension)(m)(g)cos(theta) From there i've tried to manipulate both equations by solving for m, or g, and plugging into the other equation and solving for theta but thus far I've gotten nonrealistic solutions. So I thought maybe I was doing this a little to complicated, so instead I tried to solve this as if it were a triangle. I know that FT=mass*gravity if the ball is straight up and down. i also can calculate the force of windspeed using the formula, and then from there use some law of sins to get me an angle from the triangle but im not sure if thats applicable in this scenerio. Any and all help is much appreciated and thanks in advance 2. Relevant equations The ball is experiencing three forces, the tension in the string which works along the direction of the string, The weight of the ball vertically downwards and the drag acting horizontally on the ball in the direction of the wind. Taking the xaxis horizontally in the direction of the wind and the yaxis vertically upwards the tension in the string will contribute a x and ycomponent to the equations, since it is working at an angle w.r.t. this x/y axis system. What would be the two components and their signs in this system? 



#2
Feb2807, 04:38 PM

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The link doesn't work.




#3
Feb2807, 04:45 PM

P: 9

Here it is. I just copied and pasted it.
Essay 2 Measuring Wind Speed Imagine that you are a scientist and that you have just made the most exciting trip of your life. You are now camped at the southpole of the Earth. It is a wonder filled place, but very cold. However, your thirst for knowledge and the supplies flown in regularly sustain you. In a storm, one day, you lose you last reliable anemometer (measures wind velocity) when it is struck by lightning. However being the clever scientist that you are, you decide that this minor setback was not going to deter you from making a wind speed measurement. You figure out that you need nothing more than a ball (of known mass), a piece of string and a protractor to estimate the wind speed. You know Newton's laws of motion. You also recall from the physics class you took early in your career that the resistive force felt by an object as it moves through air is approximately proportional to the square of its speed. The equation that enables one to calculate this force, R (see chapter 6, page 164 of your textbook), is Av2 ρ D 2 1 R = Here D is a dimensionless constant called the drag coefficient that depends on the shape of the object (for spheres, D has a value of about 0.50), r is the density of air, A is the crosssectional area of the object and v is the speed with which the object is moving through the air. You realize that you can use the same equation to calculate the force felt by the ball irrespective of whether the ball moves through the air or whether the wind blows against the ball. You have several balls in your possession: a baseball, a pingpong ball, a tennis ball and a golf ball. Wake up! You are not on the continent of Antarctica. You are in Auburn, WA and you have an essay due. You must devise a way to use all this information, a ball and a string to measure the speed of the wind. You must have a final expression which relates the wind speed to some quantity you can measure. What that quantity (or, quantities) is (are) depends on the method you devise. You should clearly state what assumptions you make to do your calculation. These assumptions must be realistic and should not oversimplify the problem. Using the expression you found above, estimate what these measured quantity (quantities) will be if you used a tennis ball if you had 1) A gentle wind at 15 mph 2) A strong gale at 50 mph 3) A hurricane at about 75 mph You can look up the dimensions and the mass of the balls or measure them in class. Which ball would give you the best measurement for each of the above type of winds? Explain. 



#4
Feb2807, 04:46 PM

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Wind Speed Project 



#5
Feb2807, 06:34 PM

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Looks like you've chosen the y axis along the line of action of the tension force, in which case your equation along the perpendicular x axis is not correct. "R" acts horizontally. But there is a much easier way to solve for R without involving the tension force. In which direction will the resultant of the weight and wind forces act??




#6
Feb2807, 09:56 PM

P: 9

horizontall, right?




#7
Feb2807, 10:15 PM

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#8
Feb2807, 10:39 PM

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Thanks for your help. But I am so lost. Why would I want to solve for R if my teacher gave me the formula for R?




#9
Feb2807, 10:56 PM

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