# Measuring wind speed with ball and string Newtons 2nd law

1. Jun 4, 2006

### eboarder2020

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.edu/physics/anarayanan/PHYS201Sp2006/Notes/Essay2-201.pdf" [Broken] . 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 non-realistic 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

Last edited by a moderator: May 2, 2017
2. Jun 6, 2006

### andrevdh

The ball is experiencing three forces, the tension $$T$$ in the string which works along the direction of the string, The weight of the ball $$W$$ vertically downwards and the drag $$R$$ acting horizontally on the ball in the direction of the wind. Taking the x-axis horizontally in the direction of the wind and the y-axis vertically upwards the tension in the string will contribute a x- and y-component to the equations, since it is working at an angle w.r.t. this x/y axis system. What are these two components and their signs in this system?

3. Jun 6, 2006

### eboarder2020

Hey i turned in the assignment... I asked a few friends at school for help and I got it.... The angle theta can be measured in the inverse tangent of the wind force over the mass times gravity. THanks for the help

4. Jun 6, 2006

### dav2008

Oh my!

10 characters

5. Jun 6, 2006

### andrevdh

Did they explain it to you? If not let's hope they are not there when you write exam.