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Air resistance in pendulum experiment 
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#1
Feb2308, 04:08 PM

P: 3

hey guys, i've been experimenting with a pendulum and while doing the error discussion for my experiments i got stuck with the air resistance involved...
I've used the formula to find the force applied by the air resistance to the pendulum and ended up with F=2,76*10^5 *v^2 . (v= velocity of the pendulum). Basically i just want to estimate how much of an error percentage of my results air resistance has caused, but i don't have much of an idea how to calculate this percentage. thanks for your help! 


#2
Feb2408, 06:20 AM

P: 126

I suppose you mean drag force.
F = K S v^2 K: Constant S: widest crosssection v: velocity The velocity is changing time by time. Yo may not use one velocity to calculate it. In fact the velocity of body has, V = Vm Sin(wt) where V: sudden, Vm: max, w=2(pi)f and t: time But if we use a average worth, i would put my money to :) , V = Vm/sqrt(2) this is better than others. F = K S ( Vm/sqrt(2) ) this is a suggestion. Maybe there is a better solution. 


#3
Apr2709, 08:49 PM

P: 44

How do you calculate the cross section?



#4
Apr2809, 10:14 AM

P: 44

Air resistance in pendulum experiment
What is the K?



#5
Apr2809, 02:09 PM

P: 126

Sorry, assumed you know. Known air resistance formula,
F=  1/2 p v^2 A C p: density of the fluid v: speed of the object relative to the fluid A: reference area C: drag coefficient You can look at below link (http://en.wikipedia.org/wiki/Drag_(p...Parasitic_drag) Mostly you don't calculate the effective cross sectional area. It is "reference area" in last equation. For a spherical body, widest cross section area is a circle which has the same radius of sphere. And forget K :) use the latest formula. Warning, I have no idea about finding(calculating) the effect of air resistance for pendulum. I am not sure about certainty of above(first) suggestion. Just an approach. I wanted to help you to find a good average drag force. So I find a good average velocity to use in formula. If you use a long string, even max. velocity will be more little. So drag force(air resistance) effect will be much less. But calculation is not so easy. 


#6
Apr2809, 03:36 PM

P: 44

So, basically calculating it is not even worth it if it is so small. So that probably wouldn't have an effect on our experiment. Thanks for the help.



#7
Apr2809, 03:57 PM

P: 126

Don't forget, use little angles(less then 10 degree). This is for both simple pendilum condition and required for less velocity(air resistance).



#8
Apr2809, 06:13 PM

P: 44

But how come, even when we used 45 degrees, the period, according to the photogate was still T=2π √(L/g)=1.22?
L was 37cm=.37m g=9.8m/s2 My data was like this: Theta in degrees, T 05, 1.194 10, 1.196 15, 1.201 20, 1.203 25, 1.209 30, 1.215 35, 1.223 40, 1.238 45, 1.241 I am guessing that the ball was steel.....So is wind resistance even a factor there? Is it just my error? 


#9
Apr2809, 08:46 PM

P: 126

Look at here http://en.wikipedia.org/wiki/Pendulum you will see another period equation for larger amplitudes.
Maybe because of bigger air friction :) larger degrees are not much big as expected. But look closer, T is already growing. Learn the exact acceleration of gravity and compare with yours then see which one is closer to formula. 


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